CONFORMATIONAL EQUILIBRIA FOR 3-METHYLSTYRENE - RAMAN AND FTIR SPECTRA AND AB-INITIO CALCULATIONS

The Raman spectra of liquid 3-methylstyrene show pairs of bands whose temperature-dependent intensities clearly suggest the occurrence of a conformational equilibrium. From these bands, assigned to cis and tran s conformers, a trans-cis energy difference of 0.8 +/- 0.2 kJ mol(-1) was obtained for the liquid phase. On the other hand, ab initio calcul ations at the 3-21G* and MP2/3-21G* levels predict a cis form more st able than the trans form by less than 0.15 kJ mol(-1), in good agreeme nt with gas-phase literature results.