Picosecond time-resolved resonance Raman observation of the iso-CH2I–I photoproduct from the “photoisomerization” reaction of diiodomethane in the solution phase

Abstract

We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman ͑267 nm pump and 400 nm probe excitation wavelengths͒ investigation of the initial formation and vibrational cooling of the iso-CH 2 I-I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH 2 I-I photoproduct species is predominantly responsible for the ϳ385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Similar results were obtained in both nonpolar solution ͑cyclohexane solvent͒ and polar solution ͑acetonitrile͒ solvent. The picosecond resonance Raman spectra confirm that the iso-CH 2 I-I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently undergoes vibrational cooling on the 4-50 ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transient absorption study. We discuss a possible qualitative scenario for the formation of the iso-CH 2 I-I species that is in agreement with the available gas phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH 2 I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase spectra of the iso-CH 2 I-I photoproduct as well as previously reported femtosecond transient absorption bands that are broad and weak in the 300-500 nm region over the 0.3-3 ps time scale.

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 .…”

“…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 . [6,7,9] Iodoform (CHI 3 ) is a closely related halomethane. The UV photochemistry of CHI 3 in the liquid phase has become a subject of debate in light of the recently published 100 ps tr-XD investigation.…”

Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction

“…As a consequence, it is estimated that 66 Ϯ 3.8% of initial HgI* 2 molecules decay into the ground state (in cage vibrational cooling) and release their energy to the solvent by means of collisions on the 10-ps time scale (29), whereas the remaining 34 Ϯ 3.8% dissociates into HgI ϩ I. In the 10 mM concentration of HgI 2 in methanol, we find that 33 Ϯ 3.1% of the molecules are excited by the laser pulse.…”

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 :…”