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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 ← S0 (σ* ← π) excitation in bare pyrrole is electric dipole forbidden but gains transition probability by vibronic mixing with higher electronic states. The S1 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 σ-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 Cs), renders the S1–S0 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 expe...

Authors:   Tolga N. V. Karsili; Barbara Marchetti; Roberta Moca; Michael N. R. Ashfold
Journal:   Journal of Physical Chemistry A
Year:   2013
DOI:   10.1021/jp404580v
Publication date:   15-Jul-2013
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