UV photodissociation of pyrroles: symmetry and substituent effects
| Autor: | Michael N. R. Ashfold, Barbara Marchetti, Roberta Moca, Tolga N. V. Karsili |
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| Rok vydání: | 2013 |
| Předmět: |
DYNAMICS
Radical ATOM ELIMINATION Substituent Ab initio Electronic structure ADENINE Photochemistry Molecular physics symbols.namesake chemistry.chemical_compound MOLECULES Molecular symmetry Physics::Atomic and Molecular Clusters Ethyl group Physics::Atomic Physics Physical and Theoretical Chemistry Physics::Chemical Physics Photodissociation CONICAL INTERSECTIONS chemistry STATES Rydberg formula symbols NEAR-ULTRAVIOLET PHOTOLYSIS PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY PHENOLS |
| Zdroj: | Karsili, T N V, Marchetti, B, Moca, R & Ashfold, M N R 2013, ' UV Photodissociation of Pyrroles: Symmetry and Substituent Effects ', Journal of Physical Chemistry A, vol. 117, no. 46, pp. 12067-12074 . https://doi.org/10.1021/jp404580v |
| ISSN: | 1520-5215 |
| DOI: | 10.1021/jp404580v |
| Popis: | 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 C(s)), 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 expected (by Franck-Condon arguments) given the changes in equilibrium geometry upon σ* ← π excitation. The effects of π-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 ∼10% reduction in N-H bond strength, while CN substitution (in either position) is predicted to cause a substantial (∼3000 cm(-1)) increase in the S1-S0 energy separation but only a modest (∼2%) increase in N-H bond strength. |
| Databáze: | OpenAIRE |
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