Femtochemistry of trans-azomethane: a combined experimental and theoretical study.

Abstract

The dissociation dynamics of trans-azomethane upon excitation to the S,(n, pi) state with a total energy of 93 kcalmol-1 is investigated using femtosecond-resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump-probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one-photon symmetry-forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two-photon symmetry-allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C-N bonds of the molecule break in a stepwise manner, that is, the first C-N bond breaks in approximately 70 fs followed by the second one in approximately 100 fs, with the intermediate characterized. At the parallel polarization, the first C-N bond cleavage was found to occur in 100 fs with the intensity of the symmetry-allowed transition being one order of magnitude greater than the intensity of the symmetry-forbidden transition at the perpendicular polarization. Theoretical calculations using time-dependent density functional theory (TDDFT) and the complete active space self-consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low-lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism. The initial driving force of the ultrafast bond-breaking process of trans-azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity-borrowing mechanism for the symmetry-forbidden n-pi transition. Following this torsional motion and the associated molecular symmetry breaking, an S0/S1 conical intersection (CI) can be reached at a torsional angle of 93.1 degrees (predicted at the CASSCF(8.7)/cc-pVDZ level of theory). Funneling through the S0/S1, CI could activate the asymmetric C-N stretching motion, which is the key motion for the consecutive C-N bond breakages on the femtosecond time scale.