Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride

Abstract

Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and coworkers [J. Chem. Phys. 95, 3843 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH 2 C(O)Cl was prepared in its ground electronic state (S 0 ) and excited with a laser at 248 nm to its first excited singlet state (S 1 ). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S 1 and S 2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S 0 and S 1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

“…8,[27][28][29][30][31][32][33][34] The most common occurrence is the interaction between singlet and triplet states, as in the bimolecular O( 3 P, 1 D) + H 2 → OH( 2 Π) + H reaction, 37,38 and in photodissociation of systems such as HCl, 39,40 HBr, [41][42][43][44][45] CH 3 I, [46][47][48][49][50] ICN, [51][52][53][54] BrCH 2 Cl, [55][56][57][58] or BrCH 2 COCl. [59][60][61][62][63][64] In organic photochemical reactions, the spin-orbit interaction between a triplet state and states of singlet multiplicity promotes decay of the triplet state by phosphorescence and/or intersystem crossing. 5,6,34 In order to understand the character of the molecular adiabatic states in the presence of SOC, it is convenient to consider the classic Hund's cases of a diatomic molecule.…”

Model space diabatization for quantum photochemistry

“…In the original paper on applying the fourfold way with MC-QDPT, 19 we pointed out the dependence of MC-QDPT energies on orbital rotations, and we resolved the ambiguity by defining the adiabatic energies as those calculated using the DMOs rather than the canonical molecular orbitals (CMOs). In the original work and subsequent work 26,[43][44][45][46][47][48] we found, when we checked, only small differences between the two sets of adiabatic energies; however, for a current application to thioanisole, we found differences of up to 0.8 eV when using DMOs obtained by the fourfold way and up to 0.05 eV when using DMOs obtained by the threefold way. Therefore, we developed the scheme presented here to give a diabatic potential energy matrix that, when diagonalized, gives precisely the adiabatic energies of standard QDPT with CMOs.…”

“…29 It has been successfully applied to a variety of systems at both the CASSCF and MC-QDPT levels. 26,[43][44][45][46][47][48] However, as mentioned in the Introduction, for some cases, the original fourfold way at the MC-QDPT level may give results that are quite different from the standard MC-QDPT method. These problematic cases are the motivation for developing the present MSD strategy.…”

Competitive Bond Rupture in the Photodissociation of Bromoacetyl Chloride and 2‐ and 3‐Bromopropionyl Chloride: Adiabatic versus Diabatic Dissociation

“…[3] Several groups are involved in computational work to understand this competitive bond cleavage from theoretical aspects. [6,7,[9][10][11] By using the MRCI calculation level, Fang and coworkers selected several important stationary points along the excited-state dissociation pathways of bromoacetyl chloride. [6] However, the predicted fragment branching ratio of CÀBr/ CÀCl, which was calculated to be about 6 on the basis of statistical assumption, was overestimated relative to the observed data.…”

“…As a benchmark, selective bond fission in bromoacetyl chloride and 3-bromopropionyl chloride has attracted much attention over the past decades. [1][2][3][4][5][6][7][8][9][10][11] The branching ratio of CÀBr/CÀCl bond fission is the main theme of concern. Butler and co-workers have long studied such a process: Norrish type I a bond cleavage at 248 nm by a 1 [n(O)!p*(CO)] transition.…”