Internal rotation of methyl group in electronically excited o- and m-ethynyltoluene: new correlation between the Hammett substituent constant σm and rotational barrier change.

We have determined the potential-energy function for the internal rotation of the methyl group for o- and m-ethynyltoluene in the electronic excited (S(1)) and ground (S(0)) 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 S(0) and S(1) states are shown to be 19 ± 3 and 101 ± 1 cm(-1), respectively. A conformational change occurred with rotation by 60[ordinal indicator, masculine] 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), V(3) = 19 cm(-1), V(6) = -6 cm(-1), and V(9) = 0 cm(-1) in the S(0) state, and B = 5.015 cm(-1), D = 5.392 × 10(-5) cm(-1), V(3) = 101 cm(-1), V(6) = -22 cm(-1), and V(9) = -2 cm(-1) in the S(1) 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 σ(m) and the change in the barrier of the methyl group upon excitation.