Structure and barrier to internal rotation of 4-methylstyrene in the S0- and S1-state

The rotationally resolved spectrum of the electronic origin of methylstyrene has been studied in order to obtain the geometric structure and the barriers to methyl torsion in the electronic ground and excited state. The spectrum has been analyzed using an automated assignment based on the Genetic Algorithm. The geometry is found to be quinoidally distorted upon electronic excitation, what is typical for para-disubstituted aromatic compounds. The V3 and V6 barriers and the torsional constant F for both electronic states have been determined from a combined fit of low resolution torsional bands and high resolution torsionally perturbed rovibronic spectrum.