Conformationally induced transition moment rotations in the S1←S0 electronic spectra of n-propylbenzene and n-butylbenzene

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Abstract
Laser-induced fluorescence excitation spectra of the S 1 ←S 0 electronic transitions in conformers of n-propylbenzene and n-butylbenzene, stabilised in the cold environment of a helium free jet expansion, have been recorded at two different temperatures. More than one origin band is observed in each spectrum; two for n-propylbenzene and four for n-butylbenzene. Partially resolved rotational contours of these bands have been analysed with the aid of molecular orbital calculations conducted at the HF/6-31G* and CIS/6-31G* levels to provide structural assignments. The data show that different origin bands can be assigned to different conformers of the isolated molecule, distinguished by the orientation of the side-chain with respect to the ring. Anti-conformers display pure, b-type rotational band contours. Gauche-conformers display hybrid structures, the composition of which shows that there is an electronically induced rotation of the optical transition moment in the inertial frame, which is sensitive to the conformation of the non-polar substituent.