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Theoretical and Experimental (400–10 000 cm−1) Study of the Vibrational Spectrum of Pentachlorophenol
Journal of Molecular Spectroscopy 195, 308 –316 (1999) Article ID jmsp.1999.7818, available online at http://www.idealibrary.com on
Theoretical and Experimental (400 –10 000 cm 21) Study of the Vibrational Spectrum of Pentachlorophenol Boguska Czarnik-Matusewicz,* Asit K. Chandra,† Minh Tho Nguyen,† and The´re`se Zeegers-Huyskens† *Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Street, 50 383 Wroclaw, Poland; and †Department of Chemistry, University of Leuven, 200F Celestijnenlaan, B-3001, Heverlee, Belgium Received October 26, 1998; in revised form January 5, 1999 DEDICATED TO PROFESSOR C. SANDORFY
Geometric and vibrational spectroscopic data (bond distances and angles, vibrational frequencies, infrared intensities) of pentachlorophenol–OH (PCP–OH) and pentachlorophenol–OD (PCP–OD) are calculated by density functional theory (B3LYP) using the 6-311G(d, p) basis set. Except for the vibrations involving the OH bond, the agreement between the experimental and calculated fundamental frequencies between 3600 and 400 cm 21 is very good. The theoretical method failed, however, to reproduce quantitatively the experimental intensities. The infrared spectra between 3600 and 10 000 cm 21 are studied, and the overtones or combination bands are assigned by comparing the spectra of PCP–OH and PCP–OD. The difference between the experimental and theoretical frequencies of the n(OH) and n(OD) frequencies can be mainly accounted for by the neglect of the anharmonicities of these vibrations in calculations. The binary or ternary combinations characterized by the highest coupling constants and the highest intensities are those involving the n(OH), d(OH), g(OH), and n(C–O) vibrations. © 1999 Academic Press INTRODUCTION
In recent years, considerable efforts have been devoted to the understanding of the vibrational spectra of simple aromatic derivatives. It has been recently shown that the experimentally observed frequencies of phenol are reasonably well reproduced by quantum chemical calculations at moderate computational levels (1, 2). Phenol derivatives and more particularly pentachlorophenol (PCP) are very often used as model proton donors for the study of the thermodynamic and spectroscopic properties of hydrogen-bonded systems (3– 6). There are, however, in the literature only empirical assignments for this molecule (7, 8). In the present paper, we report calculated geometric and vibrational spectroscopic data of PCP, and the experimental parameters are compared with the calculated ones. To have a deeper insight into the nature of the vibrations, their anharmonicities, and their coupling, the experimental near-infrared spectra are also studied. It is worth mentioning that the overtone spectrum of phenol has been recently investigated (9) and that the results are in good agreement with those obtained from nonresonant ionization-detected infrared spectrum of jetcooled phenol (10). In phenol derivatives, the interpretation of the near-infrared spectra is often difficult due to the presence of numerous combinations involving the CH stretching or bending modes which are not present in PCP. 308 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
TABLE 1 B3LYP/6-311G(d, p) Optimized and Experimental Geometric Parameters of PCP
Note. Bond lengths in Å, angles in degrees. a From Ref. (14).
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COMPUTATIONAL METHODS
The geometry of the PCP molecule was fully optimized using the density functional theory (DFT) with the hybrid B3LYP (11, 12) exchange correlation functionals and the 6-311G(d, p) basis functions. The Gaussian package (13) was used for all the calculations. Harmonic vibrational frequencies were calculated at the optimized geometries using the same level of theory to characterize the stationary points. Anharmonic contributions whose evaluation requires the calculation of successive energy derivatives beyond the second order have been neglected.
EXPERIMENTAL
FIG. 1. Atom labeling in PCP.
The mid-infrared spectra (4000 – 400 cm 21) were recorded with the Bruker 66 spectrometer (resolution 2 cm 21, globar source, DTGS detector, KBr beamsplitter), and cells equipped
TABLE 2 Calculated B3LYP/6-311G(d, p) and Experimental Infrared Data for the Fundamental Vibrations of PCP–OH
a IR absorbance maxima normalized to A 5 100 for the strongest band; bn 5 stretching; d 5 in-plane deformation, g 5 out-of-plane deformation; R 5 ring vibration; cfrom ref. 7; donly one absorption was observed between 713 and 696 cm 21; en.o. 5 not observed; ffrom ref. 8.
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TABLE 3 Calculated B3LYP/6-311G(d, p) and Experimental Infrared Data for the Fundamental Vibrations of PCP–OD
c
a same remarks as below Table 2; bonly one absorption is observed between 712 and 678 cm 21; from ref. 7.
with KBr windows were used. The near-infrared spectra (10 000 – 4000 cm 21) were obtained using the same spectrometer (resolution 2 cm 21, tungsten source, cooled InSb detector, CaF 2 beamsplitter), and cells equipped with quartz-infrasil windows having a path length between 1 and 5 cm were used. The spectra resulted from 32 to 64 scans. All the spectra were taken at room temperature in carbon tetrachloride at a concentration of 0.09 mol liter 21. The absence of absorption at 3300 cm 21 indicates that at this concentration the self-association of PCP is negligible. PCP from Across Chemica was crystallized from a petroleum– ether mixture. Carbon tetrachloride from Janssen Chimica was dried on molecular sieves. PCP–OD was prepared by several exchanges with methanol–OD and about 90% deuteration was achieved.
RESULTS AND DISCUSSION
1. Optimized Geometry of PCP The results of B3LYP/6-311G(d, p) geometry optimization and the experimental bond distances and bond angles (14) are indicated in Table 1. The theoretical results show that PCP is planar in agreement with the X-ray diffraction data. The computed H . . . Cl2 distance of 2.347 Å indicates the presence of a weak intramolecular OH . . . Cl hydrogen bond in the isolated molecule. In the crystalline state, the molecules are held together by intermolecular bifurcated hydrogen bonds: the OH group of a first PCP molecule being bonded to both the O and Cl5 atoms of a second one. The experimental C5–C6 distance (1.438 Å) is longer than the other C–C distances (1.365–1.371 Å), and this might be due to an effect of inter-
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FIG. 2. FT-IR spectrum (3300 –2200 cm 21) of PCP–OH. C 5 0.09 mol liter, path length 5 1 cm.
molecular hydrogen bonding. In the isolated molecule, the C5–C6 distance is calculated to be shorter (1.397 Å). The experimental C–O distance (1.329 Å) is shorter than the computed one (1.344 Å), and this difference results probably from the same effect (Fig. 1). 2. Fundamental Vibrational Transitions in PCP The experimental spectra of PCP–OH and PCP–OD in the fundamental region (4000 –300 cm 21) have been previously discussed (4, 5) and therefore are not reproduced in this paper. In contrast, the infrared intensities have not been previously determined. The B3LYP/6-311G(d, p) and experimental vibrational frequencies and intensities of PCP–OH are listed in Table 2. Table 3 contains the same parameters for PCP–OD. Seven absorptions are predicted between 220 and 70 cm 21; these modes can be described as out-of-plane ring and C–Cl deformations and are not sensitive to O-deuteration. They will not be discussed further in this work. The n(OH) and n(OD) vibrations are 100% pure and this appears also from the computed value of the isotopic ratio n(OH)/n(OD), which is 1.370, near to the theoretical value of 1.374. This is not the case for the d(OH) and d(OD) vibrations characterized by an isotopic ratio of 1.226. The contribution of the d(OH) mode to the ring mode increases from 1573 to 1309 cm 21, the mode predicted at 1222 cm 21 having a predominant d(OH) character. The shifts of the ring vibrations between 1560 and 1300 cm 21 in PCP–OD provide a further support for this mixing. Such a
strong coupling explains why in molecular complexes between PCP and proton acceptors the d(OH) vibration seems to be rather insensitive to complex formation with proton acceptors. This mode is shifted to higher wavenumbers for other phenol complexes (15). The main contribution to the d(OD) vibration is computed at 997 cm 21, but the modes predicted at 972 and 679 cm 21 also contain an important d(OD) contribution. The main contribution to the g(OH) vibration is predicted at 445 cm 21. This vibration is also coupled with the out-of-plane ring modes at 354 and 310 cm 21. The same remark also holds for the g(OD) vibration whose main component is predicted at 311 cm 21, this vibration being also mixed with the out-of-plane ring modes at 360 and 342 cm 21. The n(OH) vibration of phenol is predicted at 3827 cm 21 and the g(OH) vibration at 365 cm 21 (1). This effect is attributed to the weak intramolecular OH . . . Cl interaction. In ortho-chlorophenol, where both the cis and trans conformers exist, the shift resulting from the formation of the intramolecular hydrogen bond is 58 cm 21; the enthalpy of the corresponding OH . . . Cl bond is about 6.5 kJ mol 21 (16). The scaling factor, defined as the ratio of the experimental to the calculated wavenumbers, computed for all the modes except the main contributors to the n(OH), d(OH), and g(OH) vibrations is 0.990 for PCP–OH and 0.988 for PCP–OD. As discussed in other works, B3LYP calculations with sufficiently large basis sets provide frequencies that even without any scaling are very close to the experimental data (17–19). The
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TABLE 4 Overtones and Combination Bands (3800 –2200 cm 1) for PCP–OH and PCP–OD in Carbon Tetrachloride Solution, Assignment of the Absorptions, and Anharmonicity or Coupling Constants
scaling factors are, however, lower for the vibrations involving the OH groups, and this will be discussed in the next paragraph. It must also be pointed out that the spectra have been measured experimentally in carbon tetrachloride and that no experimental data are available in the gas phase. For phenol, the n(OH) vibration turns out to be about 40 cm 21 higher in the gas phase than in carbon tetrachloride, but the ring vibrations differ by only 5 cm 21 from reach other (20, 21). This is likely to be the case for PCP–OH. The infrared intensities computed at this level of theory represent only a qualitative trend, and large differences between computed and experimental intensities are observed for the modes between 1300 and 1000 cm 21. 3. Overtones and Combination Bands The mechanical anharmonicities can be characterized by the anharmonicity constants X 12 , X 13 , . . . which can be computed by the second-order perturbation theory from the experimental frequencies of the fundamental (n 01), the first (n 02), and the second (n 03) overtone by the expressions X 12 5 n 01 2 n 02 / 2
[1]
X 13 5 n 02 / 2 2 n 03 /3.
[2]
The harmonic frequency ( n e ) is given by the expression
n e 5 n 01 1 2X 12.
[3]
If two anharmonicity constants are used but still within the frame of second-order perturbation theory, then X 123 5 n 01 2 n 02 / 2 2 4.5Y 123
[4]
Y 123 5 1/ 2~ n 02 2 n 01 2 n 03 /3!.
[5]
The value of Y 123 must be very much smaller than X 123 for the approximation to be valid. A large value of Y 123 implies that X 12 Þ X 23 (22, 23). The coupling constants (X coupl), even in the polyatomic case, are given by the equation X coupl 5 ~ n 101 1 n 201 ! 5 n comb
[6]
where the superscripts 1, 2, and comb refer to two different normal modes and the combination between them. (24, 25). Overtones and combination bands can also be observed in the mid-infrared region. The experimental spectrum of PCP–OH recorded in extended absorbance scale between 3600 and 2200 cm 21 is shown in Fig. 2. The experimental wave-
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VIBRATIONAL SPECTRUM OF PENTACHLOROPHENOL
FIG. 3.
313
FT-IR spectrum (5200 –3600 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm.
numbers, the assignment, and the anharmonicity or coupling constants (X) computed from Eqs. [1] and [6] are listed in Table 4. The assignment is made by comparison with PCP–OD where the absorptions at 2614, 2467, and 2252 cm 21 connected with the d(OH) vibration are not observed. The overtones or combinations of ring vibrations are characterized by about the same anharmonicities or coupling constants in both molecules. The anharmonicities of the ring modes between 1560 and 1530 cm 21, which can be described as the n8 ring modes of aromatic molecules, are characterized by an anharmonicity similar to that of benzene (between 4 and 5 cm 21) (26). The d(OH) vibration is characterized by an experimental anharmonicity of only 21 cm 21. This apparent anomaly can be explained by a strong coupling between the d(OH) and ring vibrations, as shown by our calculations. Very few results on the anharmonicity of bending vibrations of OH groups are available in the literature. The anharmonicity of the bending vibration of water is between 10 and 20 cm 21 (27, 28). Taking an average value of 15 cm 21, one can compute a more realistic value of about 1215 cm 21 for the uncoupled d(OH) vibration of PCP. This value will be further used for the estimation of the coupling constants between different modes. The experimental spectra of PCP–OH and PCP–OD between 3750 and 10 000 cm 21 are reproduced in Figs. 3–5.
The data for PCP–OH (wavenumbers of the overtones or combination bands observed between 3750 and 10 100 cm 21, assignment of the absorptions, their molar extinction coefficients, and the anharmonicity or coupling constants) are summarized in Table 5. The assignments of some combination tones may appear as somewhat speculative and are based mainly on the fact that the combinations involving the n 01(OH) or n 02(OH) levels are not observed in PCP–OD. The experimental anharmonicities X 12 of the n(OH) and n(OD) vibrations computed by Eq. [1] of PCP–OH and PCP–OD are 90 and 51 cm 21, respectively. The Y 123 values calculated by Eq. [5] are weak: 2.7 cm 21 for PCP–OH and 1.7 cm 21 for PCP–OD. It is worth mentioning that in gaseous phenol, the Y 123 values are strictly equal to 0 (10). The X 12 values are somewhat higher than in phenol–OH and phenol–OD (85 and 45 cm 21) (9) or other phenol derivatives (29). This result shows that the formation of the OH . . . Cl intramolecular hydrogen bond results in a weak increase (5 cm 21) of the anharmonicity of the n(OH) vibration. For stronger intermolecular hydrogen bonds formed between phenols and carbonyl or nitrogen bases, this increase is higher and depends on the enthalpy of complex formation (25, 30 –32). The difference probably results from the existence of the intramolecular hydrogen bond.
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FIG. 4.
FT-IR spectrum (6500 –5200 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm.
FIG. 5.
FT-IR spectrum (8400 –7000 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm. Copyright © 1999 by Academic Press
VIBRATIONAL SPECTRUM OF PENTACHLOROPHENOL
TABLE 5 Near-Infrared Data (3750 –10 000 cm 21) for PCP in Carbon Tetrachloride Solution, Assignment of the Absorptions, Anharmonicity or Coupling Constants, and Molar Extinction Coefficients
The results of Tables 2 and 3 indicate that the scaling factors for the n(OH) (0.940) and n(OD) (0.952) vibrations are lower than those for the other vibrational modes as mentioned above. This can be accounted for by the higher anharmonicity of the n(OH) and n(OD) vibrations. The harmonic n(OH) and n(OD) vibrations computed from Eq. [3] are 3704 and 2709 cm 21 , respectively, and the B3LYP/ 6-311G(d, p) frequencies are 3749 and 2737 cm 21 . The differences can be ascribed mainly to the fact that the experimental spectra have been recorded in carbon tetrachloride where the stretching frequencies are lower than in the gaseous state. For unsubstituted phenol, the difference between experiment and B3LYP/6-3111G(d, p) calculation of the n(OH) frequency, which amounts to 180 cm 21 if the experimental gas phase value is used, has also been mainly attributed to the large anharmonicity associated with the OH vibration (1). There is also a significant difference between the experimental (412 cm 21 ) and computed (445
315
cm 21 ) frequency for the g(OH) vibration (scale factor, 0.926). Owing to its weak intensity, the first overtone of this vibration could not be observed and consequently the anharmonicity correction could not be calculated. It is interesting to note here that the HF/6-31G(d, p) frequencies are approximately 10% too high, and it is generally recognized that half of this can be attributed to the lack of correlation at the Hartree–Fock level and the other half to the neglect of anharmonicities (33) (Table 6). Inspection of the results of Table 5 reveals that at the exception of the combination n(OH) 1g(OH) observed at 3938 cm 21 the highest coupling constants and the highest intensities are found for the binary combinations n(OH) 1 d(OH) at 4712 cm 21 and n(OH) 1 n(C–O) at 4796 cm 21 . It is worth noting that in free aliphatic alcohols, the coupling constant of the combination n 01 (OH) 1 d(OH) is about 5 cm 21 but increases to 30 – 60 cm 21 in the associated species (34). The intermediate value of 27 cm 21 found in this work probably results from the existence of the OH . . . Cl interaction which is weaker than the intermolecular OH . . . O hydrogen bonds in alcohols. Figure 6 shows the variation of the molar extinction coefficients as a function of the frequencies for the funda1 ) and triple mental transitions, the double (n 01 (OH) 1 n 01 1 ) or three excitations involving two different (n 02 (OH) 1 n 01 1 2 01 different (n (OH) 1 n 01 1 n 01) levels. For the ternary combinations, the highest coupling constants and highest intensities are found for the combination n 01 (OH) 1 g 02 (OH) at 4326 cm 21 , n 01 (OH) 1 g 01 (OH) 1 g R at 4513 and 4563 cm 21 , n 01 (OH) 1 d 02 (OH) at 5912 cm 21 , n 01 (OH) 1 n 01 (C–O) 1 d 01 (OH) at 6020 cm 21 , n 02 (OH) 1 g 01 (OH), n 02 (OH) 1 d 01 (OH) at 8060 cm 21 , and n 02 (OH) 1 n(C–O) at 8136 cm 21 . It can thus be concluded that the double or triple excitation of vibrations involving the stretching, in-plane, or out-of-plane vibrations of the OH bond and of the stretching vibration of the adjacent C–O bond are more effective than the excitation of other vibrational modes.
TABLE 6 Overtones and Combination Bands (3750 –7600 cm 21) Observed for PCP–OD in Carbon Tetrachloride Solution and Anharmonicity or Coupling Constants
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1 FIG. 6. e max (mol cm 22) as a function of the experimental frequencies (cm 21). h, n 01 (fundamental transitions); E, n 01(OH) 1 n 01 (double excitations); ‚, 1 2 1 n 01(OH) 1 n 01 1 n 01 (triple excitations); {, n 02(OH) 1 n 01 (triple excitations).
ACKNOWLEDGMENTS The authors thank the FWO (Fund for Scientific Research Flanders Belgium) for financial support. B.C.M. thanks the Flemish Community for a postdoctoral fellowship obtained in the frame of the bilateral cooperation between Flanders– Poland. A.K.C. thanks the University of Leuven for a postdoctoral fellowship.
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14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
34.
C. Y. Peng, P. Y. Ayala, W. Cheng, M. W. Wong, J. L. Anders, E. S. Repogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzales, and J. A. Pople, “GAUSSIAN 94,” Revision C3, Gaussian Inc., Pittsburgh, PA, 1995. T. Sakurai, Acta Crystallogr. 15, 1164 (1962). C. Dorval and T. Zeegers-Huyskens, Spectrochimica Acta A 29, 1805 (1973). A. W. Baker and A. T. Shulgin, Can. J. Chem. 43, 650 (1965). Z. Latajka and Y. Bouteiller, J. Chem. Phys. 101, 9793 (1994). G. Rauhut and P. Pulay, J. Phys. Chem. 99, 3093 (1995). V. Barone and C. Adamo, J. Phys. Chem. 99, 15092 (1995). J. C. Evans, Spectrochim. Acta 16, 1382 (1960). H. D. Bist, J. C. D. Brand, and D. R. Williams, J. Mol. Spectrosc. 24, 402 (1967). A. Foldes and C. Sandorfy, J. Mol. Spectrosc. 20, 262 (1966). A. Foldes and C. Sandorfy, Can. J. Chem. 48, 2197 (1970). A. Foldes and C. Sandorfy, Can. J. Chem. 49, 505 (1971). C. Sandorfy, in “The Hydrogen Bond, Part II” (P. Schuster, G. Zundel, and C. Sandorfy, Eds.), p. 619, North-Holland, New York, 1976. L. Goodman, A. G. Ozkabak, and S. N. Thakur J. Phys. Chem. 95, 9044 (1991). A. Burneau and J. Corset, J. Chim. Phys. 171 (1972). P. Botschwina, Chem. Phys. 40, 33 (1979). G. Durocher and C. Sandorfy, J. Mol. Spectrosc. 22, 347 (1967). N. Leroux, C. Samyn, and T. Zeegers-Huyskens, J. Mol. Struct. 448, 209 (1998). Z. Pawelka and T. Zeegers-Huyskens, Vib. Spectr. 18, 41 (1998). M. Rospenk and T. Zeegers-Huyskens, J. Phys. Chem. 101, 8428 (1997). W. B. Person, K. Szczepaniak, M. Szczesniak, and J. E. Del Bene, “Recent Experimental and Computational Advances in Molecular Spectroscopy” (R. Fausto, Ed.), p. 141, Kluwer Academic Publishers, Netherlands, 1993. M. Asselin and C. Sandorfy, Can. J. Chem. 49, 1539 (1971).
Copyright © 1999 by Academic Press
Theoretical and Experimental (400 –10 000 cm 21) Study of the Vibrational Spectrum of Pentachlorophenol Boguska Czarnik-Matusewicz,* Asit K. Chandra,† Minh Tho Nguyen,† and The´re`se Zeegers-Huyskens† *Faculty of Chemistry, University of Wroclaw, 14 Joliot-Curie Street, 50 383 Wroclaw, Poland; and †Department of Chemistry, University of Leuven, 200F Celestijnenlaan, B-3001, Heverlee, Belgium Received October 26, 1998; in revised form January 5, 1999 DEDICATED TO PROFESSOR C. SANDORFY
Geometric and vibrational spectroscopic data (bond distances and angles, vibrational frequencies, infrared intensities) of pentachlorophenol–OH (PCP–OH) and pentachlorophenol–OD (PCP–OD) are calculated by density functional theory (B3LYP) using the 6-311G(d, p) basis set. Except for the vibrations involving the OH bond, the agreement between the experimental and calculated fundamental frequencies between 3600 and 400 cm 21 is very good. The theoretical method failed, however, to reproduce quantitatively the experimental intensities. The infrared spectra between 3600 and 10 000 cm 21 are studied, and the overtones or combination bands are assigned by comparing the spectra of PCP–OH and PCP–OD. The difference between the experimental and theoretical frequencies of the n(OH) and n(OD) frequencies can be mainly accounted for by the neglect of the anharmonicities of these vibrations in calculations. The binary or ternary combinations characterized by the highest coupling constants and the highest intensities are those involving the n(OH), d(OH), g(OH), and n(C–O) vibrations. © 1999 Academic Press INTRODUCTION
In recent years, considerable efforts have been devoted to the understanding of the vibrational spectra of simple aromatic derivatives. It has been recently shown that the experimentally observed frequencies of phenol are reasonably well reproduced by quantum chemical calculations at moderate computational levels (1, 2). Phenol derivatives and more particularly pentachlorophenol (PCP) are very often used as model proton donors for the study of the thermodynamic and spectroscopic properties of hydrogen-bonded systems (3– 6). There are, however, in the literature only empirical assignments for this molecule (7, 8). In the present paper, we report calculated geometric and vibrational spectroscopic data of PCP, and the experimental parameters are compared with the calculated ones. To have a deeper insight into the nature of the vibrations, their anharmonicities, and their coupling, the experimental near-infrared spectra are also studied. It is worth mentioning that the overtone spectrum of phenol has been recently investigated (9) and that the results are in good agreement with those obtained from nonresonant ionization-detected infrared spectrum of jetcooled phenol (10). In phenol derivatives, the interpretation of the near-infrared spectra is often difficult due to the presence of numerous combinations involving the CH stretching or bending modes which are not present in PCP. 308 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
TABLE 1 B3LYP/6-311G(d, p) Optimized and Experimental Geometric Parameters of PCP
Note. Bond lengths in Å, angles in degrees. a From Ref. (14).
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COMPUTATIONAL METHODS
The geometry of the PCP molecule was fully optimized using the density functional theory (DFT) with the hybrid B3LYP (11, 12) exchange correlation functionals and the 6-311G(d, p) basis functions. The Gaussian package (13) was used for all the calculations. Harmonic vibrational frequencies were calculated at the optimized geometries using the same level of theory to characterize the stationary points. Anharmonic contributions whose evaluation requires the calculation of successive energy derivatives beyond the second order have been neglected.
EXPERIMENTAL
FIG. 1. Atom labeling in PCP.
The mid-infrared spectra (4000 – 400 cm 21) were recorded with the Bruker 66 spectrometer (resolution 2 cm 21, globar source, DTGS detector, KBr beamsplitter), and cells equipped
TABLE 2 Calculated B3LYP/6-311G(d, p) and Experimental Infrared Data for the Fundamental Vibrations of PCP–OH
a IR absorbance maxima normalized to A 5 100 for the strongest band; bn 5 stretching; d 5 in-plane deformation, g 5 out-of-plane deformation; R 5 ring vibration; cfrom ref. 7; donly one absorption was observed between 713 and 696 cm 21; en.o. 5 not observed; ffrom ref. 8.
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TABLE 3 Calculated B3LYP/6-311G(d, p) and Experimental Infrared Data for the Fundamental Vibrations of PCP–OD
c
a same remarks as below Table 2; bonly one absorption is observed between 712 and 678 cm 21; from ref. 7.
with KBr windows were used. The near-infrared spectra (10 000 – 4000 cm 21) were obtained using the same spectrometer (resolution 2 cm 21, tungsten source, cooled InSb detector, CaF 2 beamsplitter), and cells equipped with quartz-infrasil windows having a path length between 1 and 5 cm were used. The spectra resulted from 32 to 64 scans. All the spectra were taken at room temperature in carbon tetrachloride at a concentration of 0.09 mol liter 21. The absence of absorption at 3300 cm 21 indicates that at this concentration the self-association of PCP is negligible. PCP from Across Chemica was crystallized from a petroleum– ether mixture. Carbon tetrachloride from Janssen Chimica was dried on molecular sieves. PCP–OD was prepared by several exchanges with methanol–OD and about 90% deuteration was achieved.
RESULTS AND DISCUSSION
1. Optimized Geometry of PCP The results of B3LYP/6-311G(d, p) geometry optimization and the experimental bond distances and bond angles (14) are indicated in Table 1. The theoretical results show that PCP is planar in agreement with the X-ray diffraction data. The computed H . . . Cl2 distance of 2.347 Å indicates the presence of a weak intramolecular OH . . . Cl hydrogen bond in the isolated molecule. In the crystalline state, the molecules are held together by intermolecular bifurcated hydrogen bonds: the OH group of a first PCP molecule being bonded to both the O and Cl5 atoms of a second one. The experimental C5–C6 distance (1.438 Å) is longer than the other C–C distances (1.365–1.371 Å), and this might be due to an effect of inter-
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FIG. 2. FT-IR spectrum (3300 –2200 cm 21) of PCP–OH. C 5 0.09 mol liter, path length 5 1 cm.
molecular hydrogen bonding. In the isolated molecule, the C5–C6 distance is calculated to be shorter (1.397 Å). The experimental C–O distance (1.329 Å) is shorter than the computed one (1.344 Å), and this difference results probably from the same effect (Fig. 1). 2. Fundamental Vibrational Transitions in PCP The experimental spectra of PCP–OH and PCP–OD in the fundamental region (4000 –300 cm 21) have been previously discussed (4, 5) and therefore are not reproduced in this paper. In contrast, the infrared intensities have not been previously determined. The B3LYP/6-311G(d, p) and experimental vibrational frequencies and intensities of PCP–OH are listed in Table 2. Table 3 contains the same parameters for PCP–OD. Seven absorptions are predicted between 220 and 70 cm 21; these modes can be described as out-of-plane ring and C–Cl deformations and are not sensitive to O-deuteration. They will not be discussed further in this work. The n(OH) and n(OD) vibrations are 100% pure and this appears also from the computed value of the isotopic ratio n(OH)/n(OD), which is 1.370, near to the theoretical value of 1.374. This is not the case for the d(OH) and d(OD) vibrations characterized by an isotopic ratio of 1.226. The contribution of the d(OH) mode to the ring mode increases from 1573 to 1309 cm 21, the mode predicted at 1222 cm 21 having a predominant d(OH) character. The shifts of the ring vibrations between 1560 and 1300 cm 21 in PCP–OD provide a further support for this mixing. Such a
strong coupling explains why in molecular complexes between PCP and proton acceptors the d(OH) vibration seems to be rather insensitive to complex formation with proton acceptors. This mode is shifted to higher wavenumbers for other phenol complexes (15). The main contribution to the d(OD) vibration is computed at 997 cm 21, but the modes predicted at 972 and 679 cm 21 also contain an important d(OD) contribution. The main contribution to the g(OH) vibration is predicted at 445 cm 21. This vibration is also coupled with the out-of-plane ring modes at 354 and 310 cm 21. The same remark also holds for the g(OD) vibration whose main component is predicted at 311 cm 21, this vibration being also mixed with the out-of-plane ring modes at 360 and 342 cm 21. The n(OH) vibration of phenol is predicted at 3827 cm 21 and the g(OH) vibration at 365 cm 21 (1). This effect is attributed to the weak intramolecular OH . . . Cl interaction. In ortho-chlorophenol, where both the cis and trans conformers exist, the shift resulting from the formation of the intramolecular hydrogen bond is 58 cm 21; the enthalpy of the corresponding OH . . . Cl bond is about 6.5 kJ mol 21 (16). The scaling factor, defined as the ratio of the experimental to the calculated wavenumbers, computed for all the modes except the main contributors to the n(OH), d(OH), and g(OH) vibrations is 0.990 for PCP–OH and 0.988 for PCP–OD. As discussed in other works, B3LYP calculations with sufficiently large basis sets provide frequencies that even without any scaling are very close to the experimental data (17–19). The
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TABLE 4 Overtones and Combination Bands (3800 –2200 cm 1) for PCP–OH and PCP–OD in Carbon Tetrachloride Solution, Assignment of the Absorptions, and Anharmonicity or Coupling Constants
scaling factors are, however, lower for the vibrations involving the OH groups, and this will be discussed in the next paragraph. It must also be pointed out that the spectra have been measured experimentally in carbon tetrachloride and that no experimental data are available in the gas phase. For phenol, the n(OH) vibration turns out to be about 40 cm 21 higher in the gas phase than in carbon tetrachloride, but the ring vibrations differ by only 5 cm 21 from reach other (20, 21). This is likely to be the case for PCP–OH. The infrared intensities computed at this level of theory represent only a qualitative trend, and large differences between computed and experimental intensities are observed for the modes between 1300 and 1000 cm 21. 3. Overtones and Combination Bands The mechanical anharmonicities can be characterized by the anharmonicity constants X 12 , X 13 , . . . which can be computed by the second-order perturbation theory from the experimental frequencies of the fundamental (n 01), the first (n 02), and the second (n 03) overtone by the expressions X 12 5 n 01 2 n 02 / 2
[1]
X 13 5 n 02 / 2 2 n 03 /3.
[2]
The harmonic frequency ( n e ) is given by the expression
n e 5 n 01 1 2X 12.
[3]
If two anharmonicity constants are used but still within the frame of second-order perturbation theory, then X 123 5 n 01 2 n 02 / 2 2 4.5Y 123
[4]
Y 123 5 1/ 2~ n 02 2 n 01 2 n 03 /3!.
[5]
The value of Y 123 must be very much smaller than X 123 for the approximation to be valid. A large value of Y 123 implies that X 12 Þ X 23 (22, 23). The coupling constants (X coupl), even in the polyatomic case, are given by the equation X coupl 5 ~ n 101 1 n 201 ! 5 n comb
[6]
where the superscripts 1, 2, and comb refer to two different normal modes and the combination between them. (24, 25). Overtones and combination bands can also be observed in the mid-infrared region. The experimental spectrum of PCP–OH recorded in extended absorbance scale between 3600 and 2200 cm 21 is shown in Fig. 2. The experimental wave-
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VIBRATIONAL SPECTRUM OF PENTACHLOROPHENOL
FIG. 3.
313
FT-IR spectrum (5200 –3600 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm.
numbers, the assignment, and the anharmonicity or coupling constants (X) computed from Eqs. [1] and [6] are listed in Table 4. The assignment is made by comparison with PCP–OD where the absorptions at 2614, 2467, and 2252 cm 21 connected with the d(OH) vibration are not observed. The overtones or combinations of ring vibrations are characterized by about the same anharmonicities or coupling constants in both molecules. The anharmonicities of the ring modes between 1560 and 1530 cm 21, which can be described as the n8 ring modes of aromatic molecules, are characterized by an anharmonicity similar to that of benzene (between 4 and 5 cm 21) (26). The d(OH) vibration is characterized by an experimental anharmonicity of only 21 cm 21. This apparent anomaly can be explained by a strong coupling between the d(OH) and ring vibrations, as shown by our calculations. Very few results on the anharmonicity of bending vibrations of OH groups are available in the literature. The anharmonicity of the bending vibration of water is between 10 and 20 cm 21 (27, 28). Taking an average value of 15 cm 21, one can compute a more realistic value of about 1215 cm 21 for the uncoupled d(OH) vibration of PCP. This value will be further used for the estimation of the coupling constants between different modes. The experimental spectra of PCP–OH and PCP–OD between 3750 and 10 000 cm 21 are reproduced in Figs. 3–5.
The data for PCP–OH (wavenumbers of the overtones or combination bands observed between 3750 and 10 100 cm 21, assignment of the absorptions, their molar extinction coefficients, and the anharmonicity or coupling constants) are summarized in Table 5. The assignments of some combination tones may appear as somewhat speculative and are based mainly on the fact that the combinations involving the n 01(OH) or n 02(OH) levels are not observed in PCP–OD. The experimental anharmonicities X 12 of the n(OH) and n(OD) vibrations computed by Eq. [1] of PCP–OH and PCP–OD are 90 and 51 cm 21, respectively. The Y 123 values calculated by Eq. [5] are weak: 2.7 cm 21 for PCP–OH and 1.7 cm 21 for PCP–OD. It is worth mentioning that in gaseous phenol, the Y 123 values are strictly equal to 0 (10). The X 12 values are somewhat higher than in phenol–OH and phenol–OD (85 and 45 cm 21) (9) or other phenol derivatives (29). This result shows that the formation of the OH . . . Cl intramolecular hydrogen bond results in a weak increase (5 cm 21) of the anharmonicity of the n(OH) vibration. For stronger intermolecular hydrogen bonds formed between phenols and carbonyl or nitrogen bases, this increase is higher and depends on the enthalpy of complex formation (25, 30 –32). The difference probably results from the existence of the intramolecular hydrogen bond.
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FIG. 4.
FT-IR spectrum (6500 –5200 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm.
FIG. 5.
FT-IR spectrum (8400 –7000 cm 21) of PCP–OH (——) and PCP–OD (. . .). C 5 0.09 mol liter, path length 5 5 cm. Copyright © 1999 by Academic Press
VIBRATIONAL SPECTRUM OF PENTACHLOROPHENOL
TABLE 5 Near-Infrared Data (3750 –10 000 cm 21) for PCP in Carbon Tetrachloride Solution, Assignment of the Absorptions, Anharmonicity or Coupling Constants, and Molar Extinction Coefficients
The results of Tables 2 and 3 indicate that the scaling factors for the n(OH) (0.940) and n(OD) (0.952) vibrations are lower than those for the other vibrational modes as mentioned above. This can be accounted for by the higher anharmonicity of the n(OH) and n(OD) vibrations. The harmonic n(OH) and n(OD) vibrations computed from Eq. [3] are 3704 and 2709 cm 21 , respectively, and the B3LYP/ 6-311G(d, p) frequencies are 3749 and 2737 cm 21 . The differences can be ascribed mainly to the fact that the experimental spectra have been recorded in carbon tetrachloride where the stretching frequencies are lower than in the gaseous state. For unsubstituted phenol, the difference between experiment and B3LYP/6-3111G(d, p) calculation of the n(OH) frequency, which amounts to 180 cm 21 if the experimental gas phase value is used, has also been mainly attributed to the large anharmonicity associated with the OH vibration (1). There is also a significant difference between the experimental (412 cm 21 ) and computed (445
315
cm 21 ) frequency for the g(OH) vibration (scale factor, 0.926). Owing to its weak intensity, the first overtone of this vibration could not be observed and consequently the anharmonicity correction could not be calculated. It is interesting to note here that the HF/6-31G(d, p) frequencies are approximately 10% too high, and it is generally recognized that half of this can be attributed to the lack of correlation at the Hartree–Fock level and the other half to the neglect of anharmonicities (33) (Table 6). Inspection of the results of Table 5 reveals that at the exception of the combination n(OH) 1g(OH) observed at 3938 cm 21 the highest coupling constants and the highest intensities are found for the binary combinations n(OH) 1 d(OH) at 4712 cm 21 and n(OH) 1 n(C–O) at 4796 cm 21 . It is worth noting that in free aliphatic alcohols, the coupling constant of the combination n 01 (OH) 1 d(OH) is about 5 cm 21 but increases to 30 – 60 cm 21 in the associated species (34). The intermediate value of 27 cm 21 found in this work probably results from the existence of the OH . . . Cl interaction which is weaker than the intermolecular OH . . . O hydrogen bonds in alcohols. Figure 6 shows the variation of the molar extinction coefficients as a function of the frequencies for the funda1 ) and triple mental transitions, the double (n 01 (OH) 1 n 01 1 ) or three excitations involving two different (n 02 (OH) 1 n 01 1 2 01 different (n (OH) 1 n 01 1 n 01) levels. For the ternary combinations, the highest coupling constants and highest intensities are found for the combination n 01 (OH) 1 g 02 (OH) at 4326 cm 21 , n 01 (OH) 1 g 01 (OH) 1 g R at 4513 and 4563 cm 21 , n 01 (OH) 1 d 02 (OH) at 5912 cm 21 , n 01 (OH) 1 n 01 (C–O) 1 d 01 (OH) at 6020 cm 21 , n 02 (OH) 1 g 01 (OH), n 02 (OH) 1 d 01 (OH) at 8060 cm 21 , and n 02 (OH) 1 n(C–O) at 8136 cm 21 . It can thus be concluded that the double or triple excitation of vibrations involving the stretching, in-plane, or out-of-plane vibrations of the OH bond and of the stretching vibration of the adjacent C–O bond are more effective than the excitation of other vibrational modes.
TABLE 6 Overtones and Combination Bands (3750 –7600 cm 21) Observed for PCP–OD in Carbon Tetrachloride Solution and Anharmonicity or Coupling Constants
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1 FIG. 6. e max (mol cm 22) as a function of the experimental frequencies (cm 21). h, n 01 (fundamental transitions); E, n 01(OH) 1 n 01 (double excitations); ‚, 1 2 1 n 01(OH) 1 n 01 1 n 01 (triple excitations); {, n 02(OH) 1 n 01 (triple excitations).
ACKNOWLEDGMENTS The authors thank the FWO (Fund for Scientific Research Flanders Belgium) for financial support. B.C.M. thanks the Flemish Community for a postdoctoral fellowship obtained in the frame of the bilateral cooperation between Flanders– Poland. A.K.C. thanks the University of Leuven for a postdoctoral fellowship.
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