Consistent assignment of the vibrations of monosubstituted benzenes

Abstract

We investigate the consistency of the labeling and assignments of the vibrations of the monosubstituted benzenes in the electronic ground state. In doing so, we also identify some inconsistencies in the labeling of the benzene modes. We commence by investigating the behavior of the benzene vibrations as one hydrogen is replaced by an artificial atomic substituent of increasing mass via quantum chemical calculations; the wavenumber variations with mass give insight into the assignments. We also examine how well the monohalobenzene vibrations can be described in terms of the benzene ones: consistent with some recent studies, we conclude that this is futile in a significant number of cases. We then show that "isotopic wavenumbers" obtained by artificially changing the mass of the fluorine atom in fluorobenzene are in very good agreement with the wavenumbers obtained via explicit calculation for the relevant monohalobenzene (chlorobenzene, bromobenzene, and iodobenzene) vibrations. As a consequence, we propose that the vibrations of monofluorobenzene be used as the basis for labelling the vibrational assignments of monosubstituted benzenes. As well as the four monohalobenzenes, we also apply this approach to the vibrations of aniline, toluene, benzonitrile, phenylacetylene, phenylphosphine, and nitrobenzene. This has allowed a much more consistent picture of the vibrational assignments to be obtained across ten monosubstituted benzenes.

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…The few Tol-h 8 modes which are between 2900 and 3000 cm -1 are straightforwardly assigned to methyllocalized vibrations, with the corresponding Tol-d 3 ones also being consistent. The a 2 modes are all very well described by Wilson modes, and this seems to be generally true, 1 and their assignment seems clear for both Tol-h 8 and Tol-d 3 , given the consistency of agreement with previous work. With regard to the b 1 modes, these have mixed-Wilson character and so are best described by the M i labels given, notwithstanding a number of consistent assignments in previous work.…”

“…Additionally, it is common practice to assume that the CH 3 group is a point mass, and so to consider the molecule as having C 2v point group symmetry; this again causes uncertainty in how to label the modes under a Mulliken scheme, since the molecule is more properly considered as C s symmetry and different point groups lead to different numbering. Finally, as noted previously by us 1 and others, the vibrations of the monosubstituted benzenes are quite different from those of benzene in a significant number of cases, making it difficult or even impossible to assign a single Wilson label to the mode. Our labelling scheme overcomes all of these drawbacks, allowing the vibrational motion to be deduced from the label, and also allowing direct comparison of vibrational activity with other monosubstituted benzenes, since the same ring-localized motions have the same label.…”

“…Oddly, it is not possible to identify a corresponding feature in the Tol-d 3 spectrum; indeed, we find that the remainder of the bands in this wavenumber region (marked region c for Tol-h 8 and region d for Tol-d 3 ) are difficult to assign with any certainty for both Tol-h 8 and Tol-d 3 . It may be seen, by comparing with the high wavenumber region of the spectra in Figure 4 when exciting via the S 1 Fermi resonance features at ∼460 cm -1 (see above), that there are a number of features possible in this region; however, a definitive assignment was not possible, and we attribute this to the effects of anharmonicity, Fermi and Darling-Dennison resonances and vibronic interactions.…”

“…As we highlighted in Ref. 1, there have been a number of inconsistencies in applying the Wilson labels to some of the vibrational modes in benzene, which has been propagated into work on substi- 3213 3076 3213 3076 2 2 2 3199 3069 3199 3077 3 3 7 a 3190 3069 3190 3060 4 5 9 a 1672 1576 1672 1568 5 6 18a 1474 1431 1476 1444 6 8 13 1258 1227 1278 1253 7 9 8a 1218 1186 1222 1189 8 10 19a 988 984 981 967 9 11 12 1009 982 1004 983 10 12 1 776 762 749 740 11 13 6a 520 508 491 476 a 2 12 14 17a 1027 1010 1024 1006 13 15 10a 808 774 807 770 14 16 16a 351 348 351 351 b 1 15 20 5 1039 1049 1039 ...…”

“…1 This scheme treats the substituent as a point mass and allows the identification of the ring-localized vibrations by a label, M i , where the label indicates the vibration of fluorobenzene which most closely resembles the monosubstituted benzene vibration: this can be done "by eye," or via a Duschinsky matrix approachthe reader is referred to Ref. 1 for further details. (The substituent-localized vibrations are assumed not to interact with the ring vibrations to a first approximation.)…”