Roaming decomposition mechanism recently discovered for molecules in the gas phase is an active area of investigation. Here, ultrafast transient absorption spectroscopy reports on direct, excited-state isomerization of BBr3 and PBr3 in methylcyclohexane and dichloromethane. This isomerization occurs within the first 100 fs after excitation of these molecules into their lowest excited singlet S1-states and can best be described via roaming radical dynamics in solution. Multiconfigurational and multireference quantum chemicals calculations suggest that the isomerization is mediated by a conical intersection between the S1-state of XBr3 and the ground S0-state of iso-XBr3 (X = B, P). Following the initial relaxation from the Franck-Condon point, roaming dynamics starts on a more planar region of the S1 surface, and in the vicinity of the conical intersection the XBr2 and Br fragments become separated to a distance larger 3 �. After passage through the conical intersection, the partially dissociated bromine atom �wanders' around the XBr2 fragment, slipping off-center of the XBr2 bisector plane, and ultimately forming the Br�Br bond with bond length significantly exceeding the equilibrium one in the iso-XBr3 isomer product.

Ultrafast photoionization of boron and phosphorus tribromides in solution as a general property of nonmetal bromides

248th National Meeting (2014)

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