Vibrational relaxation in jet-cooled alkylbenzenes. I. Absorption spectra

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Fluorescence excitation spectra of a series of alkylbenzenes cooled in a supersonic free jet have been obtained for the first 1000 cm-1 region of the 1B2(ππ*)←1A 1 ultraviolet absorption spectrum. The series includes all n-alkylbenzenes up to n-hexyl together with isopropyl- and ten -butylbenzene. As with toluene, the spectra in this region for all alkylbenzenes is found to be dominated by vibrations of five ring modes: 6a, 6b, 1, 12, and 18a. Three of these-the "system modes" (6b, 12, 18a)-are found to be largely invariant to changes in the alkyl chain length and type-the "bath." For n-alkylbenzenes with chain length of three or higher, spectra of two distinct conformations are observed with roughly equal intensity. These conformations are distinguished by configuration about the 1-2 carbon-carbon bond of the alkyl chain. When this configuration is trans [n-(t)-alkylbenzene], the alkyl chain extends away from the phenyl ring leaving the ring free to van der Waals complex binding on both sides of the ring. The He2-n-(f)- alkylbenzene van der Waals complex is thus observed. When the conformation about the 1-2 bond is gauche [n-(g)-alkylbenzene] the alkyl chain partially covers one side of the phenyl ring. This produces a small red-shift of the spectrum (a self-induced solvent shift) and prohibits the formation of the ring-centered dihelium van der Waals complex. © 1980 American Institute of Physics.

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The Journal of Chemical Physics

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