Knowledge Bank

The rotation-torsion contour of the 2668{\AA} origin band of toluene has been successfully calculated within a full asymmetric top-free internal rotation model. The results of the calculations show that both the $295\circ K$ high resolution absorption spectrum and the low-temperature (-3K) molecular beam florescence excitation spectrum (1) may be calculated with the changes in the rotational constants:\ $\Delta AF=-0.0071cm-1$\ $\Delta B=-0.00074cm-1$\ $\Delta --0.001030cm-1$\ $\Delta AT=+0.030cm-1$\ The high-resolution $295\circ K$ spectrum refinement demonstrates that the geometry of the methyl group of toluene is not substantially changed between the ground $(1A1\prime )$ and first excited $\pi \pi \ast$ singlet state $(1A1)$. Changes in the phenyl frame are typical of those found for other monosubstituted benzenes with substituents containing no lone pair of electrons. Extensions to similar molecules is in progress and will be briefly discussed.