The rotation-torsion contour of the 2668 Å origin band of toluene. High-resolution 295 K absorption spectrum

The 2668 Å vibrational origin band of toluene was measured and analyzed using an asymmetric-top/free-internal-rotor model. The change in the methyl group geometry between the ground (^eA′₁) and the excited ^eA″₁ ($\text{ππ}{}^1$) state was found to be slight. Rough values found for the changes in the phenyl frame rotational constants are similar to those for other monosubstituted benzenes with substituents lacking lone-pair electrons. The torsional structure in the toluene origin band is blue-degraded whereas the rotational structure in each Δm = 0 torsional manifold of transitions is red-degraded. Deconvolution of the torsional structure in the calculated contour shows that strong R-branch heads for transitions emanating from each torsional level of the ground state persist in the experimental contour for 0 ≤ |m| ≤ 12.