Solvation Can Open the Photoisomerization Pathway for the Direct Photodissociation Reaction of Diiodomethane:  Transient Resonance Raman Observation of the Isodiiodomethane Photoproduct from Ultraviolet Excitation of Diiodomethane in the Solution Phase

Abstract

We report transient resonance Raman experiments that identify isodiiodomethane as the photoproduct responsible for the ∼385 nm absorption band observed following ultraviolet excitation of diiodomethane in liquid solutions. Comparison with previously reported gas-phase experiments and solution-phase resonance Raman and femtosecond transient absorption results suggest that solvation leads to appreciable production of the isodiiodomethane (H2C−I−I) photoproduct via the interaction of the initially formed CH2I and I fragments with the solvent cage. The isodiiodomethane photoproduct is likely the species or an intermediate to the species that reacts with alkenes in cyclopropanation reactions that use ultraviolet excitation of diiodomethane in liquids.

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

“…17,41 Our calculations (vide infra) also predict a very strong absorption band for iso-CH 2 Br-I. Picosecond time-resolved Raman measurements on CH 2 BrI 39 along with studies of related dihalomethane compounds 3,4,9,27,29,32,[66][67][68][69][70] confirm the assignment to iso-CH 2 Br-I.…”

“…We also note that Phillips and coworkers rule out contributions from CH 2 I in their transient Raman experiments performed in cyclohexane. 41,53,70 Thus, the absence of the ∼280 nm band in the transient spectrum makes assignment of the 360 nm absorption band in cyclohexane to CH 2 I unlikely.…”

Carbene Complexes of Nonmetals

“…[129] Nanosecond [130] and picosecond [131] transient resonance Raman investigations demonstrated that isodiiodomethane 83 is mostly responsible for the intense absorption band at about 385 nm. Both 83 and the CH 2 I 2 ϩ cation appear to contribute to the weak transient absorption band at approximately 570 nm.…”

Photochemistry of Iodoform in Methanol: Formation and Fate of the Iso‐CHI2‐I Photoproduct

“…[1][2][3] Diiodomethane (CH 2 I 2 ) has received much attention because of its intriguing condensed-phase UV photochemistry, namely the formation of a metastable iso-CH 2 I-I isomer product in a high quantum yield, as revealed by isolation in Ar and N 2 matrices [4,5] and femtosecond pumpprobe (fs-PP) spectroscopy of CH 2 I 2 in solution. [6,7] The nature of the isomer product species has been confirmed by nanosecond resonance Raman (ns-RR) spectroscopy, [8] picosecond time-resolved resonance Raman (ps-TR 3 ) spectroscopy, [9] and 100 ps time-resolved X-ray diffraction (tr-XD) [10] studies. Vibrationally hot iso-CH 2 I-I occurs within several picoseconds, and is attributed to recombination of nascent CH 2 IC and IC fragments following photodissociation of one of the two CÀI bonds in CH 2 I 2 .…”