Laser-Induced Fluorescence Spectra and Torsional Potential Energy Functions of Jet-Cooled 4,4'-Dimethyl-trans-Stilbene

摘要

The laser-induced fluorescence excitation spectra of jet-cooled 4,4'-dimethyl-trans-stilbene have been recorded, and the phenyl and methyl internal rotation levels have been assigned for the S_0 and the S_1(π,π~*) electronic states. Dispersed fluorescence spectra and vapor-phase Raman spectra were utilized to help assign the low-frequency vibrations in the electronic ground state. The methyl torsions have energy levels and potential functions very similar to those of m-xylene. The S_0 state has V_6 = 15 ± 15 cm~(-1) (negligible barrier), while S_1 has V_3 = 85 ± 10 cm~(-1) and V_6 = -30 ± 20 cm~(-1). The barrier to simultaneous internal rotation of the phenyl groups in S_0 was found to be 3100 cm~(-1). This is identical to that of trans-stilbene within experimental error. For the S_1 state, the barrier is 3140 cm~(-1) as compared to 3000 cm~(-1) for trans-stilbene. The barrier to rotation about the C=C bond in the S_0 state was estimated to be 41 ± 5 kcal/mol based on one assigned frequency. In the S_1 state the trans-twist barrier was estimated to be approximately 2400 cm~(-1).