Roaming-mediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases

Abstract

'Roaming' is a new and unusual class of reaction mechanism that has recently been discovered in unimolecular dissociation reactions of isolated molecules in the gas phase. It is characterized by frustrated bond cleavage, after which the two incipient fragments 'roam' on a flat region of the potential energy surface before reacting with one another. Here, we provide evidence that supports roaming in the liquid phase. We are now able to explain previous solution-phase experiments by comparing them with new ultrafast transient absorption data showing the photoisomerization of gas-phase CHBr3. We see that, upon S0-S1 excitation, gas-phase CHBr3 isomerizes within 100 fs into the BrHCBr-Br species, which is identical to what has been observed in solution. Similar sub-100 fs isomerization is now also observed for BBr3 and PBr3 in solution upon S1 excitation. Quantum chemical simulations of XBr3 (X = B, P or CH) suggest that photochemical reactivity in all three cases studied is governed by S1/S0 conical intersections and can best be described as occurring through roaming-mediated pathways.

Tracking dissociation dynamics of strong-field ionized 1,2-dibromoethane with femtosecond XUV transient absorption spectroscopy

“…5b-d a DI channel originating from DBE + , and a Coulomb explosion (CE) channel originating from DBE ++ (or higher charged) cations. The experimental appearance time of 70 fs for Br + production is consistent with direct dissociation upon a repulsive electronically excited state (DI); 36,37 conversely, it is faster than might be anticipated for statistical dissociation. Using a simple pseudodiatomic classical model for Coulomb explosion of point charges, we can estimate a CE fragmentation time scale of 45 fs, taking 5 Å as the bond dissociation length (chosen because in calculations on CH 3 Br, the potential energy surface on the C-Br stretch coordinate is flat at this distance 43 ).…”

Substituent effects on H3+ formation via H2 roaming mechanisms from organic molecules under strong-field photodissociation

“…This results in unexpected products with unexpected product state distributions. While roaming is widely studied in unimolecular reactions in the gas phase, roaming mechanisms in bimolecular reactions [8][9][10] as well as in the liquid phase reactions 11 are now gaining considerable attention. However, the better known roaming reactions involve decomposition or isomerization of excited state neutral molecules.…”

Probing ultrafast C–Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy

“…An alternative mechanism has been proposed, whereby ultrafast photoisomerization proceeds through a conical intersection that connects the lowest-lying singlet excited state of the parent molecule to the ground electronic state, forming the isomer en-route to dissociation 42 . Subsequent ultraviolet-visible (UV-vis) transient absorption studies of gas-phase CHBr 3 following 250 nm photolysis detected an absorption feature that forms and decays on timescales of 50 fs and 85 fs, respectively 43 . It was hypothesized that the intermediate absorption corresponds to isomer formation 43 .…”

“…Subsequent ultraviolet-visible (UV-vis) transient absorption studies of gas-phase CHBr 3 following 250 nm photolysis detected an absorption feature that forms and decays on timescales of 50 fs and 85 fs, respectively 43 . It was hypothesized that the intermediate absorption corresponds to isomer formation 43 . The proposed isomerization mechanism challenges the conventional understanding of haloalkane photochemistry, where a one-dimensional model of C–X bond extension is sufficient to capture the essential details of the dissociation rather than the full 3 N -6 degrees of freedom that are, in principle, involved 7 …”