Probing the Photoisomerization of CHBr₃ and CHI₃ in Solution with Transient Vibrational and Electronic Spectroscopy

Abstract

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C-H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C-H stretching transitions of these isomers, in which the emergent halogen atom returns to form a C-X-X bonding motif, appear about 9 ps after photolysis for iso-CHBr2-Br and in about 46 ps for iso-CHI2-I. These time scales are consistent with, but differ from, the time evolution of the transient electronic absorption spectra of the same samples, highlighting the subtle differences between monitoring the vibrational and electronic chromophores. The specificity of using vibrational transitions to track condensed-phase reaction dynamics permits reassessment of the transient electronic spectrum of photolysis in neat CHBr3, which has an additional prompt feature near 400 nm. Calculations show that this feature, which arises from a precursor to the isomer, is a charge-transfer transition of a contact pair between the nascent Br fragment and a nearby CHBr3 molecule. Dilution and solvent studies show that transition is independent of the solvent. The iso-CHBr2-Br transition wavelength, however, shifts over the range of 400 to 510 nm depending on the solvent. Time-dependent density functional calculations faithfully reproduce these trends.

Non-equilibrium reaction and relaxation dynamics in a strongly interacting explicit solvent: F + CD3CN treated with a parallel multi-state EVB model

“…6,[34][35][36][37][38] The last decade or so has seen a number of attempts by a range of workers to efficiently and accurately simulate chemical reaction dynamics in condensed-phase environments using multistate molecular mechanics and multi-state empirical valence bond (MS-EVB) approaches. 19,[39][40][41][42][43][44][45][46][47][48][49] Such approaches represent important molecular configurations using diabatic valence bond states, which are coupled using any of a wide range of schemes.…”

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

“…Our assignments are similar in spirit to the earlier literature, but more consistent with recent experimental and computational studies of other halogenated methanes such as bromoform (CHBr3). [24][25][26] Iso species of this type are important as reactive agents in solution:…”

Dynamics of Bimolecular Reactions in Solution

“…Some solvents are photochemically active at UV wavelengths typically chosen for radical formation and produce complexes or isomers that contribute transient absorption bands in the UV and visible spectral regions. For example, several halogenated solvents are known to release a halogen atom photochemically and to undergo geminate recombination to either the parent molecule or a higher-energy isomeric form (30)(31)(32)(33)(34)(35)(36), as illustrated here by CCl 4 :…”

“…Recent studies of Cl atom reactions therefore used two-photon excitation at 267 nm and dissociation of a chlorinated solvent (e.g., CCl 4 , CHCl 3 , or CH 2 Cl 2 ) as a convenient, if less efficient, source of Cl atoms (26,27,51). One-photon UV photolysis of bromoalkanes is a similar route to Br atom production (32,36), although Br 2 photodissociation might prove more effective. Section 3.3 reviews recent developments in the study of F atom reactions in solution.…”

“…Complexes may range from weakly associating species, confined in close proximity by a solvent cage, to more strongly associated dimers that can diffuse as a pair through the solvent, albeit with the possibility of a rapid exchange of the solvent molecule partner. Complexes of these types have been implicated in the chemistry of Cl atoms in chlorinated solvents (26,27,(39)(40)(41), Br atoms in bromoform (30,32,36), and CN radicals in dichloromethane (42,43,54,55). A bridged complex of a CN radical with CH 2 Cl 2 , in which the radical coordinates to both an H and a Cl atom of a solvent molecule, was calculated to be bound by 1,540 cm −1 (42).…”