Photoisomerization reaction of CH2BrI following A-band and B-band photoexcitation in the solution phase: Transient resonance Raman observation of the iso-CH2I–Br photoproduct

Abstract

We present nanosecond transient resonance Raman experiments that investigate the photoproduct species formed following A-band and B-band excitation of bromoiodomethane in room temperature cyclohexane solutions. Density functional theory calculations were also performed for several species that have been proposed as photoproducts for photodissociation of bromoiodomethane in the condensed phase. Comparison of the experimental resonance Raman spectra to density functional theory computational results and results for the closely related iso-CH 2 I-I and iso-CH 2 Br-Br species demonstrated that the iso-CH 2 I-Br species is mainly responsible for a transient absorption spectrum that appears after either A-band or B-band photoexcitation of bromoiodomethane in cyclohexane solution. This is in contrast to previous results for low temperature ͑12 K͒ solids where mainly the iso-CH 2 Br-I species was observed following A-band photoexcitation of bromoiodomethane. Further density functional theory computational results indicate that the iso-CH 2 I-Br species is noticeably more stable than the iso-CH 2 Br-I species by about 4.1 kcal/mol. This suggests that although both iso-CH 2 I-Br and iso-CH 2 Br-I species may be initially produced following ultraviolet excitation of bromoiodomethane in cyclohexane solution, only the more stable isomer has a sufficiently long lifetime to be observed in our nanosecond timescale transient resonance Raman experiments. We compare results for the bromoiodomethane ultraviolet photodissociation/photoisomerization reactions in the condensed phase to those of the closely related diiodomethane system and discuss a probable mechanism for the formation of the iso-bromoiodomethane species in the condensed phase.

Communications: Photoinitiated bond dissociation of bromoiodomethane in solution: Comparison of one-photon and two-photon excitations and the formation of iso-CH2Br–I and iso-CH2I–Br

“…4,5 Excitation at 193 nm favors C-Br cleavage ͑branching ratio of 3.5:1 C-Br:C-I͒. 5 The same trends are observed in solution phase experiments including steady state 6 and timeresolved resonance Raman [7][8][9] and femtosecond transient absorption at 266 nm. 10 Recent theoretical calculations provide additional insight into the dissociation of CH 2 BrI.…”

“…The iso-CH 2 I -Br species has been identified in nanosecond transient resonance Raman experiments where CH 2 BrI in cyclohexane was pumped at 217.8 or 239.5 nm and probed at 341.5 nm. 7 No significant transient resonance enhanced Raman signals were observed when the sample was probed at 398.0 or 416.0 nm. Of particular relevance to the assignment of the 370 nm absorption in Figs.…”

“…The complex is predicted to be more stable than iso-CH 2 Br-I by ϳ2.7 kcal/ mol in the most recent calculations 12 and ϳ4.1 kcal/ mol in earlier calculations. 7,9 Iso-CH 2 Br-I has been identified in nanosecond time-resolved resonance Raman measurements ͑vide infra͒. 7,9 In the work reported here we compare one-photon ͑ϳ266 nm͒ and two-photon ͑ϳ400 nm͒ excitations of bro- moiodomethane in 2-butanol.…”

“…7,9 Iso-CH 2 Br-I has been identified in nanosecond time-resolved resonance Raman measurements ͑vide infra͒. 7,9 In the work reported here we compare one-photon ͑ϳ266 nm͒ and two-photon ͑ϳ400 nm͒ excitations of bro- moiodomethane in 2-butanol. Pump-probe transient absorption measurements were performed using three different femtosecond Ti:sapphire laser systems with central wavelengths of 790-792 nm ͑ϳ60 fs, 700 J or 110 fs, 550 J͒ or 810 nm ͑ϳ60 fs, 400 J / pulse͒.…”

“…2 and 3, iso-CH 2 I -Br, not iso-CH 2 Br-I, was observed in the nanosecond resonance Raman experiment following excitation at 252.7 nm despite the fact that 252.7 nm excitation results in primary C-I bond cleavage. 7 The less stable iso-CH 2 Br-I photoproduct decays on a time scale faster than 10 ns and forms the more stable iso-CH 2 I -Br with a non-negligible yield. These measurements in cyclohexane are consistent with our observations in 2-butanol and support the assignment of the 370 nm absorption band observed following twophoton excitation at 395 nm or 6 ns after one-photon excitation at 271 nm to the iso-CH 2 I -Br species.…”