structures and vibrations of phenol · h2o and d-phenol · d2o based on ab initio calculations

Journal of Molecular Structure (Theo&em), 276 (1992) 117-132 Elsevier Science Publishers B.V., Amsterdam

117

Structures and vibrations of phenol- H, 0 and d-phenol-D, 0 based on ab initio calculations

Martin Schlitz, Thomas Biirgi and Samuel Leutwyler

Institut fiir Anorgan., Analyt. und Physikal. Chemie, Freiestr. 3, 3000 Bern (Switzerland)

(Received 6 January 1992)

Abstract

Ab initio electronic structure calculations for phenol and the hydrogen-bonded complexes phenol*H,O and d-phenol*D,O were performed at the HartreeFock 4-31G and 6-31G** levels. Both phenol and phenol.H,O were fully structure optimized. Based on the minimum- energy structures so obtained, full normal coordinate analyses were carried out. The resulting harmonic frequencies were scaled and compared to available experimental data. The agreement is satisfactory and allows for an assignment of a majority of the bands observed in the experimental spectra. Comparison with previous calculations on (H,O), reveals a considerable increase in the strength of the hydrogen bond on going from (H,O), to phenol*H,O.

INTRODUCTION

Hydrogen bonding is important in a variety of phenomena basic to life (solvation effects, protein folding, etc.). In order to simulate bioactive molecules in aqueous surroundings using molecular dynamics or Monte- Carlo methods, reliable interaction potentials between water and hydroxy groups in several chemical environments are needed. More accurate information is now becoming available: Molecular beam techniques in combination with spectroscopic methods on the experimental side and ab initio electronic structure calculations on the theoretical side provide together an appropriate means of investigating intermolecular binding properties of isolated molecular clusters and exploring hydrogen bonding in different chemical environments.

Although for H,O-H,O interactions a large number of high quality ab initio studies have been carried out [l-8] and accurate model potentials [l, S-111 are available, less is known about hydrogen bonding interactions

Correspondence to: S. Leutwyler, Institut fiir Anorgan, Analyt. und Physikal. Chemie, Freiestr. 3, 3669 Bern, Switzerland.

0166-1269/92/$05.CUl 0 1992 Elsevier Science Publishers B.V. All rights reserved.

118 M. Schiitz et al.lJ. Mol. Struct. (Theochem) 276 (1992) 117-132

between water and aromatic hydroxy groups. Ab initio calculations of van der Waals complexes consisting of an aromatic molecule such as phenol and one to four water molecules are computationally much more demand- ing than calculations of the equivalent pure water clusters at the same level of theory. Conversely, water clusters for which one hydrogen atom is substituted by an aromatic chromophore are experimentally more acces- sible than pure water clusters: methods such as resonant two-photon ioni- sation (R2PI), dispersed fluorescence and cluster ion dip spectroscopy (CIDS) can be applied providing mass/isomer selective information on intermolecular vibronic states in the electronic ground and excited states. Therefore, accurate experimental information for such systems is available that can be related to ab initio results.

The archetype of a hydroxy-aromatic water complex is phenol * H,O. Phenol l H,O is small enough to allow use of reasonable double-zeta basis sets. However, the complex is large enough to enable the application of the experimental techniques mentioned above. Several experimental studies of the S, electronic ground-state vibrations of phenol - (H,O), (n = 1 to 3) have been made. Recently, Stanley and Castleman [12] and Ebata et al. [13] have explored the ground-state vibrations in the range 50&1500 cm-’ using IDS. Ebata et al. also resolved the low-lying intermolecular fundamentals of phenol * H, 0 in the 50-200 cm-’ region using dispersed fluorescence at a resolution of 10 cm-‘. Three low-frequency intermolecular modes were observed, two of them occurring also in the IDS spectra as combinations with intramolecular modes which were assigned to a stretching vibration 0” = 150cm-‘, a bending vibration /3; = 104cm-’ and an overtone of a second bending vibration 2/l;’ = 138 cm-l. Furthermore, much experimental work concerning the S, electronic excited state of phenol - (H,O), clusters (n = 1 to 4) has been done [14-161.

In contrast to the experimental situation, there have been relatively few ab initio studies on such systems [17]. Recently, Pohl et al. [18] have carried out full ab initio geometry optimizations onp-cresol . (H20)n (n = 0 to 3) at the HartreeFock (HF) level with minimal STO-3G basis sets. They were able to rationalise their experimental findings using the resulting minimum-energy structures. To our knowledge, no ab initio based vibrational analyses of comparable systems have been done.

In this paper we report results from ab initio HF calculations on phenol and phenol * H,O. Structures were fully minimized for both inter- and intra-molecular degrees of freedom. Normal coordinate calculations were performed for phenol, phenol - H,O and d-phenol * D, 0. A detailed comparison with experimental data is made. Agreement of the calculated harmonic frequencies with experimental data is fairly good; the assignment of the spectral bands to the calculated normal modes is evident.

M. Schiitz et al./J. Mol. Struct. (Theochem) 276 (1992) 117-132 119

COMPUTATIONAL METHODS

The calculations were performed using GAUSSIAN-M [19] on an IBM 3090-180E, GAUSSIAN-W [20] on both a VAX-6420 and a workstation IBM R/S 6000 320 and CADPAC 4.0 [21] on a Cray Y-MP. Fully optimized structures were calculated for phenol and phenol*H,O at the HF level with 4-31G and 6-31G** split-valence basis sets using the most stringent optimization con- vergence criteria (Very Tight Option in Gaussian). No attempts have been made so far to correct the basis-set-superposition error (BSSE). Full vibrational analysis was then carried out for each of the minimum-energy structures obtained, using analytic second derivatives. The harmonic frequencies obtained were then scaled with experimental values using a least- squares fitting procedure.

The basis sets were chosen based on a previous study on (H,O), clusters [22] where, of the basis sets studied, 4-31G was found to be the most suitable for predicting vibrational frequencies and 6-31G”” was superior for bond lengths and binding energies.

RESULTS AND DISCUSSION

Phenol

A substantial number of ab initio studies have been devoted to phenol, some including geometry optimizations [23-271, although only a few involved calculation of vibrational frequencies and eigenvectors. Puebla and Ha [23] have reported completely optimized molecular structures of phenol and dihydroxybenzenes at the HF 4-31G level, including the vibrational frequencies of the out-of-plane modes. However, no assignment of the calculated frequencies and no comparison with experimental data was made. In the present study we recalculated minimum-energy structures of phenol at the HF 4-31G and 6-31G** level as a prerequisite for the computa- tion of binding energies of phenol-water complexes. The geometry optimizations were performed under relaxation of all internal degrees of freedom, assuming planarity of the molecule. Thereafter harmonic frequencies and vibrational eigenvectors were calculated and related to experimental values in order to determine appropriate scaling factors for later use in calculations on phenol-water.

Geometry The obtained HF 6-31G** minimum-energy structure of phenol is

depicted in Fig. 1; bond lengths and bond angles are shown. A comparison of the calculated rotational constants and the corresponding experimental values [28] is given in Table 1. The values obtained with both the 4-31G and

120 M. Schiitz et al./J. Mol. Struct. (Theochem) 276 (1992) 117-132

0.943

> 110.9

I 1.352

1.307 1.381

1.075

Fig. 1. 6-31G** minimum-energy structure of phenol (bond lengths in angstrom, bond angles in degrees).

6-31G** calculations are in good agreement with experiment. The calculations yield rotational constants which are l-Z% too high, i.e. bond lengths which are slightly too short. This is the expected difference between a calculated (r,) structure and the experimentally determined vibrationally averaged (F,,) structure.

Vibrations The assignment of the calculated normal modes obtained for the phenol

4-31G and 6-31G** minimum-energy structures to the experimental data is based primarily on a comparison of the vibrational eigenvectors with the normal mode diagrams in an earlier assignment worked out for several monosubstituted benzenes [29]. To determine proper scaling factors for the harmonic frequencies a least-squares fit to the experimental infrared (IR) data of Bist et al. [30] was performed: the optimum scaling factors are 0.90195 for the 4-31G and 0.90283 for the 6-31G”” basis set.

The harmonic and the scaled frequencies of the 43lG and 6-31G** calculation are compiled and related to the corresponding experimental values of Bist et al. in Table 2. The agreement between experiment and the scaled ab initio results is satisfactory, with differences being within rt 10%.

M. Schiitz et al/J. Mol. Struct. (Theochem) 276 (1992) 117-132 121

TABLE 1

Rotational constants (in megahen) of phenol, phenol - H,O and d-phenol * D,O

Exp.” 4-31G Ab 6-31G** Ab

(%) (X)

Phenol A 5650.521 5766.162 2.047 5749.660 1.755 B 2619.239 2645.208 0.991 2659.331 1.531 C 1789.855 1813.343 1.312 1818.322 1.590

Phenol - H, 0 A 4438.107 4371.3 B 1110.252 1086.2 C 892.025 873.51

D-Phenol * D,O A 4346.88 4301;7 B 1024.31 1000.6 C 835.85 817.83

*Ref. 28. bA = (calculated experimental)/experimental. -

The individual differences in the scaled 4-3lG and 6-31G** frequencies are compared with the experimental values in Fig. 2. The mean and the root- mean-square (RMS) deviations from experiment are quite similar for both basis sets, being A = 1.3% and A(RMS) = 3.7% for 4-31G, and A = - 0.6% and A(RMS) = 3.1% for 6-31G**, respectively. In the context of this work, it is interesting that the 4-3lG basis gives better predictions for the a torsion, bend and stretching frequencies.

Minor inconsistencies appear in the sequence of the frequencies between experiment and the 4-31G results, which are partly resolved if the larger basis set is used. While experimentally the sequence of the normal modes is{ . . . . 5, 17a, 1, 18a, . . ., 7a, 3, 14, . . .}, for th e scaled 4-31G frequencies this changes to (. . ., 1, 18a, 17a, 5, . . ., 7a, 14, 3, . . .} and to {. . ., 1, 17a, 5, 18a, . . ., 14, 7a, 3, . . .} for 6-31G**. Perturbations (e.g. Fermi resonances) not included in the harmonic vibrational analysis are a plausible explanation for these discrepancies.

Phenol * H,O

Structure of complex In order to avoid exhaustive searches covering large parts of the energy

hypersurfaces, we adopted the (H,O), minimum-energy structures of previous calculations [22] to devise favourable starting configurations for the structural optimizations. The minimum-energy structures obtained

122

TABLE 2

M. Schiitz et al.lJ. Mol. Struct. (Theochem) 276 (1992) 117-132

Calculated and experimental vibrational frequencies of phenol

Label Exp.” Ab initio Scaled (0.90195) R)

Ab initio Scaled 4-31G 6-31G** (0.90283) R,

a” 11 244.0 263.8 237.9 - 2.5 256.2 231.3 - 5.2 a” torsion 309.0 327.2 295.2 - 4.5 314.2 283.7 - 8.2 a’ 18b 404.0 434.2 391.6 - 3.1 440.7 397.9 - 1.5 a” 16a 409.5 479.8 432.8 5.7 461.2 416.4 1.7 an 16b 503.0 581.0 524.0 4.2 568.1 512.9 2.0 a’ 6a 527.0 589.2 531.4 0.8 574.3 518.5 - 1.6 a’ 6b 619.0 707.2 637.9 3.1 678.6 612.6 - 1.0 a” 4 687.0 798.7 720.4 4.9 767.8 693.2 0.9 a” 10b 752.0 884.7 798.0 6.1 846.7 764.4 1.7 a’ 12 823.0 894.1 806.4 - 2.0 892.0 805.3 - 2.1 ay 10a 817.2 971.1 875.9 7.2 924.2 834.4 2.1 a” 17b 881.0 1060.2 956.2 8.5 996.0 899.2 2.1 a’ 1 1000.0 1114.5 1005.3 0.5 1085.2 979.7 - 2.0 a1 18a 1026.0 1141.1 1029.2 0.3 1122.9 1013.8 - 1.2 a” 17a 995.2 1145.6 1033.3 3.8 1090.1 984.2 - 1.1 a” 5 973.0 1177.8 1062.3 9.2 1111.5 1003.5 3.1 a’ 15 1070.0 1191.0 1074.2 0.4 1176.9 1062.5 - 0.7 a’ OH bend 1177.0 1249.6 1127.1 - 4.2 1197.3 1080.9 - 8.2 a’ 9b 1150.0 1300.5 1173.0 2.0 1291.0 1165.6 1.4 a’ 9a 1169.0 1317.6 1188.4 1.7 1282.1 1157.5 - 1.0 a’ 7a 1262.0 1380.6 1245.3 - 1.3 1404.2 1267.7 0.5 a’ 14 1343.0 1396.8 1259.9 - 6.2 1370.6 1237.4 - 7.9 a’ 3 1277.0 1518.5 1369.6 7.2 1488.7 1344.0 5.2 a’ 19b 1472.0 1648.8 1487.1 1.0 1635.3 1476.4 0.3 a’ 19a 1502.0 1685.9 1520.6 1.2 1671.2 1508.8 0.5 a’ 8b 1610.0 1794.7 1618.7 0.5 1798.0 1623.3 0.8 a’ 8a 1604.0 1812.0 1634.3 1.9 1810.6 1634.7 1.9 a’ 13 3027.0 3337.3 3010.1 -0.6 3326.4 3003.2 - 0.8 a’ 7b 3049.0 3352.2 3023.5 -0.8 3343.7 3018.8 - 1.0 a’ 2 3063.0 3363.3 3033.5 - 1.0 3354.1 3028.2 - 1.1 a’ 20b 3070.0 3380.5 3049.1 - 0.7 3370.9 3043.4 - 0.9 a’ 20a 3087.0 3397.1 3064.0 - 0.7 3379.6 3051.2 - 1.2 a’ OH stretch 3656.0 4022.6 3628.1 - 0.8 4197.2 3789.3 3.6

ZPEb

A (X) A (RMS) (%)

24859.6 24613.7

1.3 - 0.6 3.7 3.1

“Ref. 30. b Zero-point energy.


16a

123

A [“A] 4-31G m A ~1 6-31G**

Fig. 2. Comparison of the individual differences of the scaled 4-31G and 6-31G** frequencies relative to the experimental values. A = (calculated - experimental)/experimental.

after relaxing both intra- and intermolecular degrees of freedom, assuming only planarity of the aromatic molecule, are of C, symmetry and resemble the original (H,O), configurations. The relevant structural parameters of phenol * H, 0, uncomplexed phenol and (H, 0), obtained with both the basis sets used are given in Table 3. Interatomic distances and bond angles of phenol * H,O are given in Table 3 and compared with those for (H, 0), reported in ref. 22 (see Fig. 3). The following remarks can be made; As for (H,O),, the hydrogen bond in the complex studied is almost linear with deviations from linearity (represented by 4) increasing in going from 4-31G to 6-31G**. As expected, complexation leads to a slight elongation (0.005-- 0.01 A) of the bridging O-H bond (r2) in phenol, whereas the O-H bond of the water molecule (rs) remains unaffected. The length of the hydrogen bond given by the O-O distance R increases with increasing basis-set size (2.77 A with 4-31G, and 2.91 A with 6-31G**) and is about 3% smaller than the corresponding value reported for (H, 0), . There is a significant decrease of ca. 0.01 A in the C-O bond length of phenol (rl) in the complex which may imply a slight increase in the double-bond character of this bond. Further- more, the intramolecular angles e1 and 8, are both slightly larger by (ca. lo) in phenol * H,O compared to the corresponding monomer values. The in- clination of the C, axis of the water molecule to the O-O direction (represented by 8) is generally smaller in the phenol * H,O complex than in

124

TABLE 3


Structural parameters of the phenol - H,O complex

4-31G 6-31G**

Phenol - H,O Uncomplexed (H,O), Phenol - H, 0 Uncomplexed (H,O),

Bond length (A) rl 1.363

2 0.961 2.767 r3 0.950

Bond angle (O)

A1 0 115.61 122.58 B” 161.23 0.21

0 2 112.37

1.374 1.345 1.352 0.950 0.958 0.949 0.943 0.948

2.832 2.996 3.013 0.951 0.951 0.944 0.943 0.944

114.74 111.32 111.58 110.93 106.20 122.56 122.51 122.45

0.00 3.34 6.00 147.84 135.79 124.06

111.23 111.96 196.83 105.97 106.2

(H,O),. There is a considerable difference for this coordinate between the 4-31G and 6-31G** estimates (ca. 25’). We explored the energy hypersurface along this coordinate and found no evidence for a second (local) minimum for neither the 4-3lG or the 6-31G** basis set. We are confident that the resulting 4-3lG and 6-31G”” phenol * H,O minimum-energy structures represent the global minima of the related energy hypersurfaces.

Based on previous experiences [22] we presume that the 6-31G** structure is more reliable than the 4-31G geometry. Of the calculated rotational constants of the phenol * H,O complex listed in Table 1, B and C are particularly sensitive to the hydrogen-bond parameters R and /I.

Vibrations Vibrational frequencies and eigenvectors were computed for the phenol *

H,O minimum-energy structures obtained with the 4-31G and 6-31G** basis sets. Six additional normal modes arise upon complexation with one water

Fig. 3. Labelling of the bonds and angles in the phenol - H,O complex.


42~4 cm-l 191 .O cm-l

116.2 cm-l 262.1 cm-l

Fig. 4. Intermolecular normal modes (slanted projection) and corresponding frequencies for phenol * H,O (4-3lG).

molecule and these can be assigned as two a” rocking modes (p, and pJ, two a’ wagging modes (a, and &), one a” twisting mode (z) and one a’ stretching mode (rr). These intermolecular fundamentals are shown in slanted and atop projections in Figs. 4 and 5. The low frequency rock (p, = 42.4cm-‘) involves mainly rigid-body hindered rotation of phenol, but includes a small -0-H torsional component in the direction of the hydrogen bond deformation. In contrast, the high-frequency rock (p2 = 231.3cm-‘) has a large -0-H torsional component on phenol. In the (r mode considerable mixing of the stretching and wagging motions is apparent, which is in agreement with the experimental findings of Stanley and Castleman [12]. In the 4-31G basis set the twist mode (z = 118.2 cm-l) is a pure torsion of the hydrogen bond, but the 6-31G** calculation shows increased rock character. Otherwise little mixing of motions occurs in the normal modes.

In order to obtain adequate values for correlation with experimental results, the ab initio harmonic frequencies were scaled using the scaling factors determined for phenol (see above). The unscaled and scaled vibrational frequencies (including the zero point vibrational energy (ZPE)) for both of the basis sets used are collected in Table 4 and related to the experimental results reported in refs. 12 and 13. Agreement between the experimental and the scaled ab initio frequencies is rather good, the devia-

126 M. Schiitz et al.jJ. Mol. Struct. (Theochem) 276 (1992) 117-132

42.4 cm-l

w--y

73.9 cm-l

327 \ \ 3

118.2 cm-l

x.+ _-

(a)

191 .O cm-l

Tb \ \ \ \ \ ‘a.

231.3 cm-l

IL__ ?

262.1 cm-l d

*

\ \ \ \ J \ (f-9

Fig. 5. Intermolecular normal modes (atop projection) and corresponding frequencies for phenol * H,O (4-3lG).

tion being less than 10% for most of the fundamentals. The 4-31G result (scaled) is somewhat closer to the experimental value (mean value l.SS%, RMS 5.0%) than is the corresponding 6-31G** result (mean error - 3.78%, RMS 7.0%). While the 431G basis set overestimates the frequency of c, the 6-31G** basis underestimates this value. Furthermore, both basis sets un- derestimate the & wag frequency of the complex by up to 24% in the case of the 6-31G** basis set.

Intermolecular modes Comparing the computed intermolecular vibrational frequencies with

the low-frequency region of the dispersed fluorescence spectrum of phenol l H,O reported in ref. 13, the stretch mode c is clearly identified as the intense band situated 150 cm-l to the red from the origin. A second peak appears close by in the spectrum at 138 cm-‘” which can be assigned on the basis of the calculations as the overtone of the low-frequency wag PI. The fundamental of /I, is not apparent in the spectrum; therefore 28, must borrow its intensity from c by Fermi resonance. The assignment of the weak band situated 104 cm-’ from the origin is less clear. The twist mode z has similar frequency (106.6 cm-‘, 4-31G) but is symmetry forbidden in C, geometry and the overtone of the low-frequency rocking mode p1 is


expected at 2 x 38 = 76 cm-‘, quite far from the observed value. A possible explanation could stem from the hypothesis that higher order contribu- tions to the binding energy not included in the present calculation (BSSE, correlation, and more flexible basis sets) lead to a slight deformation from C, symmetry, with the twist mode z then becoming symmetry allowed. Based on the scaled 6-31G** frequencies, a second interpretation is possible for the 104 and 138cm-’ bands: the 104 cm-’ band may be assigned as Zp,, and the 138cm-’ band as the combination p1 + z, which is calculated at 123.6 cm-‘. However, this assignment is improbable since & itself is not observed, and 28, is far from being in Fermi resonance with any other mode.

Measurement of the ground-state frequencies of d-phenol - D,O would help solve this problem. Thus, using the 4-31G and 6-31G** force fields calculated for phenol - H,O, we carried out a vibrational analysis of d- phenol - D, 0 (C, H, OD - D, 0). The scaled intermolecular frequencies of the deuterated and undeuterated species are given in Table 5. The largest shifts due to deuteration are expected for z, p2 and /I2 which involve little motion of the phenol moiety, with the further modes being scarcely affected. The frequency of the twist mode z is expected to decrease by ca. 30% with the symmetry-allowed overtone 22 now probably gaining intensity from the stretch mode by Fermi resonance. For the high-frequency wagging mode &, the 4-31G calculation predicts a decrease of 51 cm-’ (22%). The resulting frequency lies quite close to 0. This fundamental may also be observed experimentally as a weak band borrowing intensity from the 0 mode. For the 6-31G** basis set a large increase in mixing between the stretching and wagging motions is noted for the (T mode on going from the undeuterated to the deuterated complex. To summarise, the calculations predict qualitative changes in the shape of the spectra due to deuteration.

Intramolecular modes All intramolecular modes involving non-negligible motion of the phenolic

OH group undergo substantial changes in frequency due to hydrogen bonding with H,O. The scaled frequency shifts of the phenol modes which are most affected, i.e. those that change by more than 2%, are given in Table 6. A huge shift occurs for the torsional mode of the OH group which exhibits increases of 488cm-’ and 359cm-’ in the 4-31G and 6-31G** basis sets, respectively. This implies frequency changes of 2.6 and 2.3, respectively! A comparison of the vibrational eigenvectors reveals that, due to complexation, this normal mode acquires some character of the “anti- symmetric” counterpart to pz with the movement of the phenolic hydrogen atom and the water oxygen atom being out of phase. The restoring force along this coordinate is not only due to a change in overlap between the aromatic x system and the p, orbital of the phenolic oxygen atom, as is the

128

TABLE 4

M. Schiitz et al./J. Mol. Struct. (Theochem) 276 (1992) 117-132

Calculated and experimental frequencies of phenol - H,O

Symmetry/label Exp.” Ex~.~ Ab initio Scaled A Ab initio Scaled A

4-31G 0.90195 (%) 6-31G** 0.90283 (%)

a” p1

a’ A au r

a’ u

au pz

a’ Bz a” 11

a’ 18b

a” 16a

a” 16b

a’ 6a

a’ 6b

a” 4

a” torsion

a” lob

a’ 12

a” 10a

a” 17b

a’ 1

a’ 18a

a” 17a

a” 5

a’ 15

a’ 9b

a’ 9a

a’ OH bend

a’ la

a’ 14

a’ 3

a’ 19b

a’ 19a

a’ H,O bend

a’ 8b

a’ 8a

a’ 13

a’ 7b

a’ 2

a’ 20b

a’ 20a

a’ OH stretch

a’ H,O sym. str.

a” H,O asym. str.

(69) (70.5)

104

150 151

527.4 528

618.3 618

825.1 825

812.5 813

999.5 1000

1024.7 1026

1070.1

1150.9

1272 1274

42.4 38.2 33.6 30.3 73.9 66.7 - 4.7 58.8 53.1

118.2 106.6 103.4 93.3

191.0 172.2 14.1 154.8 139.7

231.3 208.6 214.8 193.9

262.1 236.4 223.8 202.1

271.0 244.4 262.8 231.3

469.1 423.1 464.8 419.6

482.3 435.0 463.0 418.0

585.0 527.7 571.9 516.3

593.3 534.2 1.0 575.6 519.7

107.1 638.3 3.1 679.0 613.0

803.3 124.5 772.4 697.4

868.4 783.3 711.9 642.1 884.6 797.9 848.3 765.9

900.7 812.3 - 1.7 895.6 808.6

980.4 884.3 8.8 935.2 844.3

1062.1 957.9 1000.5 903.3

1114.1 1004.9 0.5 1084.5 979.1

1141.0 1029.1 0.4 1123.8 1014.6

1146.6 1034.2 1092.1 986.0

1172.8 1057.8 1107.6 999.9

1195.9 1078.7 0.8 1178.2 1063.7

1273.9 1149.0 - 0.2 1203.2 1086.3

1316.4 1187.3 1282.6 1158.0

1337.5 1206.3 1311.1 1183.7

1391.7 1255.3 - 1.5 1416.0 1278.4

1435.0 1294.3 1403.7 1267.3

1533.7 1383.3 1505.7 1359.3

1650.7 1488.8 1639.6 1480.3

1690.7 1524.9 1674.8 1512.1

1752.1 1580.3 1763.3 1592.0

1790.2 1614.7 1795.7 1621.2

1813.9 1636.1 1811.6 1635.6

3339.8 3012.3 3334.8 3010.8

3350.0 3021.5 3343.5 3018.6

3366.8 3036.6 3357.2 3031.0

3376.6 3045.5 3361.5 3040.3

3391.7 3059.2 3375.9 3047.8

3816.4 3442.2 4076.3 3680.2

3972.3 3582.8 4144.6 3741.9

4117.9 3714.1 4259.6 3845.7

- 24.1

- 7.5

- 1.8

- 1.0

- 2.1

3.9

- 2.1

- 1.0

- 0.6

- 5.6

0.3

M. Schiitz et aE./J. Mol. Struct. (Theochem) 276 (1992) 117-132 129

TABLE 4 (eontinu~)

Symmetry/label Exp.” Exp.b Ab initio Scaled A Ab initio Scaled A 4-31G 0.90195 (%) 6-31G** 090283 (%)

ZPE 30506.7

A (%) A (RMS) (%)

*Ref. 13. bRef. 12. ’ Estimated as half the overtone frequency.

30311.4

1.36 - 3.73 5.0 7.0

case for the phenol monomer, but it also involves a strong component arising from the hydrogen bond.

Similarly to (H20)2, the frequency of the OH stretching mode decreases by - 5.1/ - 2.9% and that of the OH bending mode increases by 7.0/9.5% relative to the corresponding monomer frequency. Moreover, for the lower frequency phenolic mode 18b (CO bending character), a distinct increase in frequency of &O/5.5% is noted. The intramolecular modes of the water molecule are scarcely affected.

The data given in Table 4 show that the highest intermolecular and the lowest intramolecular fundamentals & and 11, have comparable frequencies. We conclude that hydrogen bonding has a major effect on the modes involving motion of the OH group, and that the simple approach of con- sidering phenol/water complexation as an interaction between two rigid, non-relaxed molecules must fail.

TABLE 5

The effect of deuteration on the intermolecular frequencies: comparison of (scaled) frequencies v of C,HSOH * H,O and C,H,OD - D,O and the corresponding frequency shifts Av for the intermolecular normal modes

4-31G 6-31G**

Phenol + H,O, d-Phenol - D, 0 Phenol * H,O, d-Phenol - D, 0 v (cm-‘) v (cm-‘)

v (cm-‘) Av (cm-‘) v (cm-‘) Av (cm-‘)

Pl 36.2 36.0 - 2.2 30.3 27.9 - 2.4 8, 66.7 61.9 - 4.3 53.1 49.6 - 3.5 T 106.6 76.2 - 30.4 93.3 69.3 - 24.0 0 172.2 165.4 - 6.8 139.7 127.9 - 11.8 PZ 208.6 157.3 - 51.3 193.9 144.3 - 49.6 Bz 236.4 134.9 - 51.5 202.1 164.9 - 37.2

130

TABLE 6

M. Schiitz et al./J. Mol. Strut. (Theo&em) 276 (1992) 117-132

Frequency shifts Av of intramolecular phenol modes (scaled)

4-31G 6-31G**

Av (cm-‘) Av (%) Av (cm-‘) Av (%)

a” torsion 488 165.3 359 126.5 a’ 18b 31.5 8.0 21.1 5.5 a’ OH bend 79.2 7.9 102.8 9.5 a’ 9b - 24.6 - 2.0 - 79.3 - 6.8 a’ 14 34.4 2.7 29.9 2.4 a’ OH stretch - 185.9 - 5.1 - 199.1 - 2.9

Binding energies The calculated restricted Hartree-Fock (RHF) 4-31G and 6-31G**

binding energies - D, and - D, (corrected for the ZPE) of phenol * H,O are compiled in Table 7 together with the corresponding values for (H, 0), . No attempt was made to include electron correlation or to correct for the BSSE. In a previous paper on ab initio calculations of small water clusters [22] it was concluded that the BSSE inherent in the 4-31G basis set leads to substantial overestimation of binding energies, this being more than 50% for (H,O),. However, inclusion of polarisation functions results in D, and Do values which are in excellent agreement with both large-scale Hartree- Fock configuration interaction (HF/CI) calculations and experimental data for (H,O),. In the latter case the overestimation due to the BSSE and the underestimation due to the lack of electron correlation may fortuitously cancel rather closely [31]. Similar results were obtained for phenol.H,O with the 4-31G basis, resulting in a binding energy of D, = 10.4 kcal molll, which is about 50% higher than the corresponding 6-31G** value of D, = 7.25 kcal mall' . Correcting for the intermolecular and the change in intramolecular ZPE has a considerable influence, lowering the binding energy by 20-25% to Do = 8.3 kcal malll (4-31G) and Do = 5.5 kcal mol-’ (6-31G**).

It is evident from the data in Table 7 that the hydrogen bond formed between phenol and water is comparatively strong. The calculated increase

TABLE I

Binding energies of phenol - H,O and (H,O), (in kcal mol-‘/cm-‘)

4-31G 6-31G**

-Q -D, -D,

Phenol * H,O 10.4/3651 8.312999 1.312531 (H, 0), 8.212818 5.7/1976 5.6/1965

-&

5.5/1929 3.411203

M. Schiitz et al.lJ. Mol. Struct. (Theochem) 276 (1992) 117-132 131

in dissociation energy Do in going from (H,O), to phenol - H,O is large (about 60%). The results of improved calculations including larger basis sets, electron correlation effects and correction for the BSSE will be of great interest.

SUMMARY

Ab initio calculations at the RHF level using the standard 4-31G and 6-31G** split-valence basis sets were performed for phenol and phenol * H,O with full structural optimization and subsequent normal- coordinate analysis. No attempt was made to correct for the basis-set-superposition errors (BSSE). Calculations on the phenol monomer were carried out as a prerequisite for further calculations on the water complex. Appro- priate scaling factors were then determined for the ab initio harmonic frequencies of both basis sets correlating the theoretical frequencies of phenol and the experimental data of Bisst et al. [30] (linear least-square, fit).

The minimum-energy structures found for phenol*H,O have C, symmetry and are similar to the (H,O), structures reported previously [l-9,22]. Some structural reorganization occures in the phenol subunit due to complexation, especially with regard to the C-O and O-H bond lengths. The inter- and intra-molecular vibrational frequencies of phenol * H,O and d-phenol * D,O, evaluated with both basis sets were scaled using the scaling factors determined previously for phenol, and were compared in detail with the experimental results reported in refs. 12 and 13. It was possible to assign two of the intermolecular fundamentals appearing in the ground-state spectra on the basis of the calculations; the third band cannot be definitely assigned at present. Furthermore, it was found that several of the intramolecular modes of phenol undergo strong spectral shifts upon complexation with water, especially those modes associated with the C-O-H subunit.

The binding energy obtained with the 6-31G** calculation including corrections for the ZPE seems to be quite reliable. We note an increase in strength of the hydrogen bond of about 60% on going from (H,O), to phenol - H,O.

ACKNOWLEDGEMENTS

This research was supported by the Schweiz. Nationalfonds (grant Nos. 20.26447.89 and 20.28995.90). Grants for computer time by the University of Bern and the HLR-Rat (Inst. fur Wiss. Rechnen, ETH Zurich) computer centres are gratefully acknowledged.

REFERENCES

1 (a) 0. Matsuoka, E. Clementi and M. Yoshimine, J. Chem. Phys., 64 (1976) 1351.

132 M. Schiitz et al.lJ. Mol. Struct. (Theo&em) 276 (1992) 117-132

10 11 12 13 14

15 16

17 18 19

20

21

22 23 24 25 26 27 28

29

30 31

(b) E. Clementi and G. Corongiu, Int. J. Quantum Chem., Quantum Biol. Symp., 10 (1983) 31; Int. J. Quantum Chem., Quantum Chem. Symp. 18 (1984) 707. (c) J. Dietrich, C. Corongiu and E. Clementi, Chem. Phys. Lett., 112 (1984) 426. (d) E. Clementi and P. Habitz, J. Phys. Chem., 87 (1983) 2815. M.J. Frisch, J.A. Pople and J.E. Del Bene, J. Chem. Phys., 89 (1984) 3664. R.D. Amos, Chem. Phys. 104 (1986) 145. R.J. Vos, R. Hendricks and F.B. van Duijneveldt, J. Comput. Chem., 11 (1990) 1. B.J. Smith, D.J. Swanton, J.A. Pople, H.F. Schaefer and L. Radom, J. Chem. Phys., 92 (1990) 1240. G. Chalasinski, J. Chem. Phys., 94 (1991) 2873. C.J. Marsden, J. Chem. Phys., 95 (1991) 1825. R. Knochenmuss and S. Leutwyler, J. Chem. Phys., 96 (1992) 5233. (a) D.F. Coker and R.O. Watts, J. Phys. Chem. 91 (1987) 2513. (b) J.R. Reimers and R.O. Watts, Chem. Phys. 85 (1984) 83; Chem Phys. Lett. 94 (1983) 222. (c) J.R. Reimers, R.O. Watts and M.L. Klein, Chem. Phys. 64 (1982) 95. F.H. Stillinger and A. Rahman, J. Chem. Phys., 60 (1974) 1545, 68 (1978) 666. S.-B. Zhu, S. Singh and G.W. Robinson, J. Chem. Phys. 95 (1991) 2791. R.J. Stanley and A.W. Castleman, Jr., J. Chem, Phys. 94 (1991) 7744. T. Ebata, M. Furukawa, T. Suzuki and M. Ito, J. Opt. Sot. Am., B, 7 (1990) 1890. H. Abe, N. Mikami, M. Ito and Y. Udagawa, J. Phys. Chem., 86 (1982) 2567; Chem. Phys. Lett., 93 (1982) 217. K. Fuke and K. Kaya, Chem. Phys. Lett., 94 (1983) 97. (a) R.J. Lipert and S.D. Colson, J. Chem. Phys. 89 (1988) 4579. (b) R.J. Lipert and S.D. Colson, Chem. Phys. Lett. 161 (1989) 303. (c) R.J. Lipert and SD. Colson, J. Chem. Phys., 94 (1989) 2358. E. Honegger, R. Bombach and S. Leutwyler, J. Chem. Phys., 85 (1986) 1234. M. Pohl, M. Schmitt and K. Kleinermanns, J. Chem. Phys., 94 (1991) 1717. M.J. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachari, C.F. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, L.R. Kahn, D.J. Defrees, R. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fluder and J.A. Pople, Gaussian-86, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Ragava- chari, M. Robb, J.S. Binkley, C. Gonzalez, D.J. Defrees, D.J. Fox, R.A. Whiteside, R. Seeger,. CF. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and J.A. Pople, Gaussian%, Revision H, Gaussian Inc., Pittsburgh, PA, 1990. R.D. Amos and J.E. Rice, CADPAC: The Cambridge Analytic Derivative Package, Issue 4.0, Cambridge, 1987. E. Honegger and S. Leutwyler, J. Chem. Phys., 88 (1988) 2582. C. Puebla and T.-K. Ha, J. Mol. Struct. (Theochem), 204 (1990) 337. P. Politzer and N. Sukumar, J. Mol. Struct. (Theochem), 179 (1988) 439. S. Marriott, T. Silvestro and R.D. Topsom, J. Mol. Struct. (Theochem), 151 (1987) 15. C.W. Bock and M. Trachtman, J. Mol. Struct. (Theochem), 122 (1985) 155. H. Konshin, J. Mol. Struct, (Theochem), 92 (1983) 173. E. Mathier, D. Welti, A. Bauder and Hs.H. Giinthard, J. Mol. Spectrosc., 37 (1971) 63. T. Pedersen, N.W. Larsen and L. Nygaard, J. Mol. Struct., 4 (1969) 59. C.R. Quade, J. Chem. Phys., 48 (1968) 5490. D.H. Whiffen, J. Chem. Sot., (1956) 1350. R.R. Randle and D.H. Whiffen, Molecular Spectroscopy, Report on the conference held by the Institute of Petroleum, London, October 1954. H.D. Bist, J.C.D. Brand and D.R. Williams, J. Mol. Spectrosc., 24 (1967) 402. J.O. Baum and J.L. Finney, Mol, Phys., 55 (1985) 1097.

structures and vibrations of phenol · h2o and d-phenol · d2o based on ab initio calculations

Documents