Consistent assignment of the vibrations of symmetric and asymmetric meta-disubstituted benzenesTools Kemp, David J., Tuttle, William D., Jones, Florence M.S., Gardner, Adrian M., Andrejeva, Anna, Wakefield, Jonathan C.A. and Wright, Timothy G. (2018) Consistent assignment of the vibrations of symmetric and asymmetric meta-disubstituted benzenes. Journal of Molecular Spectroscopy, 346 . pp. 46-59. ISSN 0022-2852 Full text not available from this repository.AbstractThe assignment of vibrational structure in spectra gives valuable insights into geometric and electronic structure changes upon electronic excitation or ionization; particularly when such information is available for families of molecules. We give a description of the phenyl-ring-localized vibrational modes of the ground (S0) electronic states of sets of meta-disubstituted benzene molecules including both symmetrically- and asymmetrically substituted cases. As in our earlier work on monosubstituted benzenes [A. M. Gardner and T. G. Wright. J. Chem. Phys. 135 (2011) 114305], para-disubstituted benzenes [A. Andrejeva, A. M. Gardner, W. D. Tuttle, and T. G. Wright, J. Molec. Spectrosc. 321 (2016) 28], and ortho-disubstituted benzenes [W. D. Tuttle, A. M. Gardner, A. Andrejeva, D. J. Kemp, J. C. A.Wakefield and T. G. Wright, J. Molec. Spectrosc. 344 (2018) 46], we conclude that the use of the commonly-used Wilson or Varsányi mode labels, which are based on the vibrational motions of benzene itself, is misleading and ambiguous. Instead, we label the phenyl-ring-localized modes consistently based upon the Mulliken (Herzberg) method for the modes of meta- difluorobenzene (mDFB) under Cs symmetry, since we wish the labelling scheme to cover both symmetrically- and asymmetrically-substituted molecules. By studying the vibrational wavenumbers obtained from the same force-field while varying the mass of the substituent, we are able to follow the evolving modes across a wide range of molecules and hence provide consistent assignments. We assign the vibrations of the following sets of molecules: the symmetric metadihalobenzenes, meta-xylene and resorcinol (meta-dihydroxybenzene); and the asymmetric meta-dihalobenzenes, meta-halotoluenes, meta-halophenols and meta-cresol. In the symmetrically-substituted species, we find two pairs of in-phase and out-of-phase carbon substituent stretches, and this motion persists in asymmetrically-substituted molecules for heavier substituents; however, when at least one of the substituents is light, then we find that these evolve into localized carbon-substituent stretches.
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