High-temperature vapor-phase Raman spectra and assignment of the low-frequency modes of trans-stilbene and 4-methoxy- trans-stilbene

Abstract

The vapor-phase Raman spectra of trans-stilbene at 330°C and 4-methoxy- trans-stilbene at 350°C have been recorded using a specially constructed high temperature cell. Together with the dispersed fluorescence spectra, these spectra have been used to assign the eight low-frequency vibrational modes in each molecule. For trans-stilbene, strong polarized bands at 273 and 152 cm −1 were ascribed to the totally symmetric ν 24 (C e-phenyl bend) and ν 25 (in-plane phenyl wag), respectively. A depolarized band at 207 cm −1 was assigned to the B g phenyl flap ( ν 47), and a polarized shoulder near 125 cm −1 is believed to result from both the B g phenyl torsion ( ν 48) and the overtone of the A u phenyl flap ( ν 36). For 4-methoxy- trans-stilbene the C e-phenyl bend and in-plane phenyl wag were shifted to 234 and 144 cm −1, respectively. These assignments were critical for understanding the fluorescence spectra associated with the torsional motions.