UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

H (Rydberg) atom photofragment translational spectroscopy and ab initio electronic structure calculations are used to explore ways in which ring substituents affect the photofragmentation dynamics of gas phase pyrroles. S1 <- S-0 (sigma* <- pi) excitation in bare pyrrole is electric dipole forbidden but gains transition probability by vibronic mixing with higher electronic states. The S-1 state is dissociative with respect to N-H bond extension, and the resulting pyrrolyl radicals are formed in a limited number of (nontotally symmetric) vibrational levels (Cronin et al. Phys. Chem. Chem. Phys. 2004, 6, 5031-5041). Introducing sigma-perturbing groups (e.g., an ethyl group in the 2-position or methyl groups in the 2- and 4-positions) lowers the molecular symmetry (to C-s), renders the S-1-S-0 transition (weakly) allowed, and causes some reduction in N-H bond strength; the radical products are again formed in a select subset of the many possible vibrational levels but all involve in-plane (a') ring-breathing motions as expected (by Franck-Condon arguments) given the changes in equilibrium geometry upon sigma* <- pi excitation. The effects of pi-perturbers are explored computationally only. Relative to bare pyrrole, introducing an electron donating group like methoxy (at the 3- or, particularly, the 2-position) is calculated to cause a similar to 10% reduction in N-H bond strength, while CN substitution (in either position) is predicted to cause a substantial (similar to 3000 cm(-1)) increase in the S-1-S-0 energy separation but only a modest (similar to 2%) increase in N-H bond strength.