Structure and internal rotation in the S0 and S1 states of o-toluidine studied by high resolution UV spectroscopy

The rotationally resolved spectrum of the o-toluidine S-1 <- S-0 origin was measured using laser induced fluorescence spectroscopy. From the resulting spectrum torsional barriers to internal rotation of the methyl group were derived, which resulted in S-0 state values of V-3 = 699 +/- 11 cm(-1) and V-6 = 64 +/- 11 cm(-1) with an effective rotational constant F of 5.38 +/- 0.04 cm(-1) while for the S-1 state the result was V-3 = 40.87 +/- 0.14 cm(-1) and V-6 = -16.8 +/- 0.8 cm(-1) with F = 5.086 +/- 0.001 cm(-1). The S-1 state structure was found to be severely distorted, with the methyl group making a 7.7 degrees degree angle with the benzene ring. Evidence of an excited state precessional motion of the methyl group was found.