Rotational band contours of the first electronic transition origin bands of p-xylyl and p-fluorobenzyl

Abstract

The origin bands of the gas phase discharge emission spectra of the p-xylyl and p-fluorobenzyl radicals were photographed at high resolution. The observed band contours were fitted by computer-simulated type B rotational bands; this indicates that the transition moment in both cases is along the b in-plane short axis and corresponds to a 1 2 A 2-1 2 B 2 first electronic transition, as found previously in benzyl itself. For reasonable geometrical models of the ground state (methyl-group reduced to a mass-point in p-xylyl), A″, B″ and C″ rotational constants and their variations ΔA, ΔB and ΔC during the transition are determined. The band origins are found to be at 21700·5 ± 0·2 and 21524·0 ± 0·2 cm-1 respectively in p-xylyl and p-fluorobenzyl, with observed band maxima which are at 4606·57 Å or 21702·05 cm-1 and 4644·14 Å or 21526·48 cm-1, respectively. Four band sequences are identified in both spectra and assignments of the vibrational modes involved are discussed. In p-xylyl are also shown and discussed the observed band contours of three vibronic bands at -457·0, -636·7 and -829·9 cm-1 from the origin 00 0 band. Assignment of these bands as, respectively, 6a 0 1, 6b 0 1 and 10 1, is discussed. In p-fluorobenzyl, 6a 0 1 and 10 1 are observed at -453 and -835 cm-1, at lower resolution. The application of the present results to the study of low-temperature rotational band contours in supercooled spectra is also considered.