Nonadiabatic interactions in wave packet dynamics of the bromoacetyl chloride photodissociation.

The competitive photodissociation of bromoacetyl chloride BrCH[sub 2]COCl in the first [sup 1]A[sup ″] state (S[sub 1]) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C–Cl bond (α to the excited carbonyl) over the weaker C–Br bond (β) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C–Cl:C–Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C–O/C–X (X=Br, Cl) subspaces to emphasize the role of the C–O vibration upon [n[sub O]→π[sub CO][sup *]] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck–Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and C[sub s] constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C–O/C–X subspaces. The inclusion of the C–O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]