Royal Society Publishing

Rotational Analysis of the 2600 angstrom Absorption System of Benzene

J. H. Callomon, T. M. Dunn, I. M. Mills


The 2600 angstrom absorption system of benzene has been examined with very high resolving power, and the rotational fine structure partially analysed by comparison with computed contours. Vibronic bands involving degenerate e$_{2g}$ vibrations have contours characteristic of the vibrational angular momentum, and Coriolis coefficients $\zeta $ have been determined for the lowest e$_{2g}$ vibrations in ground and B$_{2u}$ excited states. Band contours thus provide an additional criterion for checking vibrational assignments. In particular, values of $\zeta $ determine the separation between band maximum and origin which can thus be calculated, and the vibrational analysis consequently refined in certainty and precision. Improved values of several fundamentals have been obtained, some new vibrational assignments made, and some previous ones rejected. Some important anharmonic constants have also been obtained. Vibronic relative intensities are briefly discussed. The rotational and vibrational evidence together make it certain that the equilibrium configurations of the carbon skeleton in benzene are exactly planar and hexagonal in both the ground and excited states, point group D$_{6h}$. The rotational constants then give an estimate of the increase in [Note: See the image of page 499 for this formatted text] C-C distance on excitation of +0.038 angstrom, in excellent agreement with estimates from other sources. The electronic origin of the system is revised: T$_{00}$ = 38086$\cdot $1 cm$^{-1}$.

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