Ultrafast infrared studies of chemical reaction dynamics in room-temperature liquids.

摘要

Femosecond infrared spectroscopy provides sufficient spectral and temporal resolution to support a detailed investigation of the early events of a photochemical reaction. Previously unreported transient species that arise as intermediates during the course of a reaction may have lifetimes that are too short for conventional characterization. For these species, quantum-mechanical (density functional theoretical and ab initio) electronic structure calculations provide invaluable insight into chemical properties including molecular structure and energetics. With the combination of experimental and theoretical results, it is possible to assemble a comprehensive picture of the reaction dynamics of a system that is intricately influenced by the surrounding solvent molecules.;The mechanisms of several important organometallic reactions, such as alkane C-H bond activation by eta3-Tp*Rh(CO), silane Si-H bond activation by eta5-CpMn(CO)2 and eta 5-CpRe(CO)2, as well as chlorinated methane C-Cl bond cleavage by the Re(CO)5 radical are elucidated. The results demonstrate the importance of molecular morphology change (C-H and Si-H activation), solvent rearrangement (Si-H activation), intersystem crossing (Si-H activation), and solvent caging (C-Cl cleavage) in understanding the reactivity of the organometallic species. The nature of the apparent free-energy barrier for C-H, Si-H, and C-Cl bond activation reaction is found to be cleavage of an alkane C-H bond, rearrangement of a silane molecule HSiR3 (R = alkyl group) from a nonreactive alkyl site to the reactive Si-H bond, and CI atom transfer from a chlorinated methane molecule to Re(CO)5, respectively. These results support previous ab initio calculations for C-H and Si-H bond activation reaction profiles which suggest that cleavage of an alkane C-H bond by a transition metal center, unlike that of a silane Si-H bond, involves a precursor complex. The results of C-CI bond activation by a transition metal radical call for a re-examination of existing models that invoke an intermediate with either nineteen valence electrons at the metal center or charge-transfer character.