Journal of the American Chemical Society p. 1934 - 1942 (1987)
Update date:2022-08-17
Topics:
Kjaer, A. M.
Ulstrup, J.
We have investigated the shape of the solvatochromic absorption band for "Betaine-26" 2,4,6-triphenyl-N-(di-tert-butyl-4-hydroxyphenyl)pyridinium ion in a range of polar, apolar, protic, and aprotic solvents.Multiphonon band theory, including both molecular modes and a vibrationally dispersive solvent, indicates that the solvents fall in three categories: (1) The bandshape for polar, aprotic solvents is well reproduced by that for a structureless continuous dielectric and a single high-frequency molecular mode.Solvent broadening correlates with εo-1 - εs-1, εo being the optical and εs the static dielectric constant.The molecular frequency, ωc, and displacement, Δc, are not very solvent dependent, emphasizing their molecular character, and the value ωc ca. 1600 cm-1 suggests that C-O, C-N, and C-C stretching is involved. (2) Bands for apolar, aprotic solvents correspond to the same model, ωc and Δc are again not very solvent dependent and coincide with the values for polar aprotic solvents.The solvent broadening is solvent independent, and wider than that for a structureless dielectric.This points to multipolar, dispersive, pressure, or pseudopotential forces as coupling mechanisms. (3) The bandshape for normal alcohols can only be reproduced by a model resting on two molecular modes and a vibrational high-frequency solvent "tail".Broadening, asymmetry, molecular frequencies, and deuterium isotope effects trace the protic solvent spectral entanglement to coupling between betaine-26 and a local mode group with features of both O-H stretching and bending and librational solvent motion.
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