Distance dependence of nonadiabaticity in the branching between C–Br and C–Cl bond fission following 1[n(O),π*(C=O)] excitation in bromopropionyl chloride

Abstract

These experiments on bromopropionyl chloride investigate a system in which the barrier to C-Br fission on the lowest lA" potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C-Br fission dramatically reduces the branching to C-Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH 2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C-Br fission, further suppressing C-Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH 2 hCOCI, determining the branching ratio between the competing primary C-Br and C-CI fission pathways and detecting a minor C-C bond fission pathway. While the primary C-CI:C-Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C-Br fission fragments show that essentially no C-Br fission results from promoting the molecule to the lowest lA" potential energy surface via the l[n(O),rr*(C=O)] transition; C-Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born-Oppenheimer approximation. While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the lA" surface, C-Br fission, should dominate, the experimental measurements show C-CI bond fission dominates by a ratio ofC-CI:C-Br= 1.0: <0.05 upon excitation of the l[n(O),1T*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C-Br fission pathway (C-CI:C-Br = 1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C-Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the In(O)rr*(C=O) and np(Br)a*(C-Br) configurations at the barrier to C-Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C-Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C-Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C-Br and C-CI fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward-Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

Chemical Reaction Dynamics Beyond the Born-Oppenheimer Approximation

“…Subsequent crossed laser-molecular beam experiments showed just that (57). While in bromoacetyl chloride, the initial 1 (n O π * C=O ) electronic transition resulted in a C-Cl:C-Br bond fission ratio of 1.0:0.4, in bromopropionyl chloride the same initial transition resulted in a C-Cl:C-Br bond fission ratio of 1.0:<0.05 (52,57).…”

“…Indeed, our simple electronic structure calculations on the Woodward-Hoffmann forbidden reactions in Br(CH 2 ) n COCl, described above and in (57), showed the splitting is on the order of hundreds of wavenumbers for both C-Br and C-Cl fission in bromoacetyl chloride and on the order of only tens of wavenumbers for C-Br fission in bromopropionyl chloride. In addition, there is preliminary computational evidence that upon breaking the symmetry element that makes the reaction Woodward-Hoffmann forbidden, as in the gauche conformers of bromoacetone and allyl chloride discussed in the next subsections, the splitting increases considerably.…”

“…Subsequent crossed laser-molecular beam experiments showed just that (57). While in bromoacetyl chloride, the initial 1 (n O π * C=O ) electronic transition resulted in a C-Cl:C-Br bond fission ratio of 1.0:0.4, in bromopropionyl chloride the same initial transition resulted in a C-Cl:C-Br bond fission ratio of 1.0:<0.05 (52,57). The <0.05 branching to C-Br fission represents an upper limit; in fact, a comparison of the distributions of relative kinetic energies imparted to the C-Br fission fragments in the two molecules determined from the data in Figures 4 and 5 show that essentially all of the Br atom products observed from bromopropionyl chloride merely result from an overlapping transition to an electronic state diabatically repulsive in the C-Br bond.…”

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

“…As 3-bromopropionyl chloride is substituted, the orbital distance extension between CÀBr and C=O weakens the electronic coupling between n(O)p*(C=O) and n p (Br)s*(CÀBr), thereby the probability of a nonadiabatic transition is increased. [2,3] Thus, the branching ratio is reduced to < 0.05, which deviates significantly from the statistical prediction of about 2. [3] Several groups are involved in computational work to understand this competitive bond cleavage from theoretical aspects.…”

“…[2,3] Thus, the branching ratio is reduced to < 0.05, which deviates significantly from the statistical prediction of about 2. [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.…”