2-pyridone: the role of out-of-plane vibrations on the s[sub 1]↔s[sub 0] spectra and s[sub 1]...

THE JOURNAL OF CHEMICAL PHYSICS 125, 114308 �2006�

2-pyridone: The role of out-of-plane vibrations on the S1^S0 spectraand S1 state reactivity

Jann A. Frey, Roman Leist, Christian Tanner, Hans-Martin Frey, and Samuel Leutwylera�

Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

�Received 1 June 2006; accepted 25 July 2006; published online 18 September 2006�

The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone �pyridin-2-one �2PY�� and its N–Hdeuterated isotopomer �d-2PY� have been recorded by two-color resonant two-photon ionization,laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combiningthese methods, the B origin of 2PY at 00

0+98 cm−1 and the bands at +218 and +252 cm−1 areidentified as overtones of the S1 state out-of-plane vibrations �1� and �2�, as are the analogous bandsof d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-planecoordinates that predict further �1� and �2� overtones and combinations, reproducing �80% of thevibronic bands up to 600 cm−1 above the 00

0 band. The fluorescence spectra excited at the electronicorigins and the �1� and �2� out-of-plane overtone levels confirm these assignments. The S1 nonplanarminima and S1←S0 out-of-plane progressions are in agreement with the determination of nonplanarvibrationally averaged geometries for the 00

0 and 000+98 cm−1 upper states by Held et al. �J. Chem.

Phys. 95, 8732 �1991��. The fluorescence lifetimes of the S1 state vibrations show strong modedependence: Those of the out-of-plane levels decrease rapidly above 200 cm−1 excess vibrationalenergy, while the in-plane vibrations �5�, �8�, and �9� have longer lifetimes, although they are aboveor interspersed with the “dark” out-of-plane states. This is interpreted in terms of an S1� state reactionwith a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-planevibrational states can directly surmount this barrier, whereas in-plane vibrations are much lessefficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S0� statefundamentals, overtones of the S0 state �1� and �2� out-of-plane vibrations, and �30 other overtonesand combination bands. The B3LYP/6-311+ +G�d , p� calculated anharmonic wave numbers are invery good agreement with the observed fundamentals, overtones, and combinations, with adeviation � =1.3%. © 2006 American Institute of Physics. �DOI: 10.1063/1.2338042�

I. INTRODUCTION

Cis-amide groups constitute or contribute to thehydrogen-binding sites of several nucleobases. 2-pyridone�pyridin-2-one �2PY�� is one of the smallest nucleobasemimics containing a cis-amide group and has hence beenwidely employed in chemical and biochemical contexts1–4 aswell as in gas-phase studies.5–16 Numerous experimental17–19

and theoretical20–27 investigations have treated the ketoenoltautomerization reaction of 2PY to 2-hydroxypyridine �2HP�.2-pyridone can also be considered as a model for the excited-state reactivity of the pyrimidine nucleobases uracil andthymine.28 Since their solution and gas-phase spectra arebroad and structureless,29,30 it is difficult to obtain informa-tion on their excited-state structure, vibrations, and reactivity.

The first spectroscopic study of supersonically cooled2-pyridone5 has shown two equally intense bands in the reso-nant two-photon ionization �R2PI� spectrum, at 29 831 and29 930 cm−1, which were interpreted as the electronic origins

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�A and B� of two different ground-state conformers.5 Fluo-rescence spectra excited at the A and B origins were attrib-uted to different species, although no detailed vibrational as-signments were given.5 Held et al. measured rotationallyresolved fluorescence excitation spectra of both the A and Borigins.6 They showed that the respective S0 state rotationalconstants are identical and correspond to a planar molecule,while the respective S1 state rotational constants are charac-teristic for a slightly nonplanar structure.6 They interpretedthe nonplanar S1 state in terms of a pyramidal structurearound the N atom of 2PY, resulting in a pseudoaxial and apseudoequatorial orientation of the N–H bond.6 Using micro-wave and millimeter-wave spectroscopies, Hatherley et al.determined the ground-state rotational constants and alsofound only a single planar ground-state form of 2PY.31 Re-cently, Tanjaroon et al. measured the deuterium and nitrogenquadrupole hyperfine structures and provided even more pre-cise rotational constants.32 Using fluorescence-dip infrared�FDIR� and fluorescence-dip stimulated Raman �FDSR�depletion spectroscopies, Matsuda et al. confirmed that the Aand B electronic origins of 2PY originate from the sameground state level and measured the S0 state N–H and CvOstretching vibrational frequencies.8,9

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114308-2 Frey et al. J. Chem. Phys. 125, 114308 �2006�

We have recently performed studies of 2-pyridone hy-drogen bonded to itself, �2PY�2;33,34 to2-hydroxypyridine;17,35 to 2-aminopyridine;36 to uracil,thymine, other methyluracils, and 5-fluorouracil;13,14 tofluorobenzenes;15,16 and to adenine.37 The S1←S0 excitationsof all of these species are localized on the 2PY chromophore.The spectra all feature an intense low-wave-number bandwhich cannot be assigned in terms of S1 state intermolecularvibrations and that we have interpreted in terms of the2-pyridone B origin.5,6

Despite this extensive work, the S1 state vibrational levelstructure of 2PY and d-2PY is still not well understood. Afew vibronic bands beyond the B origin have been noted, butnot analyzed or discussed.5,6,9,10 We show that the majorityof the S1 vibronic level structure �620 cm−1, including the Borigin at 00

0+98 cm−1, can be assigned in terms of overtonesand combinations of two out-of-plane modes �1� and �2�. Webase the assignments on the predicted eigenvalues of one-dimensional anharmonic model potentials that are symmetricin the respective q1� and q2� coordinates. The S1→S0 fluores-cence spectra excited at the low-lying 00

0+98, +212, and+252 cm−1 bands exhibit characteristic changes in theirFranck-Condon factors to ground-state out-of-plane overtonelevels, relative to the fluorescence spectrum from the 00

band, confirming their assignment to out-of-plane S1 statelevels. We have also measured the S1←S0 spectra of 2PYand d-2PY using the fluorescence depletion �FDEP�technique.9 This has allowed to measure additional “dark”�nonfluorescent� excited-state vibrational levels that can beassigned as higher members of the out-of-plane �1� and �2�progressions. Interestingly, these dark states are often closelyintermingled with fluorescing levels; the latter can be as-signed as in-plane S1 vibrational fundamentals.

It has recently been shown by Barone that anharmonicfrequencies calculated with the B3LYP density functionalcan reproduce vibrational frequencies of N heteroaromaticsto high accuracy.38,39 Our assignments of the 2PY fluores-cence spectra are in good agreement with the S0 state funda-mentals, overtones, and combinations predicted by B3LYPanharmonic vibrational calculations. The latter are also con-sis tent with the previous analysis of Nowak et al.40 of theinfrared spectrum of 2-pyridone in Ar and N2 matrices.

II. EXPERIMENT

2PY was heated to 80–90 °C and was expanded in20 Hz pulsed supersonic expansions through thin-walled cir-cular or slit nozzles, using Ne carrier gas at backing pres-sures of 1.2–1.6 bar. Mass-selected two-color resonant two-photon ionization �2C-R2PI� spectra were measured in theregion of 29 850–31 000 cm−1 by crossing the skimmed su-personic jet with the unfocused UV excitation and ionizationlaser beams that were brought to spatial and temporal over-laps in the source of a linear time-of-flight mass spectrom-eter. S1←S0 excitation was performed by a frequency-doubled DCM dye laser at pulse energies of typically200 �J. The ionization light at 228 nm ��43 800 cm−1,1–2 mJ� was generated by sum frequency mixing of 355 nm

tra were digitized with a LeCroy LT374 digitizer, averagedover 64 laser shots, and transferred to a personal computer�PC�.

Laser-induced fluorescence �LIF� spectroscopy was per-formed by crossing the unskimmed molecular beam with theunfocused UV laser beam �4 mm downstream of thenozzle. The total fluorescence was collected using aHamamatsu R-928 photomultiplier tube. The signal was av-eraged over 64 shots and transferred to a PC. For the dis-persed fluorescence experiments and for the lifetime mea-surements, the emitted light was collected using acombination of spherical mirror and quartz optics, dispersedwith a SOPRA F1500 UHRS 1.5 m monochromator and de-tected with a cooled Hamamatsu R928 photomultiplier. Thefluorescence decay times were measured with the same pho-tomultiplier and recorded with a LeCroy Wavepro 954 oscil-loscope �1 GHz analog bandwidth, 2 Gsamples/s�. Thepulsewidth for the fluorescence excitation laser alone �scat-tered light from the molecular beam nozzle� could be closelyreproduced as a Gaussian of 7.2 ns full width at half maxi-mum �FWHM�. The lifetimes were determined by convolut-ing this Gaussian with a single exponential decay.

The principle of FDEP spectroscopy has been describedelsewhere.9 The excitation laser beam was spatially over-lapped with a second laser that was fired 110 ns earlier anddepleted the vibrational ground state. FDEP spectra were re-corded by fixing the excitation laser at the A electronic originof 2PY or d-2PY and scanning the depletion laser from29 800 to 30 800 cm−1.

III. COMPUTATIONAL METHODS AND RESULTS

The minimum-energy structure of 2PY in the S0 statewas calculated with the B3LYP density functional and the6-311+ +G�d , p� basis set using GAUSSIAN03.41 The structureconverged to planarity �Cs symmetry� at �2�10−6 Eh a0

−1.Both harmonic normal-mode and anharmonic vibrationalcalculations were undertaken at the optimized structure. Theanharmonic vibrational wave numbers are obtained from asecond-order perturbative treatment based on quadratic, cu-bic, and semidiagonal quartic force constants.38,39 The resultsof the anharmonic calculations will be discussed in Sec.IV B.

The S1 state minimum-energy structures were calculatedat three different levels, using the configuration interactionsingles �CIS� method, the symmetry adapted cluster/configuration interaction �SAC-CI� method �using the Lev-elOne set of excitation operator thresholds�, and the com-plete active space self-consistent field �CASSCF� method.The CASSCF calculations employed the 6-31G�d , p� basisset, while for the CIS and SAC-CI calculations diffuse func-tions on heavy atoms were added �6-31+G�d , p��. The CISmethod accounts for single excitations only but includes the��*-, ��*-, and ��*-type one-electron excitations. TheSAC-CI method includes additional selected double excita-tions but truncates the triple and higher excitations. TheCASSCF active space correlated the four occupied and thethree unoccupied �-type valence orbitals and the oxygen

114308-3 2-pyridone: Out-of-plane vibrations J. Chem. Phys. 125, 114308 �2006�

The CASSCF wave function contains all possible excitationswithin this active space. All calculations were performed us-ing GAUSSIAN03.41

The CIS method predicts a moderately large out-of-planedistortion of 2-pyridone, as shown in Fig. 1�a�. It combinedthe pseudoequatorial distortion proposed by Held et al. witha boat-type displacement. With the SAC-CI and CASSCFmethods, the calculated S1 state out-of-plane distortions ofthe S1 minimum are smaller and relatively similar to eachother, as can be seen in Figs. 1�b� and 1�c�. Despite extensivesearches, none of the three methods resulted into a stablepseudoaxial6 or any other locally stable S1 state structure.

At the CASSCF level, the calculated distortion can bepartially described by a pyramidalization displacementaround the N–H bond.6 In addition, the C6–H bond is dis-placed in the opposite direction to that of the N–H bond �forthe atom numbering, see Scheme 1�. The carbonyl bond re-mains within the approximate plane of the C–C–C frame-work. The calculated deformation is characterized by the di-hedral angles �H–N–C2–C6�=11.3°, �H–C6–N–C5�=−24.1°, �H–N–C2=O�=4.5°, and �H–N–C6–H�=23.9°. The CASSCF S1 state normal modes were calculatedat the optimized S1 state structure and will be discussed be-low.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

A. S1 state vibronic spectra of 2PY and d-2PY

1. 2-pyridone

The S1←S0 vibronic spectra recorded by the FDEP, LIF,and 2C-R2PI techniques are shown in Figs. 2�a�–2�c� and therespective band positions are collected in Table I. The in-tense band at 29 831 cm−1 is assigned to the 00

0 electronicorigin, in agreement with the earlier work.5–9 Besides thebands at 98, 218, and 252 cm−1 observed already earlier by

FIG. 1. S1 state optimized structures of 2-pyridone, using the CIS/6-31+G�d , p�, SAC-CI/6-31+G�d , p�, and CAS�10,8� /6-31G�d , p� modelchemistries.

R2PI and LIF techniques, the fluorescence depletion spec-

trum reveals additional bands at +353, +432, +526/ +529,+556, +605, and +620 cm−1. These bands were first mea-sured �but not analyzed� by Matsuda et al., who noted theirvery complicated structure.9 They also noted that the absenceof these higher-energy excitations in the LIF and R2PI spec-tra implies that the fluorescence quantum yield decreasesrapidly with increasing energy. They attributed this, on onehand, to increased internal conversion and intersystem cross-ing rates, and on the other hand, to possible ��* /n�*

couplings.9 Here we analyze and assign these low-energybands and propose assignments in terms of out-of-plane ex-citations. We return to the issue of mode-specific fluores-cence quantum yields and lifetimes below.

The CASSCF normal-mode wave numbers for the S1

state are given in Table II; the calculation predicts that thefour lowest vibrations are out-of-plane modes. Only threein-plane vibrations are predicted to lie within 700 cm−1

above the origin, with respective harmonic frequencies of�5�=485 cm−1, �8�=561 cm−1, and �9�=622 cm−1. We tenta-tively assign the very weak band observed at 440 cm−1 inFigs. 2�a�–2�c� as �5�, the medium strong band at �541 cm−1

to �8�, and the weak band at �648 cm−1 as �9�. The observed

FIG. 2. �a� Fluorescence depletion spectrum of supersonically 2-pyridone,probe laser at the lowest-energy transition �29 831 cm−1�. �b� Laser-inducedfluorescence �LIF� spectrum of 2-pyridone. The extra peak at 30 465 cm−1 isthe origin of the 2-pyridone·H2O complex.5 �c� Mass-selected two-colorresonant two-photon ionization spectrum of 2-pyridone, with ionization at228 nm.

wave numbers are in good agreement with the calculations

and the assignments will be placed on firm ground below,based on the fluorescence spectra. However, it is clear thatfour to five additional intense low-wave-number bands re-main below �5� and these can only be assigned to out-of-plane vibrational excitations.

TABLE I. Experimental S1 state vibrational wave numbers of 2-pyridoneand d-2-pyridone, measured by resonant two-photon ionization �R2PI�, fluo-rescence excitation �LIF�, and fluorescence depletion �FDEP� spec-troscopies. For the two assignments, cf. text and Fig. 5.

R2PI/LIF FDEP Assignment A Assignment B

2-pyridone 29 831 000 00

98 98 102 10

218 218 104 20

252 252 202 10

353 106 /10

432 204 10

440 440 501 20

528 10220

541 541 801 80

556605 20

620 108

648 648 901 90

d-2-pyridone 29 859 000

95 95 102

214 214 104

244 244 202

357 106

417 204

430 430 501

513 10220

529 529 801�?�

540 10230

584 206

596 596 901

620 10420

641 10430

665 20230

TABLE II. Calculated S1 state harmonic and anharmonic wave numbers for2-pyridone and d-2-pyridone, at the CASSCF/6-31G�d , p� level. “oop” de-notes out of plane and “ip” denotes in plane.

Mode Description 2-pyridone d-2-pyridone

�1 oop twist deformation 150.6 148.0�2 oop boat 164.0 162.9�3 oop C5–H 361.4 341.1�4 oop N–H/C2–H anti 434.3 378.6�5 ip CvO/N–H bend 485.0 467.3�6 oop C2–H/N–H 507.7 468.1�7 oop N–H/C2–H con 527.2 508.7�8 ip 6a 560.5 547.3�9 ip 6b 622.2 614.9�10 oop C1/C3–H/C5–H 696.7 693.6�11 oop C3–H/C4–H con 719.1 718.3�12 ip breathing 866.3 848.4�13 oop C3–H/C4–H anti 936.7 904.9�14 ip C4–C5 stretch 948.0 938.1�15 ip ring deformation 1047.0 1038.0

We recall that Held et al. have interpreted their upperstates of the 00

0 and 000+98 cm−1 bands in terms of two dif-

ferent conformers with nonplanar geometries.6 They specifi-cally noted that the N–H hydrogen atom is more out of planefor the B origin than for the A origin and that the geometrycorresponding to the B origin is significantly more nonplanarthan that corresponding to the A origin “at some atom otherthan the amine hydrogen.”6 They suggested that this was dueto amine invertamers and considered the possibilities �i� thatthey are true conformers with two different equilibrium ge-ometries or �ii� that they are two different vibrationally av-eraged structures of a planar molecule whose motions aregoverned by an asymmetric double-minimum potential alongthe inversion coordinate in the S1 state.6

However, both possibilities neglect the symmetry planeimplicit to the molecular framework: Any S1 state out-of-plane deformation of the molecule breaks the original S0

state planarity in two symmetry-equivalent ways, leading totwo symmetry-equivalent nonplanar S1 minima. The obser-vation of nonplanar vibrationally averaged structures for theA and B band upper states6 implies that there is at least onesymmetric double-minimum potential along an out-of-planecoordinate. The A and B upper states may then correspond tothe v=0 and 2 levels in the same double-minimum potential,since vibrational excitations are only allowed with �v=0,2 ,4 , . . . , for a planar→nonplanar electronic transition.Alternatively, the hypothesis �i� considered by Held et al.would give rise to two different anharmonic double-minimum potentials and two pairs of symmetry-equivalentminima. We have therefore examined possible assignmentsof the vibronic bands in Fig. 2�a� in terms of differentchoices of symmetric anharmonic potentials along out-of-plane coordinates:

In assignment A, we first fitted the excited-state vibronicbands in terms of an anharmonic potential along the coordi-nate q1�. The small-amplitude limit of this coordinate corre-

FIG. 3. The lowest-energy out-of-plane normal modes of 2-pyridone: ��a�and �b�� S0 state, calculated at the B3LYP/6–311+ +G�d , p� level. ��c� and�d�� S1 state, calculated at the CASSCF/6-31G�d , p� level.

sponds to the CASSCF eigenvector �1� shown in Fig. 3�c�.

Assigning the +98 cm−1 band as 102 and that at +218 cm−1 as

104, we fitted a quartic potential of the form42 V=q1�

4+Bq1�2,

as shown in Fig. 4�a�. The next even overtone predicted bythis potential is 10

6 at 355 cm−1; it nicely fits a small transi-−1 8

FIG. 4. Two alternative fitted S1 state potentials along the two lowest-energyout-of-plane vibrational coordinates q1� and q2�, utilized for the assignmentsA and B. See text for details.

tion at 353 cm as shown in Fig. 5�a�. The 10 transition

predicted at 502 cm−1 is expected to be very weak and is notobserved.

We then fitted the +252 and +432 cm−1 bands as 202 and

204 excitations to a second potential along q2�, corresponding

to the boat-deformation �2� mode. The CASSCF eigenvectoris shown in Fig. 3�d�. The fit results in a symmetric double-minimum potential with a barrier of 436 cm−1 above v=0, asshown in Fig. 4�a�, right. The 20

6 excitation is predicted at605 cm−1, in excellent agreement with a medium strong bandat 605 cm−1 observed in the FDEP spectrum, Fig. 5�a�. Aband at +528 cm−1 can be assigned to the overtone combi-nations 10

2204, respectively, see Table I and Fig. 5. The only

remaining band is at 440 cm−1, which has been assigned asthe lowest in-plane vibration �5� above.

In assignment B, an alternative set of assignments of thelow-wave-number excitations is possible, resulting in tworather different S1 state potentials along q1� and q2�. Again, theband at 98 cm−1 is assigned as 10

2; however, the +252 cm−1

band is now attributed to the 104 overtone, see Fig. 5�b�. This

results in a very shallow symmetric double-minimum poten-tial along q1�, cf. Fig. 4�b�. Further predicted overtones are 10

at 435 cm−1 and 108 at 642 cm−1, which might be assigned to

the bands at 432 and 620 cm−1, cf. Fig. 5�b�. When the−1 2

FIG. 5. Assignments A and B for the overtone and combination excitationsinvolving the out-of-plane modes �1� and �2� of 2-pyridone. See text fordetails.

+218 cm band is attributed to the 20 overtone, the symmet-

ric potential along �2� becomes nearly harmonic, see Fig.5�b�. The 20

4 overtone is now attributed to the band at440 cm−1, which was assigned to the 50

1 transition in assign-ment A.

The symmetric double-minimum potential V�q2�� in as-signment A or the double-minimum potential V�q1�� in as-signment B give rise to nonplanar vibrationally averagedgeometries for the different vibrational levels. These provideplausible explanations for the slightly different vibrationallyaveraged rotational constants associated with the so-called Aand B origins6 in terms of the levels v=0 �for the A origin�and v=2 �for the B origin�.

2. d-2-pyridone

The fluorescence depletion and two-color R2PI spectraof d-2-pyridone are shown in Figs. 6�a� and 6�b�. The respec-tive band positions are also given in Table I. The intenseband at 29 859.5 cm−1 is assigned to the electronic origin.Our value is 12.5 cm−1 lower than the value given for the A�band of d-2-pyridone by Held et al.;6 we believe our value tobe correct to within ±0.5 cm−1 on the basis of several mea-surements. The second band at 00

0+95 cm−1—denoted the B�band—has been observed at the position observed by Held etal.6 to within our experimental error of ±0.5 cm−1. In addi-tion, the mass-selected 2C-R2PI spectrum shows furtherbands at 214, 244, 430, 529, and 596 cm−1. Over the

FIG. 6. �a� Fluorescence depletion spectrum of d-2-pyridone, probe laser atthe lowest-energy transition �29 859.5 cm−1�. Assignment A for the overtoneexcitations of the out-of-plane modes �1� and �2�, see text. �b� Mass-selectedtwo-color resonant two-photon ionization spectrum of d-2-pyridone, withionization at 228 nm.

0–700 cm spectral range, the fluorescence depletion spec-

trum reveals the same bands, as well as additional transitionsat +287, +357, +417, +513, +540, +620, +641, and+665 cm−1. Again, the nonobservation of these higher-energy excitations in the R2PI spectrum implies that theirfluorescence quantum yields decrease rapidly with excess vi-brational energy in the S1 state.

The CASSCF S1 state calculations in Table II again pre-dict that the lowest modes are out-of-plane vibrations. The00

0+95, 214, and 244 cm−1 are thus also assigned as out-of-plane excitations. We fitted the observed transitions in termsof the model potentials that led to assignment A; in contrastto 2-pyridone, we are not able to propose a satisfactory fitanalogous to assignment B. The reason is that any such fitinvolves assigning the excitation at 430 cm−1 as 10

6. How-ever, this level must be assigned to an excited-state in-planevibration, based on the fluorescence spectra shown belowand in agreement with its appearance in the R2PI spectrum.Therefore, we only show the potentials fitted to assignmentA in Fig. 5�c�.

The potentials are very similar for 2PY and d-2PY, thelatter giving rise to slightly lower vibrational energies. Toachieve this, the reduced mass along the q1� coordinate wasincreased from 1.00 for 2PY to 1.04 for d-2PY. The analo-gous reduced mass scaling for q2� is from 1.00 for 2PY to1.085 for d-2PY. The assignments derived from Fig. 5�c� areshown in Fig. 6�a�. Very satisfactory assignments can beachieved for strong as well for the weak bands. TheCASSCF calculated out-of-plane frequencies �1� and �2� showquite small deuteration shifts of the order of 1%–2% �TableII�, in good agreement with the small shifts observed. Thisexperimental observation confirms that the �1� and �2� modesinvolved are not simply dominated by a local N–H out-of-plane motion, since this would lead to much larger deutera-tion shifts.

The medium strong band observed in R2PI �and also influorescence depletion� at 430 cm−1 is assigned to the in-plane vibration �5� and is confirmed by the fluorescence spec-tra discussed below. We note that the analogous 50

1 band in2PY is considerably weaker. In both 2PY and d-2PY, the �5�level lies close to the 4�2� level, giving rise to a Fermi reso-nance �FR�. Since the two levels lie only 7 cm−1 apart in2PY but are spaced by 13 cm−1 in d-2PY, the FR coupling ind-2PY may be weaker and the �5� band in d-2PY more in-tense, since it is less strongly coupled to the dark 4�2� state.In contrast, the R2PI band that is observed at 541 cm−1 in2PY and assigned to the �8� in-plane vibration is almost com-pletely absent in the R2PI spectrum of d-2PY. We tentativelyassign �8� to the very weak band at 540 cm−1, as shown in theinsert in Fig. 6�b�; this band is prominent in the FDEP spec-trum, Fig. 6�a�. In d-2PY, the three close-lying levels513/529/540 cm−1 form a Fermi triad. For d-2PY, the FRcoupling between the two out-of-plane levels and the in-plane state seems to be much stronger. Finally, the band ob-served at 596 cm−1 is assigned to the in-plane vibration �9�,

B. Fluorescence spectra and S0 vibrationalfrequencies

1. 2-pyridone

An overview dispersed fluorescence spectrum excited atthe 00

0 band of 2PY �29 831 cm−1� is shown in Fig. 7. Fourfurther fluorescence spectra were excited at the 00

0+98, +218,+252, and 541 cm−1 bands, cf. Figs. 2�b� and 2�c�, and wererecorded out to �3000 cm−1, the lowest 1300 cm−1 beingshown in Fig. 8. The measured wave numbers and intensitiesare collected in Table III.

The vibrational bands are first assigned by comparingtheir wave number to those of the fundamentals, overtones,and combination bands predicted by the B3LYP/6-311++G�d , p� anharmonic vibrational analysis, as given in thelast column of Table III. Figure 9 shows the B3LYP normal-mode eigenvectors of all the fundamental vibrations of 2PYthat have been observed in the fluorescence spectra. Thecomplete set of B3LYP harmonic and anharmonic frequen-cies is listed in Table IV. In Sec. IV C, detailed comparisonsof our measured wave numbers will be made �i� to the infra-red spectra of 2PY in Ar and N2 matrices measured byNowak et al.40 and �ii� to the supersonic jet infrared andstimulated Raman measurements of Matsuda et al.8

a. Fluorescence from 000 The weak band at 329 cm−1

appears �120 cm−1 below the lowest calculated in-plane vi-bration �3�. The only possible assignment to an out-of-planevibration is as the 12

0 overtone, since the 220 overtone lies far

higher, see below. Also, this assignment agrees nicely withthe anharmonic calculation, which predicts 12

0 at 339 cm−1.The four strong and medium strong bands at 450, 538, 605,and 811 cm−1, cf. Figs. 7 and 8, can be identified as thein-plane modes 31

0, 510, 61

0, and 1010, based on their better than

1% agreement with the calculated anharmonic frequencies,see Table III. Further overtones and combinations of thosebands are observed at 902 cm−1 �32

0�, 1076 cm−1 �520�, and

−1 0 0

culated anharmonic wave numbers.Between 610 and 802 cm−1, no in-plane fundamental vi-

brations are predicted. Based on the observed 120 wave num-

ber, the weak band at 647 cm−1 is assigned as the 140 over-

tone. The medium strong band at 744 cm−1 can only beattributed to the 22

0 out-of-plane overtone, calculated at774 cm−1. Although this corresponds to a 3.9% deviation,there are no reasonable alternatives. The band at 854 cm−1

also does not agree with any in-plane fundamental. A surveyof all possible combinations suggests either the out-of-planecombination 11

0710, calculated at 878 cm−1, or the out-of-plane

combination 21041

0, calculated at 890 cm−1.The band at 988 cm−1 is assigned to the in-plane funda-

mental 1310, in good correspondence to the calculated

986 cm−1. Four strong in-plane bands at 1235, 1364, 1552,and 1721 cm−1 can be identified as the in-plane fundamentals191

0, 2010, 231

0, and 2510. The higher wave number part of the

spectrum in Fig. 7 is characterized by overtones and combi-nation bands involving those four fundamentals, as listed inTable III. The agreement of these combination bands andovertones with the calculated values is within 1.3%, cf.Table III.

b. Fluorescence from 000+98 cm−1 �10

2� The most strikingdifference of this spectrum compared to that from the 00

band is the complete disappearance of the overtone bands10

2220 and 10

2240 and of the associated combination bands, e.g.,

01910, 10

2220231

0, and 10222

02510. Furthermore, the Franck-

Condon factors �FCFs� of the 10222

0501 and 10

2260 bands at 1279

and 2219 cm−1 are reduced to less than half of those from theorigin. Notable also are the complete disappearance of the10

2140 and of 10

221041

0 out-of-plane excitations. The FCFs of thein-plane modes change much less in comparison. These largeintensity changes of the out-of-plane transitions in fluores-cence give strong support to the above assignment of the

0 −1

FIG. 7. Overview fluorescence spec-trum from the 00

0 or A origin of2-pyridone at 29 831 cm−1.

00+98 cm transition to an S1 state out-of-plane overtone.

c. Fluorescence from 000+218 cm−1 �10

4� In this emissionspectrum, the bands involving both the �1� and ��2 out-of-plane modes are generally enhanced, while the in-planemodes remain unaffected. Relative to the emission from theorigin, the 10

4120 band increases by a factor of 22 and the 10

by a factor of 2.5 and the 10424

0 band increases by a factor of9 and 10

4260 by a factor of 4. The combination band 10

412051

0 at866 cm−1—which is not present in the 00

0 spectrum—appearsstrongly. The combination bands 10

4220191

0, 10422

02310, and

02510 show two to three times larger intensities. A new

band at 1184 cm−1 is tentatively assigned to 10422

0301. Since

only the Frank-Condon factors of out-of-plane modes areaffected while in-plane modes show roughly the same inten-sities as from the 00

0, the S1 state 218 cm−1 level is assignedto an out-of-plane excitation.

d. Fluorescence from 000+252 cm−1 �20

2� Excitation at0 −1

FIG. 8. Fluorescence spectra excited at �a� the 000 band of 2-pyridone at

29 831 cm−1, �b� 000+98 cm−1 band, �c� 00

0+218 cm−1, �d� 000+252 cm−1, and

�e� 000+541 cm−1.

00+252 cm leads to increase of bands involving �1� while

reducing the �2� related transitions. Similar to the 000

+218 cm−1 spectrum, 20212

0 is increased by 50% while 20222

�14%� is reduced to about half of its intensity in the00

0-spectrum. Also, the 20214

0 overtone at 645 cm−1 is rela-tively strong while 20

2240 could not be observed at all. The

0 band doubles its intensity with respect to the 000 spec-

trum, and many of its overtones are also more intense.e. Fluorescence from 00

0+541 cm−1 �501� Here, all transi-

tions in the out-of-plane modes �1� and �2� and their combi-nation bands disappear except for 50

1220 which is very weak.

In contrast, the intensity of the in-plane modes �5� and �6�increases dramatically. The 50

1520 overtone grows strongly.

Three new bands are observed at 1056, 1208, and1349 cm−1. They are assigned to the 50

130160

1 combination, the50

1620 overtone, and the 50

122061

0 combination; the differenceswith respect to the calculation are 1%. This is in agreementwith the above assignment of the +541 cm−1 level to thein-plane mode �8�, which corresponds most closely to theground-state �5� vibration.

The large changes of the FCFs of the out-of-plane over-tones in emission to the S0 state as a function of the differentexcitations clearly support our previous assignments of the00

0+98, +218, and +252 cm−1 excitations to S1 state out-of-plane overtones. For all three levels, however, the Franck-Condon factors exhibit large changes in emission to both the�1� and �2� ground state levels. This implies that the excited-state �1� and �2� vibrations do not correspond directly to the S0

state �1� and �2� modes. This normal-mode rotation or Dush-insky effect is also reflected in a lack of mirror symmetrybetween the absorption and emission spectra.

2. d-2-pyridone

a. Fluorescence from 000 The fluorescence spectrum of

d-2-pyridone is similar to that of 2PY and many of the bandassignments are the same. Thus, the five bands in Fig. 10�b�at 442, 532, 601, and 796 cm−1 are assigned to the in-planemodes 31

0, 510, 61

0, 1010, and 191

0. Their deuteration shifts rela-tive to 2-pyridone �cf. Figs. 10�a� and 10�b�� are −8, −6, −4,−15, and −14 cm−1, respectively. The shifts are of the orderof 1%–2% and agree with the B3LYP calculated anharmonicwave number shifts to within 2 cm−1 �see Table V�.

As before, the weak band at 324 cm−1 appears�120 cm−1 below the lowest calculated in-plane vibration��3 and can only be assigned to the 12

0 overtone. The mea-sured deuteration shift is only −5 cm−1; the anharmonicallycalculated wave number shift of the 12

0 overtone is also small�−2 cm−1�. The band at 728 cm−1 can only be attributed tothe 22

0 out-of-plane overtone, based on its calculated anhar-monic wave number �757 cm−1�. The deuteration shift is−16 cm−1, in good agreement with the calculated deuterationshift of −18 cm−1 �see Table V�.

An unusually large deuteration shift is exhibited by theband at 804 cm−1; its wave number is decreased by−50 cm−1 relative to the band at 854 cm−1 in 2PY. There,two alternative assignments were proposed, 11

0710 or 21

anharmonic deuteration shift is −55 cm−1. The calculatedshift for the alternative 11

0710 band is −124 cm−1, much larger

than that observed.

b. Fluorescence from the +96 cm−1 �102�, +214 cm−1 �10

4�,and +244 cm−1 �20

2� levels As for 2PY, the major changewhen exciting to the +96 cm−1 level is the disappearance of

TABLE III. Experimental S0 state vibrational frequencies of 2-pyridone frofrequency, relative intensities in parentheses�.

Excitationassignment

29 83100

0+9810

120 329 �3� 328 �3� 330 �68

310 450 �28� 450 �18� 451 �22

510 538 �51� 538 �34� 539 �39

610 605 �10� 605 �11� 606 �7

140 647 �5� ¯ 647 �8

220 744 �29� ¯ 745 �78

0¯ ¯ 779 �37

1010 811 �12� 810 �10� ¯

0 /21041

0 854 �11� ¯ ¯

0¯ 866�3� 867 �54

320 902 �4� ¯ ¯

0 988 �12� 989 �8� 988 �2231

¯ ¯ ¯

520 /12

0220 1076 �13� 1077 �13� 1076 �4

0 1145 �6� 1145 �6� ¯

0¯ ¯ 1184 �1

1810 1205 �18� 1205 �15� ¯

¯ ¯ ¯

1910 1235 �66� 1235 �73� 1234 �5

1262 �16� 1261 �11� ¯

0 1280 �13� 1279 �4� 1283 �122

¯ ¯ ¯

2010 1367 �42� 1366 �43� 1367 �3

240 1483 �5� ¯ 1483 �4

2310 1552 �51� 1551 �53� 1554 �3

¯ ¯ ¯

2510 1721 �52� 1719 �52� 1721 �6

510191

0 1771 �25� 1770 �18� 1771 �351

02010 1904 �16� 1904 �11� ¯

220191

0 1979 �15� ¯ 1980�362001 �11� ¯ ¯

120251

0¯ ¯ 2049 �4

510231

0 2089 �20� 2090 �17� 2089 �2¯ ¯ ¯

260 2220 �7� 2219 �3� 2218 �3

510251

0 2257 �22� 2257 �17� 2256 �322

02310 2293 �11� ¯ 2293 �3

1920 2438 �12� 2435 �12� ¯

220250

1 2462 �28� 2460 �22� 2463 �5191

02010 2599 �22� 2597 �19� ¯

1910231

0 2783 �24� 2782 �24� ¯

1910231

0 2911 2910 �12� ¯

1910251

0 2950 �24� 2949 �23� 2951 �3201

02510 3077 �9� 3076 �6� ¯

2320 3108 �9� 3108 �8� ¯

2310251

0 3270 �18� 3267�17� ¯

2520 3427 �12� 3426 �10� ¯

3486 �11� 3486 �11� ¯

the out-of-plane overtone band 1022, compare Fig. 10�c� with

10�b�. In contrast, the fluorescence spectrum excited at the00

0+214 cm−1 band shows large intensity enhancements inboth the out-of-plane 10

4120 and 10

4220 overtone bands, as well

as for the 10424

0 overtone and the 10421

0410 combination. Excita-

tion to the 000+244 cm−1 level leads to an enhancement of the

0 transition and to a lesser degree for the 20222

0 transition.−1 1

orescence spectroscopy �wave numbers relative to the respective excitation

+25220

B3LYP, anharmonic�% deviation�

329 �49� ¯ 339.2 �+3.1% �¯ ¯ 454.8 �+0.9% �

538 �103� 538 �147� 542.1 �+0.8% �606 �12� 606 �240� 610.3 �+0.9% �645 �17� ¯

743 �14� 745 �16� 774.0 �+3.9% �777 �19� ¯

¯ 812 �25� 802.0 �−1.1% �¯ ¯ 877.9 �1.3%�/

890.0 �4.2%�868 �15� ¯

¯ ¯ 910.3 �+0.8% �986 �22� ¯ 986.3 �+0.2% �

¯ 1056 �74� 1065.1 �+0.9% �1076�29� 1076 �57� 1084.2 �+0.8% �1144 �14� 1143 �80� 1152.3 �+0.8% �

1204 �15� ¯ 1198.8 �−0.5% �¯ 1208 �41� 1220.0 �+1.0% �

1234 �48� 1235 �24� 1207.0 �−2.3% �1261 �11� ¯

1281 �11� ¯

¯ 1349 �68�1366 �25� 1367 �21� 1359.4 �+0.6% �

1550 �25� 1551 �22� 1542.9 �−0.6% �1695 �22� ¯

1721 �34� ¯ 1710.7 �−0.6% �1772 �63� 1770 �71� 1748.5 �−1.3% �1904 �43� 1904 �44� 1901.2 �−0.2% �1980 �12� ¯

2049 �20�2089 �37� 2089 �52� 2085.7 �−0.1% �2100 �32� ¯

2219 �13� ¯

2257 �39� 2256 �124� 2251.9 �−0.2% �¯ ¯

¯ ¯ 2411.5 �−1.1% �2562 �25� ¯

¯ ¯ 2564.7 �−1.3% �¯ ¯ 2748.2 �−1.3% �¯ ¯ 2894.3 �−0.6% �¯ ¯ 2916.5 �−1.1% �¯ ¯ 3064.3 �−0.4% �¯ ¯ 3081.9 �−0.8% �¯ ¯ 3250.6 �−0.6% �¯ ¯ 3408.1 �−0.6% �¯ ¯

��

1�7�0�

9�1�

2�1�

1�3�9�

c. Fluorescence from the +430 cm level �50� Excitation

of the +430 cm−1 band leads to an intense three-memberedground-state progression in the �3� mode. From this we inferthat the S1 state 430 cm−1 vibration is closely related to the�3� ground-state in-plane vibration. Comparison of theCASSCF S1 state normal-mode eigenvectors shows that thelowest in-plane S1 state vibration �5� is very similar to the S0

state �3� mode.Similar to the fluorescence spectra of 2PY, the intensity

changes of the out-of-plane vibrational overtones and com-binations in fluorescence support the above assignment ofthe +96, +215, and +244 cm−1 transitions as S1 state out-of-plane overtone levels. As for 2PY, the Franck-Condon pat-terns of the three fluorescence spectra do not allow to estab-lish a clear one-to-one correspondence between the S1 statevibrations �1� ,�2� and the S0�1� ,�2� vibrations. This suggeststhat the two out-of-plane modes are strongly mixed in the S1

state relative to their �1� ,�2� counterparts. This strong Dush-insky rotation leads to atypical Franck-Condon patterns.

Calculations of the S1↔S0 Franck-Condon factors forthe out-of-plane modes would be desirable, but are not fea-

FIG. 9. S0 state normal modes of 2-pyridone calculated at theB3LYP/6–311+ +G�d , p� level.

sible, for the following reasons: �i� Since both the �1� and �2�

vibrations are highly anharmonic, any reasonable FCF calcu-lation would have to be based on a two-dimensional �2D�fully anharmonic S1 state potential energy surface V�S1�, in-cluding quadratic, biquadratic, quartic, and possibly highereven terms of the potential V�S1� as a function of q1� and q2�.�ii� The strong Dushinsky rotation implies that even in thesimplest description, q1� and q2� are �unknown� linear combi-nations of q1� and q2�, but there is a probability that furtherground-state vibrations are needed in the linear expansion forq1� and q2�. �iii� For anharmonic potentials, the kinetic energypart of the vibrational Hamiltonian can no longer be simplywritten as a sum of diagonal kinetic energy operators alongthe q1� and q2� coordinates, but additional off-diagonal cou-pling elements arise that may be large and strongly coordi-nate dependent.

C. Summary of S0 state vibrations and comparisonto previous measurements

Above we have identified a total of 12 S0 state vibra-tions, 10 as in-plane fundamentals, and 2 via out-of-planeovertones, as well as �30 higher overtones and combinationbands. The ground-state infrared spectra of 2PY have beenmeasured in Ar and N2 matrices by Nowak et al.,40 who haveidentified 19 IR-active fundamentals. The IR frequencies inthe N2 matrix are between 2 and 9 cm−1 higher than those inthe Ar matrix, and the latter are on average �2 cm−1 higherthan the gas-phase wave numbers measured here. We there-fore compare to the Ar matrix data. Table IV collects theB3LYP harmonic and anharmonic calculated wave numberswith the supersonic jet values and those measured in Ar ma-trices.

We note that seven fundamentals ��3�, �5�, �10� , �13� , �18� ,�19� , and �23� � are observed both in the fluorescence and IRmatrix spectra, with a mutual agreement �3 cm−1. Twelvefundamentals ��4�, �7�, �8�, �9�, �11� , �16� , �17� , �20� , �21� , �24� , �25� ,and �30� � are only observed in the matrix IR spectra.40 Threemodes ��1�, �2�, and �6�� are only observed in the fluorescencespectra, and eight modes �mainly the C–H stretches� are notobserved in either spectra.

There are two notable frequency shifts between the gas-phase and matrix frequencies: �1� Nowak et al. have assigneda very strong IR band at 1705 cm−1 in the Ar matrix as �25� ,the CvO stretch �with additional H–N–CvO andOvC–C–H in-plane bend components, see Fig. 9�.40 Weassign 251

0 to an intense fluorescence band that lies 16 cm−1

higher, at 1721 cm−1. This is in agreement with the super-sonic jet work of Matsuda et al. who measured 2PY by thefluorescence-dip stimulated Raman technique8,9 and assigneda strong band at 1721 cm−1 to the CvO fundamental. �2�Nowak et al. assign the N–H stretch to a band at 3438 cm−1

in the Ar matrix. Matsuda et al. have also observed the gas-phase N–H stretch by fluorescence-dip Raman9 andfluorescence-dip IR �Ref. 8� as a single strong band centeredat 3448 cm−1. We do not observe the N–H stretch by the

D. Fluorescence and nonradiative lifetimes

The fluorescence lifetime of the 000 and the 00

0+98 cm−1

bands have been determined by Held et al. as 11±1 ns viathe widths of the rotationally resolved rovibronic lines.6 Wehave measured the fluorescence lifetimes of the 00

0 and 000

+98 bands directly and find fl�000�=11.4±0.7 ns and fl�

+98�=9.6±1 ns, in agreement with the values of Held et al.Matsuda et al. have inferred the quantum yields of about 20bands relative to that of the 00

0 band, by comparing the UVdepletion spectrum and the R2PI spectrum, using a rateequation model.9

The large difference between the LIF/R2PI spectra, onone hand, and the FDEP spectra, on the other hand, impliesthat the fluorescence lifetimes of the higher vibronic excita-tions vary strongly. In terms of the assignments given above,it is clear that excitation of either of the out-of-plane modes�1� or �2� leads to a rapid decrease in fluorescence lifetime.The states of �1� and �2� that lie �300 cm−1 above the originare nonfluorescent. In contrast, there are three fluorescent

TABLE IV. Comparison of the S0 state harmonic and2-pyridone with experimental values. All wave numbesymmetric, and “as” antisymmetric.

Mode Description Irrep H

�1 oop ring deformation a��2 oop ring deformation a��3 ip CvO bend a��4 oop N–H,C3,4,6–H bend a��5 ip ring deformation a��6 ip ring deformation a��7 oop N–H bend a��8 oop total–H bend a��9 oop C3,4–H s a��10 ip ring deformation a��11 oop C5,6–H bend s a��12 oop C3,4–H bend as a��13 ip ring deformation a��14 oop C5,6–H bend as a��15 ip ring deformation a��16 ip C–N stretch a��17 ip C3,4,5–H bend a��18 ip C3,6 ,N–H a��19 ip C–N stretch a��20 ip total–H bend a��21 ip C3,4,6 ,N–H bend a��22 ip C3,5,6 ,N–H bend a��23 ip CvC stretch a��24 ip ring deformation, N–H bend a��25 ip N–H bend, CvO stretch a��26 ip C4–H stretch a��27 ip C5,6–H stretch as a��28 ip C3–H stretch a��29 ip C5,6–H stretch s a��30 ip N–H stretch a�

aThis work.bReference 8cReference 40

states of 2PY and two of d-2PY at excess energies

�440 cm−1, i.e., the 501, 80

1, and 901 excitations. All of these

are in-plane fundamental excitations. This implies that exci-tation of the two lowest out-of-plane modes leads to a rapidnonradiative process already at very low excess vibrationalenergy. In contrast, the lower in-plane fundamental vibra-tions have fluorescence lifetimes that are in the range of sev-eral nanoseconds, 10–20 times longer than close-lying out-of-plane states.

E. Theoretical calculations and discussion of the S1state reactivity of 2PY

Sobolewski and Adamowicz �SA� have explored theexcited-state reactivity of both 2-pyridone and its tautomer2-hydroxypyridine �2HP�, using the configuration interactionsingles �CIS�, complete active space self-consistent field�CAS-SCF�, and second-order perturbation theory�CASPT2� methods.20 Their CIS/6-31G�d , p� calculationsalready predicted that the S1 state is of ��* character forboth 2HP and 2PY and that the S1 minimum of 2PY is non-

armonic B3LYP/6-311+ +G�d , p� wave numbers ofscaled. “oop” denotes out of plane, “ip” in plane, “s”

B3LYP Expt.

nic Anharmonic Supersonic jeta,b Ar matrixc

.7 169.3 165

.2 387.7 372

.0 454.8 450 452

.9 500.4 485

.6 542.1 538 541

.9 610.3 605

.6 707.9 680

.0 733.4 722

.8 766.9 761

.9 802.0 811 814

.2 854.4 840

.7 960.9

.3 986.3 988 992

.7 1025.9

.8 998.2

.0 1088.1 1085

.4 1141.4 1152

.3 1198.8 1205 1204

.0 1207.1 1235 1237/1234

.4 1359.4 1367 1348

.7 1420.1 1399

.5 1456.3

.8 1542.9 1552 1550

.9 1614.4 1623

.2 1710.7 1721/1721b 1705

.4 3024.8

.9 3073.0

.0 3075.2

.4 3070.5

.6 3428.0 3448c 3438

anhrs un

169387458484548617687731767817851937999

10001016110911641222123813871453148715791657174631673197320932223593

planar. Since they calculated the S1 state barrier to nonpla-

narity at the CIS level to be 100 cm−1, they conducted thehigher-level CASSCF and CASPT2 investigations for planargeometries only. They investigated photoinduced reactionpaths using CASSCF/3-21G geometry optimizations, calcu-lating CASPT2/DZVP energies at the CASSCF geometries.In Cs symmetry, the S1 state is calculated to be 1A��*� andthe S2 state is 1A��n�*�, calculated to lie about 0.35 eVabove the ��* state. �i� For the excited-state intramolecularproton transfer between the 2PY and 2HP forms, SA foundthat a barrier height of 1.25 eV in the 2PY→2HP directionand concluded that the optically induced 2PY→2HP reac-tion is rather unlikely in the S1 state.20 �ii� The excited-stateN–H dissociation reactions from the dark S2 state is 1A��n�*�S2 states of 2PY and 2HP pass via a barrier of �1.5 eV to afreely dissociative surface; hence both tautomeric forms are

FIG. 10. Fluorescence spectra excited at �a� the 000 band of 2-pyridone and

the correlation with those of d-2-pyridone excited at �b� the 000 band

�29 859 cm−1�, �c� 000+96 cm−1, �d� 00

0+214 cm−1, �e� 000+244 cm−1, and �f�

000+430 cm−1, cf. Fig. 6.

“well protected” in the A� state against dissociation of the

proton. �iii� They also investigated nonradiative decay chan-nels from the ��* state of 2PY and 2HP via biradical pre-fulvenic forms back down to the ground state. For 2HP, alow-barrier reaction towards a prefulvenic form, with astrong nonadiabatic coupling to the ground state, was found.For 2PY, however, SA calculated a monotonically rising en-ergy profile for the reaction towards the prefulvenic form anda much smaller configurational mixing, viz., their Fig. 9.20

In contrast to SA, Li et al. have calculated a nonplanarS1 state minimum geometry for 2PY at the CASSCF/6-31G�d , p� level, using a �10,8� active space. They reportedthe N–H bond to be bent out of the molecular plane by ±27°.We note that our CASSCF minimum energy is lower thantheirs and conclude that slightly different minimum-energystructures were obtained in the two calculations, probablydue to a different choice of active orbitals. However, theexistence of two nonplanar and enantiomeric S1 state minimais in qualitative agreement with our results and also with ouranalysis of the �1� and �2� vibrations in terms of double-minimum potentials.

Using the optimization procedure for conical intersec-tions in GAUSSIAN03, we have located a minimum-energyS1 /S0 conical intersection �MECI� that connects the S1 andS0 states at the CASSCF/6-31G�d , p� level. The active spacefor this calculation is the same as that used to determine theS1 state minimum, see above. However, the MECI optimiza-tion is based on a state-averaged CASSCF wave function,whereas the S1 minimum-energy structure optimization usesa single-state wave function; hence the absolute energies ofthe two calculations cannot be compared. The MECI geom-etry is strongly nonplanar, showing a strong pyramidalizationaround the N–H group and an out-of-plane displacement ofthe C5–H bond. The MECI geometry is shown in Fig. 11.The double-cone shape of a conical intersection can be rep-resented if the energies of the intersecting states are plottedversus two special internal coordinates: These are �a� theinterstate coupling �or derivative coupling� vector and �b� thegradient difference vector, also shown in Fig. 11. We notethat the CASSCF mode �1� of the S1 minimum structure, Fig.3�c�, points very approximately towards this MECI structure.

At the CIS/6-31+G�d , p� level, we have calculated theentire reaction path from the S1 state minimum via an S1

state barrier of �2200 cm−1 to the MECI; a graph of therespective S1 and S0 potential curves is shown in Fig. 12.However, the CIS reaction path length, barrier height, andrelative energies of the S1 minimum and the minimum-energy conical intersection should only be taken as a roughindication of the true situation, due to the lack of doubles,triples, and higher excitations in the CIS calculation. A com-plete reaction path calculation was not possible at theCASSCF level, due to instabilities of the CAS procedurenear the S1 state barrier.

V. CONCLUSIONS

The S1←S0 vibronic spectra of 2PY and d-2PY havebeen measured by mass-selective resonant two-photonionization,5 laser-induced fluorescence excitation,6 and fluo-

9 −1

the gradient difference vectors �in blue�.

brational energy, the fluorescence depletion spectra exhibit anumber of vibronic bands that are “dark” in the R2PI andLIF spectra.9 Our interpretation of these additional bands in-volves even overtone transitions in the two lowest out-of-plane vibrations �1� and �2�. The A origin is assigned as thetrue 00

0 band, while the so-called B origin is assigned as the2�1� out-of-plane overtone. We have fitted symmetric double-minimum potentials along the q1� and q2� out-of-plane coordi-nates that reproduced the observed levels for 2PY and d-2PY and predicted additional overtone transitions in nearlyquantitative agreement with a number of dark vibronic bandsin the FDEP spectrum.

Of the two assignments considered, the combination ofthe 2PY and d-2PY R2PI, LIF, and FDEP spectra argue forthe assignment denoted by A. In this model, the symmetric

rs of d-2-pyridone, from fluorescence spectroscopy,values. Also given are the experimental and calcu-

+430B3LYP,

anharmonic

Deuteration shifts

Expt. Calc.

¯ 336.5�+3.7% � −5 −2.1444 447.1�+1.1% � −8 −8.0530 535.2�+0.5% � −6 −6.9603 608.9�+1.3% � −4 −1.4729 756.9�+3.8% � −16 −18¯ −11 ¯

802 835.0�+4.7% � −58 −55.0806 788.1�−2.0% � −7 −14¯ −8 ¯

886 894.5�+1.4% � −19 −15.9976 989.4�+1.5% � −13 −14.61062 1070.5�+0.5% � −11 −13.71134 1143.6�−0.9% � −12 −8.7¯ 1217.8�+1.1% � −4 −2.2

1223 1194.0�−2.2% � −14 −13.0

FIG. 12. CIS/6-31+G�d , p� calculation of the excited-state reaction coordi-nate connecting the S1 state minimum shown in Fig. 1 to the S1 /S0 conicalintersection. The ground-state potential energy curve is calculated at therespective excited-state geometries. The CIS/6-31+G�d , p� minimum-energy conical intersection geometry is qualitatively similar to the

TABLE V. Experimental S0 state vibrational wave numbeand comparisons to anharmonic B3LYP/6-311+ +G�d , p�lated deuteration shifts.

ExcitationAssignment

29 859.510

+21420

120 324 326 326 328

310 442 443 442 ¯

510 532 532 533 528

610 601 602 600 604

220 728 728 729 ¯

0¯ ¯ 768 770

0 796 796 ¯ ¯

1010 804 ¯ 805 ¯

0¯ 858 859 ¯

320 882 ¯ ¯ ¯

0 975 975 97152

0 1065 1062 1064 106151

0610 1133 ¯ 1134 ¯

620 1202 1202 ¯ ¯

1910 1221 1221 1222 1221

FIG. 11. Minimum-energy conical intersection geometry at the CASSCF�10,8� level, see text. �a� with derivative coupling vectors �in red�, �b� with

CASSCF�10,8� geometry shown in Fig. 11.

potential associated with �1� has a single minimum along q1�but is strongly anharmonic with a large quartic term. Thesymmetric potential associated with �2� has two minimaalong the q2� coordinate. For this reason, both the v=0 �ori-gin� and the 2�1� overtone levels have different nonplanarvibrationally averaged structures, in qualitative agreementwith the results obtained by Held et al.6 Our one-dimensionalmodel potentials should be viewed as a first step; at least atwo-dimensional potential will be necessary to explain �i� theunusual S1↔S0 Franck-Condon factors of the out-of-planevibrations and �ii� the details of the higher-energy vibrationallevel structure. The latter becomes complicated in the vicin-ity of the barrier along q2�, estimated at �430 cm−1.

The S1→S0 fluorescence spectra of 2-pyridone andd-2-pyridone have been excited at different excited state lev-els. Ten in-plane and two out-of-plane fundamental vibra-tions and more than 30 combinations and overtones havebeen identified in the S0 state. Excitation at the levels that arehere assigned to the out-of-plane overtone excitations at +98,+218, and +252 cm−1 gives transitions with strongly en-hanced Franck-Condon factors to the ground-state overtones12

0, 140, 22

0, and 240, thereby confirming the excited-state as-

signments as out-of-plane vibrations. The intensity changesof both �1� and �2� overtones in the emission spectra as afunction of S1 state excitation imply that the �1� and �2� vibra-tions are rotated relative to the �1� and �2� ground-state modes.This pronounced Dushinsky effect is in agreement with thecomplete lack of mirror symmetry between absorption andemission spectra.

A rapid decrease of fluorescence lifetimes occurs for theout-of-plane vibrational overtones �250 cm−1 for 2PY �seealso Ref. 10� and d-2PY. This is related to an S1 state in-tramolecular reaction that is strongly enhanced when higherout-of-plane vibrational levels are excited. In contrast, exci-tation to the S1 levels at 00

0+440 cm−1 for d-2PY and at 000

+528 cm−1 for 2PY leads to fluorescence spectra with en-hanced transitions to the in-plane S0 state vibrations �3�, �5�,and �6�. Thus there are “bright” S1 state in-plane vibrationallevels above the dark out-of-plane states, and the in-planevibrational states have 10–20 times longer fluorescence life-times than out-of-plane states that lie at the same or higherinternal energy. This vibrational state specificity directly sug-gests that the S1 state reaction involves a nonplanar transitionstate or conical intersection. Indeed, we have calculated anS1 /S0 conical intersection at the CASSCF level. The geom-etry of 2PY at the conical intersection is markedly nonplanar,being pyramidalized at the N and at the C5 atoms.

The anharmonic B3LYP/6–311+ +G�d , p� wave num-bers are in very good agreement with our own and withprevious jet and matrix spectroscopic Raman and IR data andassignments,40,43 showing �1% rms deviation. This confirmsthe reliability of combing the B3LYP density functional witha perturbation theoretical anharmonic treatment38,39 forthe calculations of accurate ground-state vibrational wavenumbers.

ACKNOWLEDGMENT

This work was supported by the Schweiz. Nationalfonds�Projct Nos. 2000-68081.02 and 200020-105490�.

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