Fouriertransform microwave spectroscopy of triplet carbon monoxides,C2O, C4O, C6O, and C8OYasuhiro Ohshima, Yasuki Endo, and Teruhiko Ogata Citation: J. Chem. Phys. 102, 1493 (1995); doi: 10.1063/1.468881 View online: http://dx.doi.org/10.1063/1.468881 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v102/i4 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

Fourier-transform microwave spectroscopy of triplet carbon monoxides,C2O, C4O, C6O, and C8O

Yasuhiro Ohshima and Yasuki EndoDepartment of Pure and Applied Sciences, College of Arts and Sciences, The University of Tokyo, Komaba,Meguro-ku, Tokyo 153, Japan

Teruhiko OgataFaculty of Liberal Arts, Shizuoka University, Ohya, Shizuoka 422, Japan

~Received 26 September 1994; accepted 18 October 1994!

Rotational spectra of CnO with n52, 4, 6, and 8 have been observed by using a Fabry–Perot typeFourier-transform microwave spectrometer cooperated with a pulsed discharge nozzle. Themolecules have been generated by an electric discharge of carbon suboxide diluted in Ar, andadiabatically cooled to'2 K in a subsequent supersonic expansion. All the observed spectra forthese species are characterized as linear molecules in the3S2 electronic ground state. Since all thethree spin sublevels have been detected even in the free-jet condition, the spin–spin couplingconstants have been determined precisely as well as other spectroscopic constants. The couplingconstants show rapid increase asn becomes larger, indicating smaller energy gaps between theexcited1S1 state and the3S2 ground state for the longer species. Along with the recent observationof singlet CnO ~n55, 7, and 9! @Ogata, Ohshima, and Endo, J. Am. Chem. Soc.~submitted!#, thepresent study has established the existence of a complete set of the linear carbon-chain series CnOup ton59 in the gas phase. The effective CvC bond lengths evaluated from the rotational constantsdecrease gradually to a converging value of'1.28 Å as n becomes larger. No apparentquasilinearity has been observed in the centrifugal-distortion constants of all the members, incontrast to the relevant series of the pure carbon clusters, Cn , some of which~n53 and 7! haveshown substantial nonrigidity for the bending vibration. ©1995 American Institute of Physics.

a

ir

a

l

n

hn

lcc

tC

tant

m-

rst

hex-c-by

ndra-

s.orec-

r-d

I. INTRODUCTION

Carbon clusters, Cn , with relatively small sizes havebeen subjected to extensive investigations in recent yeComplexities in the properties of these species have nbeen largely unveiled, as both experimental and theoretdata have been accumulated on the molecular geometintramolecular vibrational dynamics, thermodynamicchemical reactivities, and so on.1 The small carbon clustersalso play an important role in the nucleation processeslarger clusters such as extremely stable fullerenes towcarbon particles and soot. They are considered as carriersubstantial amount of the total carbon budget in interstespace. Geometrical characterization has been achievedthe species withn up to 10, mainly through spectroscopy ithe gas phase and low-temperature matrices.2–16All the ob-served species have linear molecular structures, and the mstable electronic configurations have been found as singletriplet, depending onn5odd or even.

Besides the pure Cn clusters, carbon clusters mixed withetero atoms, e.g., O, N, and S, are of particular importawith respect to reactivities of Cn with molecules containingsuch hetero atoms. It should be another interesting probto address how the molecular properties, such as stableformations, electronic structures, and vibrational dynamiare changed by the ‘‘doped’’ hetero atoms. The seriesmolecules, CnO, are prototype systems of these mixed cabon clusters. Spectroscopy of the CnO series has significanimportance in astrophysics. Besides extremely abundantthe next longer members, C2O and C3O, were indeed de-tected toward the dark molecular cloud, TMC-1.17,18 Longer

J. Chem. Phys. 102 (4), 22 January 1995 0021-9606/95/102(4)Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

rs.owcalies,s,

ofrds oflarfor

ostt or

ce

emon-s,ofr-

O,

carbon-chain species have thus been considered as imporcandidates for radioastronomical detection.19

In contrast to Cn , only shorter members of the CnO se-ries have been studied so far; molecules, of which the geoetries have been characterized experimentally, aren52, 3, 4,and 6, besides the stable shortest member of CO. The fiidentification of the second shortest member, C2O, wasachieved through its infrared~IR! spectrum in the low-temperature matrix.20 This study was followed by an obser-vation of electronic transitions in the gas phase, where tmolecule was generated by flash photolysis of carbon suboide, C3O2.

21 Precise molecular constants in the ground eletronic state have been determined subsequentlymillimeter-wave ~MMW ! spectroscopy.22 High-resolutionabsorption spectra have also been observed in the mid- anear-IR regions by using semiconductor diode lasers asdiation sources.23,24All the studies have established that C2Ohas a linear geometry in the3S2 electronic ground state. Inaddition, an electron spin resonance~ESR! spectrum in ma-trices has been measured to yield the magnetic constant25

The small hyperfine coupling constants thus determined f13C-substituted species have shown that the unpaired eltrons lie mainly in app orbital, which is compatible with acumulene-type configuration, i.e., :CvCvO.

For the next member, C3O, an IR absorption band ob-served in an Ar matrix was attributed to the species.26 Themolecule was detected in the gas phase by microwave~MW!spectroscopy as an unstable pyrolysis product.27 The bondlengths in the linear singlet molecule were precisely detemined from a set of rotational constants for singly substitute

1493/1493/8/$6.00 © 1995 American Institute of Physics Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

t

-

r

oi

-

e

-ir

m

d

f

ed

a

theia-theor

in

s

tm,ithe

-ri-

dicredal

forf

er,-

e-e-s

rto

I,

d,

epinofst

nt

1494 Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

species. The IR spectra of matrix isolated C3O, including anumber of isotopic species, were remeasured to yield tvibrational fundamental frequencies and harmonic forcfield.28 Rotational transitions for high-J values were also ob-served in the MMW region.29,30For longer members of CnO,all the experimental investigations have been restricted tomeasurements in low-temperature matrices. An ESR studyC4O and C6O has revealed that both species are in the3S2

electronic ground state,31 as in the case of the shorter member with evenn, C2O. Infrared absorption features in an Armatrix were claimed as the bands due to C4O.

32 Severalquantum chemical calculations have been performed to pdict the electronic and structural properties of CnO withn52–6.33–40

For the detailed knowledge on the electronic and gemetrical structures, bonding nature, and vibrational dynamof bending, we have started spectroscopic investigationslonger members of CnO. Here, we have adopted newly developed pulsed-discharge nozzle Fourier-transform micrwave ~PDN-FTMW! spectroscopy. In this method, transienspecies to be studied are generated in an electric dischargan adiabatically cooled supersonic free-jet, and thus the stdistribution is restricted to levels with relatively lower internal energies. This is a crucial advantage for species wlonger carbon-chain frameworks, since they possess lapartition functions due to small rotational constants anmany low-frequency bending vibrations. Rotational spectrocopy is also preferable for the detection of longer CnO, be-cause of larger dipole moments expected for the species wlargern.38

In the course of the study, we found that the CnO mol-ecules are formed quite efficiently when carbon suboxideused as a precursor. All the members of CnO up ton59 haveso far been detected, where all the CnO molecules identifiedhave linear molecular geometries. In this paper, we report tresults on the paramagnetic species with even carbon atoi.e., n52, 4, 6, and 8. Rotational transitions in the3S2 elec-tronic ground state were observed in the centimeter-waregion, giving the magnetic fine-structure constants in addtion to the rotational and centrifugal-distortion constants. Thelectronic configurations, effective CvC bond lengths, andrigidity for bending vibrations of the radicals are discusseon the basis of the precisely determined molecular constanResults on the singlet species withn55, 7, and 9 will bereported as a separated paper.41

II. EXPERIMENT

The MW spectrometer employed in the present studyof a Fourier-transform-type with a Fabry–Perot cavity, whicis similar to the one originally designed by Balle anFlygare.42 Detailed explanation of the FTMW spectrometehas already been made elsewhere.43 Recently, some modifi-cations have been made on this spectrometer; FID signalsnow detected by a quadrature heterodyne receiver, andquency scanning are semiautomated by the aid of a persocomputer.44 The spectrometer is cooperated with a pulsedischarge nozzle to produce short-lived molecular speciwhich has been successfully applied to studies of free racals, such as CnS ~n53–5! ~Refs. 45–47! and HCnS

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

hee

heof

e-

-csof

o-tin

ate

thgeds-

ith

is

hes,

vei-e

dts.

ish

r

arere-nalds,i-

~n53,4!.44 The dc discharge was conducted by applyingpulsed high voltage~'21.5 kV with 100ms duration! to anelectrode, just after the gas sample was pulsed throughnozzle. Discharge products thus generated were then adbatically cooled in a subsequent supersonic expansion. Inpresent experimental condition, the internal temperature fthe triplet CnO molecules was estimated to be'2 K bycomparing intensities of the observed lines in different spsublevels.

The CnO radicals were produced by a discharge of a gamixture, 0.3%–0.7% of C3O2 diluted in Ar. The stagnationpressure of the sample gas was kept in the range of 4–7 awhich gave almost the same signal intensities. Samples wlarger C3O2 concentration were found to be preferable for thproduction of longer members of CnO. Carbon suboxide wasprepared by dehydration of malonic acid by heating its mixture with phosphorus pentaoxide, and the product was pufied by repeated vacuum distillation.

Three mutually perpendicular Helmholtz coils are placeoutside the FTMW chamber to cancel the Earth’s magnetfield. Each observed line was checked whether it disappeawithout the discharge, and also it was affected by an externmagnetic field. Observed linewidths of C2O, C4O, C6O, andC8O were in the range of 20–40 kHz~FWHM!, which werealmost the same as those of diamagnetic species, exceptseveral low-N transitions showing additional broadening o'10 kHz, which arose from the residual magnetic field.

III. RESULTS

Before starting the search for the longer CnO series, weexamined the rotational transitions of the shortest membC2O. According to the molecular constants reported by Yamadaet al.,22 two lines were predicted to lie in the frequencycoverage of our spectrometer, 4–26 GHz. They areN(J)51~2!20~1! at 22 258.1660.03 MHz and 1~0!–0~1! at9561692 MHz, where the errors represent two standard dviations. The former line was located at the position as prdicted, 22 258.175 MHz. The signal intensity of this line wafairly strong, as shown in Fig. 1~a!, and the experimentalconditions were optimized by monitoring it. Since the lattetransition had much larger frequency uncertainty, we hadsearch a region6100 MHz, and found it at 9647.567 MHz.The transition frequencies of these lines are listed in Tablealong with the previously reported data in the MMWregion.22

The frequency region in 18.8–19.3 GHz was searchefor transitions of C4O, and a number of lines were detectedas shown in Fig. 2. All of these lines were attributed todischarge products including already known C3O and itsisotopomers.27 Several of them were found to be sensitive tothe external magnetic field, indicating that the carrier of thlines should possess an unquenched electronic and/or sangular momentum. Among these paramagnetic lines, twothe strongest at 18 810.072 and 19 119.800 MHz were molikely to be the transitions of C4O. The lines were tentativelyassigned toN(J)54~4!–3~3! and 4~5!–3~4! of C4O, respec-tively, from the prediction based on the rotational constafrom ab initio molecular structures38,39 and the spin–spincoupling constant determined in the Ne matrix.31 The inten-

No. 4, 22 January 1995Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

-

aTa

t

1495Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

sity of the former line was about one-half of that of the lattewhich was consistent with the population difference betwethe F2(N5J) andF1(N5J21) spin sublevels in the adiabatically cooled free-jet condition. This assignment was cofirmed by finding the subsequent transitions of theF1 andF2ladders, and transitions in the remainingF3(N5J11) sub-level were also detected in the corresponding positions.

FIG. 1. Rotational spectra of CnO with n52, 4, 6, and 8;~a! The N(J)51~2!–0~1! transition of C2O at 22 258.175 MHz;~b! 4~5!–3~4! of C4O at19 119.800 MHz;~c! 7~8!–6~7! of C6O at 12 514.009 MHz;~d! 9~10!–8~9!of C8O at 7 827.627 MHz. The spectra were obtained after 10, 100, 200,1000 shots accumulations at a repetition rate of 5 Hz, respectively.vertical scales forn54, 6, and 8 are factored by 2, 10, and 70 relative to thfor n52.

TABLE I. Observed transition frequencies of C2O in the 3S2 state ~inMHz!.

N8 J8 N9 J9 nobs nobs2ncalc

1 0 0 1 9 647.567a 0.0001 2 0 1 22258.175a 20.0002 2 1 1 46182.189 0.0022 3 1 2 45826.706 20.0223 2 2 1 70105.960 20.0033 3 2 2 69272.927 20.0053 4 2 3 69069.476 20.0044 3 3 2 92718.800 0.0194 4 3 3 92363.286 0.0294 5 3 4 92227.853 20.0115 4 4 3 115 656.566 0.0035 5 4 4 115 453.024 20.0005 6 4 5 115 354.035 0.0096 5 5 4 138 677.586 20.0086 6 5 5 138 542.092 20.0016 7 5 6 138 464.858 0.0067 6 6 5 161 729.433 20.0197 7 6 6 161 630.306 20.0187 8 6 7 161 567.126 20.0058 7 7 6 184 794.969 20.0008 8 7 7 184 717.580 0.0038 9 7 8 184 664.004 0.019

aMeasured in the present study, whose weight in the analysis is 4. OMMW transitions are taken from Ref. 22, with unit weight.

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

r,en

n-

Fi-

nally, 13 lines in total have been observed in the 6–24 GHzregion, as listed in Table II. A typical example of the ob-served lines is shown in Fig. 1~b!.

Although the lines of C4O were weaker than those ofC2O by a factor of'5, they were observed with a fairly goodsignal to noise ratio even with a moderate data acquisition~100 shots, 20 s!. This fact encouraged us to identify the nextlonger member of the triplet CnO series, C6O. Again, thetransition frequencies were predicted on the basis of the ro-tational constant from theab initiomolecular structure38 andthe spin–spin coupling constant in the Ne matrix.31 Theseconstants were used after scaling by comparing the observedand the predicted values for C4O. This prediction was fairlyreliable as we could find the expected transitions with only0.3% deviation. Finally, 24 lines were observed for all thethree spin sublevels in the 4–17 GHz region, as listed inTable III. The intensities of the C6O spectra were smaller bya factor of'5 than those of C4O, as shown in Fig. 1~c!.

Unlike the shorter members of the triplet CnO series,C8O was detected unexpectedly during the search for thesinglet species with oddn. A weak paramagnetic line de-tected at 7044.011 MHz was realized to be one of theF1ladder of C8O, i.e., 8~9!–7~8!. Although noab initio calcu-lation nor matrix ESR experiment has been reported so faron this radical, the rotational constant was able to be esti-mated with sufficient accuracy by an empirical extrapolation

ndhet

her

FIG. 2. Stick diagram for lines observed in the discharge of the C3O2/Ar gassample. Paramagnetic lines are indicated with open circles.

TABLE II. Observed transition frequencies of C4O in the 3S2 state ~inMHz!.

N8 J8 N9 J9 nobs nobs2ncalc

1 2 0 1 6033.511 0.0022 1 1 0 25 487.747 0.0002 2 1 1 9405.052 0.0032 3 1 2 10 152.516 0.0023 2 2 1 12 776.575 0.0013 3 2 2 14 107.563 20.0023 4 2 3 14 570.267 20.0024 3 3 2 18 062.594 20.0014 4 3 3 18 810.072 20.0024 5 3 4 19 119.800 0.0005 4 4 3 23 049.847 0.0015 5 4 4 23 512.570 0.0015 6 4 5 23 732.453 0.000

No. 4, 22 January 1995 Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

e

tel

o

l

ht

a

–-

1496 Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

using the observed values for the shorter members of tripCnO, as has been done by Oka for the sake of the astronocal search of longer members of cyanopolyynes.48 Lines intheF2 andF3 sublevels were observed within 1 MHz of thpredicted frequencies. The intensities of the C8O lines in thelowest spin sublevel were again weaker by a factor of'5than those of C6O, as shown in Fig. 1~d!. In addition, thespin–spin coupling constant is much larger than those ofshorter species, giving an unfavorable population decreasthe upper spin sublevels. It was thus required to accumuup to fifteen thousand shots~more than 1 h!! to achieve ad-equate signal to noise ratios for high-N transitions in theF2or F3 ladder. Finally, 30 transitions have been observedthe 4–14 GHz region, as listed in Table IV.

IV. ANALYSIS

Since the observed spectral patterns of C2O, C4O, C6O,and C8O are compatible with a linear molecule in a3S elec-tronic state, the following Hamiltonian is used:49

H5B0N22DJ~N

2!21~2/3!l~3Sz22S2!

1~1/3!lD@~3Sz22S2!,N2#11gN–S, ~1!

whereN andS are rotational and electron-spin angular mmenta, respectively, and [A,B]15AB1BA. For C2O, twolines observed in the present study have been analyzed awith the previously reported MMW data,22 which yielded themolecular constants listed in Table V. The accuracy of tspin–spin coupling constant,l, has been improved almos5000 times by the inclusion of the newly observed low-Ntransitions, especiallyN(J)51~0!–0~1!. Other constants areessentially the same as those obtained from the MMW d

TABLE III. Observed transition frequencies of C6O in the 3S2 state ~inMHz!.

N8 J8 N9 J9 nobs nobs2ncalc

2 3 1 2 4 596.212 20.0023 3 2 2 5 098.544 20.0023 4 2 3 6 149.874 0.0004 3 3 2 5 600.866 0.0024 4 3 3 6 798.061 0.0014 5 3 4 7 718.496 0.0025 4 4 3 7 446.229 0.0015 5 4 4 8 497.574 0.0015 6 4 5 9 302.524 0.0016 5 5 4 9 276.632 0.0016 6 5 5 10 197.083 20.0026 7 5 6 10 901.418 0.0007 6 6 5 11 091.621 20.0017 7 6 6 11 896.595 0.0007 8 6 7 12 514.009 0.0008 7 7 6 12 891.743 20.0018 8 7 7 13 596.102 20.0038 9 7 8 14 138.823 0.0009 8 8 7 14 678.164 20.0039 9 8 8 15 295.613 0.0009 10 8 9 15 774.299 0.00010 9 9 8 16 452.366 0.00210 10 9 9 16 995.120 0.00210 11 9 10 17 418.940 20.001

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

letmi-

hein

ate

in

-

ong

e

ta

alone. The standard deviation of the fit is 14 kHz, which isalmost the same as the experimental accuracy of the MMWmeasurement.

Sets of the molecular constants determined for C4O,C6O, and C8O are also listed in Table V. Since transitions inall the three spin sublevels have been observed, the spinspin and spin–rotation coupling constants have been determined independently with acceptable accuracies. This is incontrast with the study of C4S,

46 whose spin–spin couplingconstant is much larger~113.6 GHz!, and transitions in theF2 or F3 ladder have not been detected because of muchsmaller population in the free-jet condition. The standard de-viations of the fits are 2.1, 1.6, and 3.2 kHz for C4O, C6O,and C8O, respectively, which are all within the experimentalaccuracies of the present FTMW measurements.

V. DISCUSSIONS

A. Electronic configuration and molecular structure

The present identification of the triplet carbon monox-ides, C4O, C6O, and C8O, has confirmed the existence of thelinear CnO series up ton59 in the gas phase, along with theprevious studies on the shorter members~n<3! ~Refs. 20–30! and our separated report on the oddn species.41 All theevenn species are in the3S2 electronic ground state, whilethe odd ones in the1S1 state. The symmetry alternation inthe electronic ground states is explained well with a simpleconsideration of the molecular orbitals of the CnO series. The

TABLE IV. Observed transition frequencies of C8O in the 3S2 state ~inMHz!.

N8 J8 N9 J9 nobs nobs2ncalc

5 6 4 5 4 694.628 0.0016 7 5 6 5 477.536 20.0007 8 6 7 6 260.659 0.0038 8 7 7 6 410.278 20.0028 9 7 8 7 044.011 20.0019 8 8 7 6 559.864 0.0039 9 8 8 7 211.563 20.0019 10 8 9 7 827.627 0.00010 9 9 8 7 379.066 20.00310 10 9 9 8 012.846 20.00210 11 9 10 8 611.521 20.00011 10 10 9 8 198.024 0.00911 11 10 10 8 814.135 0.00411 12 10 11 9 395.717 20.00012 11 11 10 9 016.681 20.00012 12 11 11 9 615.413 20.00112 13 11 12 10 180.232 0.00213 12 12 11 9 835.043 20.00413 13 12 12 10 416.695 20.00213 14 12 13 10 965.076 20.00114 13 13 12 10 653.091 20.00214 14 13 13 11 217.977 20.00114 15 13 14 11 750.268 20.00215 14 14 13 11 470.799 20.00615 15 14 14 12 019.257 20.00315 16 14 15 12 535.822 0.00016 15 15 14 12 288.173 0.00516 16 15 15 12 820.545 0.00516 17 15 16 13 321.738 20.00317 18 16 17 14 108.037 0.003

No. 4, 22 January 1995 Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

1497Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

TABLE V. Molecular constants for C2O, C4O, C6O, and C8O in the3S2 states.a

C2O C4O C6O C8O

B0/MHz 11 545.597 0~7! 2 351.262 5~2! 849.757 09~7! 400.641 83~8!DJ/kHz 5.819~8! 0.128~6! 0.009 3~5! 0.001 6~2!l/MHz 11 496.870~9! 11 680.181 2~12! 17 352.74~10! 34 096.~7!lD/kHz 25.4~6! 1.13~18! 0.98~5! 1.32~5!g/MHz 217.820~3! 24.755 9~8! 21.043~4! 0.46~8!

aNumbers in parentheses denote one standard deviation of the least-squares fitting in units of the last significantdigits.

ye

u

iet

c

sp

m

s

d-

h

u

molecules possess 2n12 electrons in thep orbitals, whichprimarily consist of the 2p atomic orbitals of carbon andoxygen. The doubly-degenerate HOMO is thus half-fillewith evenn, while that of oddn species is fully filled. The~p!2 configuration provides3S2, 1D, and 1S1 electronicstates, among which the first one is the lowest in energ50

The closed-shell~p!4 configuration correlates to a singlelectronic state,1S1. The parallel argument has been applieto explain the electronic structures of the pure carbon clters, Cn , besides the difference in the number ofp electrons~2n22 in the case of Cn!.

1

The molecular structure of the linear CnO series is rep-resented schematically in a valence bond picture as that wcumulative double bonds, i.e., :CvCv•••vCvO. The r sbond lengths derived forn53 and 5 have confirmed thestructure.27,41 For the triplet species with evenn, no defini-tive information on the bond lengths is available becausethe lack of the data on the isotopically substituted specHowever, the determined rotational constants are consiswith the results from theab initio calculations, which havealso predicted the cumulative structures. The differenfrom the observed rotational constants are only20.6%,20.6%, and10.7%, respectively, forn52 at MRD-CIlevel,34 n54 at CASSCF level,39 and n56 at HF/DZPlevel.38 So far, no theoretical prediction has been reportedthe molecules withn>7. However, the rotational constanthave been predicted with sufficient accuracies by a simextrapolation using the results on the shorter memberseven or oddn series, indicating no apparent deviation fro

TABLE VI. Molecular constants and effective CvC bond length of theCnO series.a

n B0/MHz DJ/kHz R~CvC!b/Å

2 11 545.597 0~7! 5.819~8! 1.370 43c 4 810.886 4~2! 0.778 3~3! 1.280 64 2 351.262 5~2! 0.128~6! 1.292 05d 1 366.847 09~6! 0.035 0~5! 1.277 76 849.757 09~6! 0.009 3~5! 1.283 07d 572.941 05~5! 0.004 75~15! 1.277 18 400.641 83~8! 0.001 6~2! 1.280 29d 293.736 11~4! 0.000 96~7! 1.277 0

aNumbers in parentheses denote one standard deviation of the least-sqfitting in units of the last significant digits.bAll the CvC bond length are assumed to be equal in a molecule, wR~CvO! fixed at 1.155 Å.cReference 30.dReference 41.

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

d

.

ds-

ith

ofs.ent

es

on

leof

the cumulative double-bond structure for the longer memberof the CnO series up ton59.

B. Effective C vC bond lengths

The rotational constants and the centrifugal-distortionconstants determined for the CnO series up ton59 are sum-marized in Table VI. Since all the CnO species have thecumulative double-bond structures, an effective CvC bondlength was evaluated on the assumption of the uniform bonlengths in each molecule. Because only the main isotopomers have been observed except forn53 and 5, it is im-possible to determine both the CvC and CvO bond lengthsindependently. Thus, the CvO bond has been fixed to 1.155Å, in order to reproduce the averaged values of ther s~CvC! bond lengths forn53 and 5.27,41The derived effec-tive CvC bond lengths are listed in Table VI, and also plot-ted againstn in Fig. 3. The values for the pure carbon clus-ters, Cn ,

4,8,11,14–16are also shown in this figure. This figureclearly shows that there are two series for the molecules witeven and oddn. For the case of oddn, the bond lengths,starting at 1.281 Å forn53, decreases slightly asn in-creases, and are soon converged to'1.277 Å forn>7. Onthe other hand, the bond lengths for evenn decrease rapidlyby 0.08 Å from 1.370 Å forn52–4, and then approachgradually also to'1.28 Å. The trend is well correlated tothat for the Cn clusters. For the species with smallern, the

ares

ithFIG. 3. Effective CvC bond lengths in the CnO and Cn molecules plottedagainstn. Bond lengths for Cn are taken from Ref. 4~n53!, Ref. 8~n54!,Ref. 11~n55!, Ref. 14~n56!, Ref. 15~n57!, and Ref. 16~n59!.

No. 4, 22 January 1995 Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

e

b

ne

ded

n

n

onlue

rd

udy.in

nin

lsonsm

stn-e

uen ismo-ap-in

val-mens

d so

on-

dg-ion

anseytheo-

detheld.

1498 Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

bond lengths of the Cn clusters are slightly larger than thosof CnO, but the difference becomes smaller asn increases. Inother words, the effect of the terminal oxygen to the CvCbond lengths becomes smaller as the carbon chain becolonger.

C. Spin–spin and spin–rotation coupling constants

The spin–spin coupling constants for the triplet CnO se-ries are plotted against the numbers ofp electrons in Fig. 4.For CnO, the constants ofn52 and 4 are almost the samebut the value increases rapidly for largern. Since the un-paired electrons in the triplet CnO are located in thep or-bital, the spin density becomes distributed as the chaincomes longer, which should cause the decrease in the spspin dipolar interactions between the two unpaired electroThe observed trend is therefore explained by the increasthe second-order contributions to the effectivel constantsfrom the excited electronic states, rather than the first-orcontribution of the spin–spin dipolar interaction within th3S2 ground state. The dominant part in the second-orcontributions is the spin–orbit interaction between the3S2

and1S1 states, both of which arise from the same electroconfiguration, as in the case of O2.

51 The energy differencebetween the two electronic states should be smaller aschain becomes longer, while the spin–orbit interaction wounot be affected so much, in order to explain the observincrease in thel constants. Although there is no experimetal information on the position of the1S1 electronic state forany CnO with evenn, ab initio calculations predict that1S1

lies above3S2 by 1.58 eV~POL-CI! ~Ref. 33! or 1.10 eV~MRD-CI! ~Ref. 34! in the case ofn52. The MP3/DZP cal-culation has predicted the differences to be 0.47 and 0.11for n54 and 6, respectively.38

The increase of thel constants withn is also evident inthe case of the Cn clusters.

8,13,14 There is good correlationbetween CnO and Cn with the same number ofp electrons,as shown in Fig. 4. It is quite natural that the absolute valu

FIG. 4. Spin–spin coupling constants of the triplet CnO and Cn moleculesplotted against the numbers ofp electrons. The constants for Cn are those inthe Ne matrices~n54 and 8! ~Ref. 13! and that in the gas phase~n56! ~Ref.14!.

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

mes

,

e-in–s.of

er

er

ic

thelded-

eV

es

for Cn are much smaller than those for CnO; the latter spe-cies contain the oxygen atom with the spin–orbit interactifive times larger than that of carbon. The fact that the vafor C2O is almost the same as C4O in spite of the larger1S1/3S2 energy gap will be partly explained by the largespin–orbit interaction originating from the larger unpaireelectron density on the oxygen atom in C2O. The spin–spincoupling constants for CnO with n52, 4, and 6 from the ESRmeasurements in the Ne matrices25,31are smaller by 3%–5%than those in the gas phase determined in the present stProbable origins of the matrix effect have been discussedsome detail by Smith and Weltner.25 The zero-field-splittingparameter for C8O is larger than 2 cm21, which will causeanother difficulty in the ESR detection of this radical, evethough the molecule is able to be produced efficientlymatrices.

The spin–rotation constant in the ground state is acontributed by the second-order effects of the interactiowith electronic excited states. The contribution comes frothe spin orbit and electronic Coriolis interactions with3Pelectronic states in this case.51 The contributions are propor-tional to the matrix element ofASO and the rotational con-stant, and inversely proportional to the3P/3S2 energy gap.The lowest3P state for the CnO species, which arises fromthe ~s!1~p!3 electronic configuration, corresponds to the firspin-allowed excited state from the ground state. If we cosider the contribution from the first excited state only, thconstant forn52 is calculated to be235 MHz by using theobserved energy gap and theASO constant for theA 3Pstate21,24 with the pure precession approximation. The valis twice as large as the observed one, even though the sigthe same. This may be because the electronic angularmentum is assumed to be unity in the pure precessionproximation, which should be quenched to some extentthe actual system. The spin–rotation constants forn52, 4,and 6 have the same negative signs, and the normalizedues relative to the rotational constants are also in the saorder. If all the constants are dominated by the interactiobetween theA 3P and X 3S2 states, this implies that theenergy difference between the two states does not affectemuch asn increases, in contrast to the case of1S1/3S2.However, situations seem to be more complicated, and ctributions form3P states other than theA state should be-come substantial as the carbon chain becomes longer, juing from the sudden change of the sign of the spin–rotatconstant forn58.

D. Centrifugal-distortion constants and rigidity ofCnO

The centrifugal-distortion constants can be used asexperimental source on the molecular vibration. In the caof a linear molecule such as CnO, the constant depends onlon the stretching force constants and the bond lengths, ifintramolecular vibrations are dominated by the harmonic ptential and the vibrational amplitudes are small.52 However,as the vibrational anharmonicity or the bending amplitubecomes larger, the distortion constant deviates fromvalue determined considering only the stretching force fieOne of the typical examples is C3, which is well known as a

No. 4, 22 January 1995 Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

s

n

-m

n

ra

e

y

h

h

e

ese

n

ia-

ryp-

etin

-n

x--

u

1499Ohshima, Endo, and Ogata: FTMW spectra of triplet C O

quasilinear molecule with bending vibration of'63 cm21.2,5

The observedDJ constant of the molecule is seven timelarger than the calculated value.3 For another example, C3O2,which has a lower bending vibration of only'18 cm21, theratio of the observed and calculated distortion constanreaches almost 15.53

We have thus examined the centrifugal-distortion costants for the CnO series. The harmonic force fields of C2Oand C3O have been determined form the IR spectra in thlow-temperature matrices.20,28 The force constants of C3Ohave also been evaluated by anab initio calculation at aconsiderably high level.37 Since no information has been reported on the harmonic force constants for the longer mebers of CnO, the force field of C3O is transferred to thelonger species with an assumption that all the force constafor the CvC bonds are the same except for those neighboing to the CvO bond. Constants for nonadjacent bond paiare set to be zero. The ratios of the observed centrifugdistortion constants to those thus calculated are plottedFig. 5. Even though the values, which are in the range1.1–1.5, indicate some discrepancies from the semirigmolecule approximation, no evident quasilinearity has beobserved for all the CnO series up ton59.

The trend is sharply contrasted to that of the Cn clusters,though the both series compose of the CvCv•••vC frame-works. Besides the well established quasilinearity of the C3molecule,2–5 C7 has shown extremely large increase of throtational constant upon the bending excitation.15 The flop-piness of the Cn clusters has been explained qualitatively bthe characteristics of HOMOs.13,15Molecules with~8n14!pelectrons are predicted to possess low-frequency bendingbrations, which have been proven by the examples like C3,C7, and C3O2. However, C5O and C9O, which belong to thecategory of these molecules, have been found to be ratrigid. Compared with the other CnO species, the centrifugal-distortion constants of C6O and C8O are considerablysmaller. It is impossible to assume that these species havestronger stretching force constants in order to explain tfact, since the effective bond lengths of both the species alonger than those of the neighboring oddn species. Contri-bution from the bending vibrations might be smaller for thspecies, but the origin is not obvious at present.

FIG. 5. Ratios of the observed and calculated centrifugal distortion costants for the CnO molecules plotted againstn. Errors represented are en-tirely due to those for the observed ones. See text for details of the evaltion of the calculated values.

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

ts

-

e

-

tsr-sl-inofidn

e

vi-

er

theere

E. Formation mechanism

It will be of importance to discuss in some detail theformation mechanism of the CnO series under the presentexperimental condition. The observed intensities of CnOhave been decreased by a factor of about five asn increasesby 2, for both the singlet~oddn! and triplet~evenn! series.By comparing the molecules with even and oddn, the inten-sities ofn52 and 3 were almost the same, and that was trufor n54 and 5, and so forth. Population of the triplet specieare distributed to the three spin sublevels. In addition, sincthe radical species with evenn should be more reactive thanthe closed-shell species with oddn, they will have largerprobability of disappearance by collisions with the wall ofthe discharge nozzle. Judging from these facts, the formatioefficiencies of CnO seem to decrease monotonically asnincreases with no specific preference.

The C2O radical is formed directly by the decompositionof the parent molecule,

C3O2→C2O1CO. ~2!

For C3O, however, direct formation process,

C3O2→C3O1O, ~3!

seems to be insufficient because of the large bond dissoction energy.54 Instead, the reactions involving the reactiveand abundant C2O radical may be dominant, such as

C2O1C2O→C3O1CO, ~4!

or

C3O21C2O→C3O12CO. ~5!

The formation mechanisms similar to Eq.~4! can be specu-lated for the molecules withn.3, in which the carbon chainincreases by unity during the successive reactions. Contrato the present experimental conditions, an alternative aproach has been adopted to form the triplet CnO in the Nematrix, where Cn clusters generated by laser ablation ofgraphite were irradiated by the UV light to react with thecoadded CO.31

F. Relaxation processes in free jet

For the triplet CnO series, the relative intensities of theobserved lines belonging to the three spin sublevels havbeen well reproduced with the Boltzmann distribution aabout 2 K, which is the same as the rotational temperaturethe free jet. This fact indicates that the thermal equilibriumare achieved between the different spin sublevels of the triplet molecules formed in the supersonic expansion. Ainfinite-order-sudden approximation calculation by Alex-ander and Dagdigian55 has shown that the inelastic collisionrate constantsbetweenthe different spin ladders~i.e.,Fi↔F j

with iÞ j ! are smaller than thosewithin the ladder, especiallyin states with high-N values. Therefore, the collisions afterthe radical formation are frequent enough in the present eperimental condition, for the two internal degrees of freedom, i.e., spin and rotation, with relatively smaller energyintervals to be effectively thermalized. The adiabatic coolingconcerning vibrations may not be effective compared with

n-

a-

n

No. 4, 22 January 1995Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

e

e

re

se

L

c

n

s

W

g

.

.

.

s.

,

s.

1500 Ohshima, Endo, and Ogata: FTMW spectra of triplet CnO

spin or rotation. However, the vibrationally excited state oC3O in the n5 bending mode with'130 cm21 internalenergy27,37 was not detected, giving the population in thstate less than 1024 relative to that of the ground state. Thissets the vibrational temperature well below 20 K in thpresent experimental condition.

VI. CONCLUSION

In this paper, we have reported the first identification othe carbon-chain triplet molecules, CnO, with n54, 6, and 8in the gas phase, along with the observation of low-N tran-sitions of C2O. These radicals were generated by an electdischarge of supersonically cooled carbon suboxide dilutin Ar. Their rotational spectra, which are characteristic tlinear molecules in the3S2 electronic ground state, havebeen observed by using a FTMW spectrometer, to yield spetroscopic constants including magnetic fine-structure costants. Together with the separated report on the singlet CnOmolecules with oddn, geometrical and electronic structureof the CnO series up ton59 have been discussed on thbasis of the precisely determined constants. Variations in tCvC bond lengths and the spin–spin coupling constanshow close correlations with those for the relevant pure Cn

clusters, but aspects of the low-frequency bending vibratioare significantly different with each other between the twseries.

ACKNOWLEDGMENTS

We thank R. Van Zee and W. Weltner for their commenon the matrix ESR spectrum, which was previously attributed to C10 in Ref. 13. The present work was supported bgrants in aid by the Ministry of Education, Science, and Cuture of Japan~Nos. 63470007, 02740242, and 04233205!.

1As an excellent review, see W. Weltner, Jr. and R. J. Van Zee, Chem. R89, 1713~1989!.

2L. Gausset, G. Herzberg, A. Largerqvist, and B. Rosen, Astrophys. J.142,45 ~1965!.

3K. Matsumura, H. Kanamori, K. Kawaguchi, and E. Hirota, J. ChemPhys.89, 3491~1988!; K. Kawaguchi, K. Matsumura, H. Kanamori, andE. Hirota, ibid. 91, 1953~1989!.

4K. W. Hinkle, J. J. Keady, and P. F. Bernath, Science241, 1319~1988!.5C. A. Schmuttenmaer, R. C. Cohen, N. Pugliano, J. R. Heath, A.Cooksy, K. L. Busarow, and R. J. Saykally, Science249, 897 ~1990!.

6W. R. M. Graham, K. I. Dismuke, and W. Weltner, Jr., Astrophys. J.204,301 ~1976!.

7L. N. Shen and W. R. M. Graham, J. Chem. Phys.91, 5115~1989!.8J. R. Heath and R. J. Saykally, J. Chem. Phys.94, 3271~1991!.9K. R. Thompson, R. L. DeKock, and W. Weltner, Jr., J. Am. Chem. So93, 4688~1971!.

10P. F. Bernath, K. H. Hinkle, and J. J. Keady, Science244, 562 ~1989!.11J. R. Heath, A. L. Cooksy, M. H. W. Gruebele, C. A. Schmuttenmaer, aR. J. Saykally, Science244, 564 ~1989!.

12N. Moazzen-Ahmadi, A. R. W. McKellar, and T. Amano, J. Chem. Phy91, 2140~1989!.

13R. J. Van Zee, R. F. Ferrante, K. J. Zeringue, W. Weltner, Jr., and D.Ewing, J. Chem. Phys.88, 3465 ~1988!. In this reference, several ESRsignals observed in rare-gas matrices were assigned to slightly bent C10.The authors of this reference have recently informed us that the assiment is incorrect and the signals would be due to unknown paramagnespecies other than pure carbon clusters.

J. Chem. Phys., Vol. 102,Downloaded 07 Sep 2013 to 131.94.16.10. This article is copyrighted as indicated in the abstract.

f

f

icdo

c-n-

hets

nso

t-yl-

ev.

.

.

.

d

.

.

n-tic

14H. J. Hwang, A. Van Orden, K. Tanaka, E. W. Kuo, J. R. Heath, and R. JSaykally, Mol. Phys.79, 769 ~1993!.

15J. R. Heath, R. A. Sheeks, A. L. Cooksy, and R. J. Saykally, Science249,895 ~1990!; J. R. Heath and R. J. Saykally, J. Chem. Phys.94, 1724~1991!.

16J. R. Heath and R. J. Saykally, J. Chem. Phys.93, 8392 ~1990!; A. VanOrden, H. J. Hwang, E. W. Kuo, and R. J. Saykally,ibid. 98, 6678~1993!.

17M. Ohishi, H. Suzuki, S. Ishikawa, C. Yamada, H. Kanamori, W. M.Irvine, R. D. Brown, P. D. Godfrey, and N. Kaifu, Astrophys. J.380, L39~1991!.

18H. E. Matthews, W. M. Irvine, P. Friberg, R. D. Brown, and P. D. Godfrey,Nature310, 125 ~1984!; R. D. Brown, P. D. Godfrey, D. M. Cragg, E. H.N. Rice, W. M. Irvine, P. Friberg, H. Suzuki, M. Ohishi, N. Kaifu, and M.Morimoto, Astrophys. J.297, 302 ~1985!.

19N. G. Adams, D. Smith, K. Giles, and E. Herbst, Astron. Astrophys.220,269 ~1989!.

20M. E. Jacox and D. E. Milligan, J. Chem. Phys.43, 3734~1965!.21C. Devillers and D. A. Ramsay, Can. J. Phys.49, 2839~1971!.22C. Yamada, S. Saito, H. Kanamori, and E. Hirota, Astrophys. J.290, L65

~1985!.23C. Yamada, H. Kanamori, H. Horiguchi, S. Tsuchiya, and E. Hirota, JChem. Phys.84, 2573~1986!.

24M. Fujitake, R. Kiryu, and N. Ohashi, J. Mol. Spectrosc.154, 169~1992!;N. Ohashi, R. Kiryu, S. Okino, and M. Fujitake,ibid. 157, 50 ~1993!.

25G. R. Smith and W. Weltner, Jr., J. Chem. Phys.62, 4592~1975!.26R. L. DeKock and W. Weltner, Jr., J. Am. Chem. Soc.93, 7106~1971!.27R. D. Brown, F. W. Eastwood, P. S. Elmes, and P. D. Godfrey, J. AmChem. Soc.105, 6496~1983!; R. D. Brown, P. D. Godfrey, P. S. Elmes,M. Rodler, and L. M. Tack,ibid. 107, 4112~1985!.

28R. D. Brown, D. E. Pullin, E. H. N. Rice, and M. Rodler, J. Am. Chem.Soc.107, 7877~1985!.

29T. B. Tang, H. Inokuchi, S. Saito, C. Yamada, and E. Hirota, Chem. PhyLett. 116, 83 ~1985!.

30W. Klebsch, M. Bester, K. M. T. Yamada, G. Winnewisser, W. JoentgenH.-J. Altenbach, and E. Vogel, Astron. Astrophys.152, L12 ~1985!.

31R. J. Van Zee, G. R. Smith, and W. Weltner, Jr., J. Am. Chem. Soc.110,609 ~1988!.

32V. G. Maier and H. P. Reisenauer, Angew. Chem.100, 590 ~1988!.33S. P. Walsh, J. Chem. Phys.72, 5679~1980!.34C. F. Chabalowski, R. J. Buenker, and S. D. Peyerimhoff, J. Chem. Phy84, 268 ~1986!.

35R. L. DeKock, R. S. Grev, and H. F. Schaefer III, J. Chem. Phys.89, 3016~1988!.

36R. D. Brown and E. H. N. Rice, J. Am. Chem. Soc.106, 6475~1984!.37P. Botschwina, J. Chem. Phys.90, 4301~1989!.38D. W. Ewing, J. Am. Chem. Soc.111, 8809~1989!.39H. Kannari, K. Aoki, K. Hashimoto, and S. Ikura, Chem. Phys. Lett.222,313 ~1994!.

40R. D. Brown, D. McNaughton, and K. G. Dyall, Chem. Phys.119, 189~1988!.

41T. Ogata, Y. Ohshima, and Y. Endo, J. Am. Chem. Soc.~submitted!.42T. J. Balle and W. H. Flygare, Rev. Sci. Instrum.52, 33 ~1981!.43M. Iida, Y. Ohshima, and Y. Endo, J. Chem. Phys.94, 6989~1991!.44Y. Hirahara, Y. Ohshima, and Y. Endo, J. Chem. Phys.101, 7342~1994!.45Y. Ohshima and Y. Endo, J. Mol. Spectrosc.153, 627 ~1992!.46Y. Hirahara, Y. Ohshima, and Y. Endo, Astrophys. J.408, L113 ~1993!.47Y. Kasai, K. Obi, Y. Ohshima, Y. Hirahara, Y. Endo, K. Kawaguchi, and A.Murakami, Astrophys. J.410, L45 ~1993!.

48T. Oka, J. Mol. Spectrosc.72, 172 ~1978!.49Y. Endo and M. Mizushima, Jpn. J. Appl. Phys.21, L379 ~1992!.50G. Herzberg,Molecular Spectra and Molecular Structure, Vol. III. Elec-tronic Spectra and Electronic Structure of Polyatomic Molecules~VanNostrand Reinhold, New York, 1966!, Chap. 3.

51K. Kayama and J. C. Baird, J. Chem. Phys.46, 2604~1967!.52W. Gordy and R. L. Cook,Microwave Molecular Spectra~Interscience,New York, 1970!, Chap. 8.

53L. Fusina, I. M. Mills, and G. Guelachvili, J. Mol. Spectrosc.79, 101~1980!.

54S. V. Filseth, Adv. Photochem.10, 1 ~1977!.55M. H. Alexander and P. J. Dagdigian, J. Chem. Phys.79, 302 ~1983!.

No. 4, 22 January 1995Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions