Perturbations of the fully resolved electronic spectra of large molecules by the internal rotation of attached methyl groups. Influence of complex formation

Rotationally resolved S-1<--S-0 fluorescence excitation spectra of three me thylindoles and their single atom van der Waals complexes with argon have b een obtained. Each spectrum is extensively perturbed by the hindered intern al rotation of the methyl group. Analyses of these perturbations show that the barriers to such motion are substantially increased by complex formatio n. Barriers of this type are primarily electronic in origin. Thus, even at the relatively large distances (3.5 Angstrom) found in van der Waals comple xes, the attachment of a weakly bound argon atom has a significant effect o n the electron distribution in the indole ring.