Internal rotation effects in the rotationally resolved S1(1Lb)←S0 origin bands of 3-methylindole and 5-methylindole

The rotationally resolved ultraviolet (UV) excitation spectra of the S-1(L-1(b))<--S-0 origin bands of 3-methylindole and 5-methylindole have been measured and analyzed. As a result of an internal rotation of the methyl group, each spectrum consists of rotational lines of overlapping 0a(1)<--0a(1) and 0e<--0e torsional transitions. Like indole, 3-methylindole and 5-methylindole undergo axis reorientation upon electronic excitation. The Hamiltonian used to describe all observed spectral features includes a pure rotational part, a pure torsional part, and terms describing the interaction between the internal rotation and the overall rotation. It also accounts for the axis reorientation effect. Values for the barrier heights of the methyl torsion, the angle of the methyl top axis with the inertial axes, and the rotational constants are obtained for both the S-0 and the S-1 state. From an analysis of the intensities of the rotational transitions, the direction of the transition moment and the axis reorientation angle are obtained. Due to quantum interference effects in the 5-methylindole spectrum the sign of these angles could be determined. (C) 1998 American Institute of Physics. [S0021-9606(98)02120-5]