Global sampling of the photochemical reaction paths of bromoform by ultrafast deep-UV through near-IR transient absorption and ab initio multiconfigurational calculations

Abstract

Ultrafast deep-ultraviolet through near infrared (210-950 nm) transient absorption spectroscopy complemented by ab initio multiconfigurational calculations offers a global description of the photochemical reaction pathways of bromoform following 255-nm excitation in methylcyclohexane and acetonitrile solutions. Photoexcitation of CHBr3 leads to the ground-state iso-CHBr3 product in a large quantum yield (∼35%), formed through two different mechanisms: concerted excited-state isomerization and cage-induced isomerization through the recombination of the nascent radical pair. These two processes take place on different time scales of tens of femtoseconds and several picoseconds, respectively. The novel ultrafast direct isomerization pathway proposed herein is consistent with the occurrence of a conical intersection between the first excited singlet state of CHBr3 and the ground electronic state of iso-CHBr3. Complete active space self-consistent field calculations characterize this singularity in the vicinity of a second order saddle point on the ground state which connects the two isomer forms. For cage-induced isomerization, both the formation of the nascent radical pair and its subsequent collapse into ground-state iso-CHBr3 are directly monitored through the deep-ultraviolet absorption signatures of the radical species. In both mechanisms, the optically active (i.e., those with largest Franck-Condon factors) C-Br-Br bending and Br-Br stretching modes of ground-state iso-CHBr3 have the largest projection on the reaction coordinate, enabling us to trace the structural changes accompanying vibrational relaxation of the non-equilibrated isomers through transient absorption dynamics. The iso-CHBr3 photoproduct is stable in methylcyclohexane, but undergoes either facile thermal isomerization to the parent CHBr3 structure through a cyclic transition state stabilized by the polar acetonitrile medium (∼300-ps lifetime), and hydrolysis in the presence of water.

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

“…This isomerization pathway could be termed 'roamingmediated', because it involves the migration of the incipient bromine fragment and leads to the iso product species; however, this could not be confirmed without comparison to the same reaction in the gas phase. The isomeric form occupies a shallow minimum on the potential energy surface (PES)-somewhat below the C-Br bond radical dissociation asymptote [12][13][14][15] -and has been proposed 16 to be accessible on the way towards molecular bromine from photoexcited bromoform in the gas phase. This isomerization pathway in the gas phase can only occur through the internal rearrangement of atoms, that is, directly and without the involvement of any other species.…”

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

“…4 to account for the experimental observations. Pal et al 25 recently argued for a direct route to iso-bromoform via a conical intersection from the S1 state of photoexcited CHBr3, but we have no direct (experimental or theoretical) evidence for such a pathway in CCl4 so have not included it in the scheme. with CCl3 either as a fast geminate recombination process, or after diffusion, which is suggested to take hundreds of nanoseconds 18,23,39 and is therefore unlikely to be important on the timescale of this work.…”

“…Isomeric (iso) forms of the parent compound have been extensively characterised in several photolysis studies of halomethanes in matrices and in liquids, 24,25,42,43 and shown to give rise to broad, strong visible absorption bands. A recent computational study of CHBr3 isomerization showed significant charge-separation character in iso-CHBr2-Br, which exhibits mixed covalent and ionic character, with (CHBr2) + Br -contributing the major (62%) resonance structures.…”

“…Our assignments are similar in spirit to the earlier literature, but more consistent with recent experimental and computational studies of other halogenated methanes such as bromoform (CHBr3). [24][25][26] Iso species of this type are important as reactive agents in solution:…”

Solvent Effects on Nonradiative Relaxation Dynamics of Low-Energy Ligand-Field Excited States: A CuCl42– Complex

“…23 The signal output of a TOPAS (LightConversion) optical parametric amplifier was used to generate 1300 nm excitation ("pump") pulses with the energy of 40 μJ pulse −1 . A white-light continuum in the 340−760 nm spectral range was used for probing.…”