Solvent effects on vibrational phase relaxation and frequency shifts in isobutylene

The Raman line shapes and frequencies of the ν4 asymmetric (C=CH2) stretching mode at 1657 cm−1 and the ν9 symmetric (C–CH3) stretching mode at 805 cm−1 (both of symmetry A1) have been measured in dilute solutions of isobutylene in several polar and nonpolar solvents. The frequency shifts in the different solutions can be interpreted in terms of repulsion, dispersion, induction, and dipolar forces between molecular bonds. The induction and dipolar forces play only a minor role. The blue shift due to the repulsion forces and the red shift due to the attractive dispersion forces are nearly equal for ν9, while the dispersion forces are much more important for ν4. Analogous behavior has already been described in our earlier work dealing with pure compressed isobutylene. The frequency shift and the linewidth are connected via the first and second spectral moments, and the order of magnitude of the width can be predicted from the frequency shifts. However, quantitative calculations are more difficult for linewidths than for shifts. Qualitatively, the linewidth behavior is consistent with the result for the vibrational frequencies concerning the relative importance of the repulsive and attractive energies.