Nonadiabaticity and the competition between alpha and beta bond fission upon 1[n,π*(C=O)] excitation in acetyl- and bromoacetyl chloride

Abstract

This work investigates how molecular dissociation induced by local 1[n(O),π*(C=O)] electronic excitation at a carbonyl functional group can result in preferential fission of an alpha bond over a weaker bond beta to the functional group and how nonadiabaticity in the dynamics drives the selectivity. The experiment measures the photofragment velocity and angular distributions from the photodissociation of acetyl chloride and bromoacetyl chloride at 248 nm, identifying the branching between bond fission channels and the mechanism for the selectivity. The anisotropic angular distributions measured shows dissociation occurs on a time scale of less than a rotational period, resulting in primary C–X (X=Cl, Br) bond fission, but no significant C–C bond fission. While the selective fission of the C–Cl over the C–C alpha bond can be predicted from the adiabatic correlation diagram for this special class of Norrish type I cleavage, the preferential fission of the C–Cl alpha bond over the C–Br bond beta to the carbonyl group would not be predicted on the adiabatic potential energy surface. In bromoacetyl chloride, fission of the C–Cl and C–Br bonds occurs with a branching of 1.0:1.1 (approximately 1.0:0.5 from the 1nπ* transition) compared with a predicted statistical branching ratio of 1:30. This preferential α-bond fission is attributed to a dissociation mechanism on the coupled [n,π*(C=O)] and [n(X),σ *(C–X)] electronic states, a model consistent with the lack of C–C fission and the measured kinetic energy and angular distributions. The selectivity results from the relative strengths of the electronic coupling between the initially excited [n,π*(C=O)] bound configuration and the two [n(X),σ *(C–X)] states, the weaker coupling inhibiting the adiabatic crossing over the barrier to C–Br bond fission. The results demonstrate the need to go beyond the Born–Oppenheimer approximation to gain predictive ability in any reactive system where the electronic configuration changes along the reaction coordinate, particularly at barriers due to configuration crossings. In addition, the Cl product angular distribution determines the orientation of the 1[n(O),π*(C=O)] transition dipole moment and shows it is governed by the C2v symmetry of the localized carbonyl electronic orbitals and not by the asymmetric substitution at the carbonyl group. Spectra of the Br atoms from direct dissociation at 193 nm help separate the contribution from the overlapping nσ *(C–Br) transition at 248 nm.

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

“…[14][15][16] The obtained diabatic couplings for CÀCl bond fission along the minimum energy path were essentially one order of magnitude larger than those for CÀBr. Such a trend is consistent with the report of the Butler group, [2,3] but the later calculations are underestimated. The stronger nonadiabaticity in the CÀBr channel leads to the formation of excited-state CH 2 COCl (ffi 2 A') + Br products through a diabatic pathway; the precursor may not be trapped in the bound diabatic surface with a slow bond breaking rate through adiabatic dissociation, as proposed previously.…”

“…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.…”

“…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.…”

“…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. [1][2][3][4][5] Competitive dissociation channels were found to favor cleavage of the stronger CÀCl bond over the weaker CÀBr bond; both reactions proceeded on the S 1 potential energy surface (PES). The branching ratio of CÀBr/CÀCl bond fission was 0.4 against the statistical prediction of 30.…”

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). 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.…”