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

• Published in: Nature chemistry Vol. 7; no. 7; pp. 562 - 568
• Main Authors: Mereshchenko, Andrey S., Butaeva, Evgeniia V., Borin, Veniamin A., Eyzips, Anna, Tarnovsky, Alexander N.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2015; Nature Publishing Group
• Subjects: 119/118; 140/125; 639/638/440/94; 639/638/440/949; 639/638/440/950; Analytical Chemistry; Biochemistry; Bromides; Chemistry; Chemistry/Food Science; Inorganic Chemistry; Organic Chemistry; Photochemicals; Photochemistry; Physical Chemistry; Potential energy
• ISSN: 1755-4330; 1755-4349; 1755-4349
• DOI: 10.1038/nchem.2278

‘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 CHBr 3 . We see that, upon S 0 –S 1 excitation, gas-phase CHBr 3 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 BBr 3 and PBr 3 in solution upon S 1 excitation. Quantum chemical simulations of XBr 3 (X = B, P or CH) suggest that photochemical reactivity in all three cases studied is governed by S 1 /S 0 conical intersections and can best be described as occurring through roaming-mediated pathways. Roaming — a new and unusual reaction mechanism in gas-phase chemical transformations — is now shown to occur in solution. Following ultraviolet excitation of geminal tribromides, what initially seems to be the simple fission of a bond is in fact isomerization occurring through the roaming of molecular fragments.