| Abstrakt: |
We have determined the potential-energy function for the internal rotation of the methyl group for o- and m-ethynyltoluene in the electronic excited (S1) and ground (S0) states by measuring the fluorescence excitation and single-vibronic-level dispersed fluorescence spectra in a jet. The 0-0 bands were observed at 35 444 and 35 416 cm-1, respectively. The methyl group in o-ethynyltoluene is shown to be a rigid rotor with a potential barrier to rotation of 190 ± 10 cm-1 in both states. No change in the conformation occurred upon excitation. Barrier heights of m-ethynyltoluene in the S0 and S1 states are shown to be 19 ± 3 and 101 ± 1 cm-1, respectively. A conformational change occurred with rotation by 60º upon excitation. The potential parameters were as follows: reduced rotational constant (B) of 5.323 cm-1, centrifugal-distortion constant (D) of 6.481 × 10-5 cm-1, V3 = 19 cm-1, V6 = -6 cm-1, and V9 = 0 cm-1 in the S0 state, and B = 5.015 cm-1, D = 5.392 × 10-5 cm-1, V3 = 101 cm-1, V6 = -22 cm-1, and V9 = -2 cm-1 in the S1 state. For m-methylstyrene, m-tolunitrile, and m-ethynyltoluene, which all have a multiple-bonded carbon in the substituent, we found a new correlation between the Hammett substituent constant σm and the change in the barrier of the methyl group upon excitation. [ABSTRACT FROM AUTHOR] |
