Photodissociation dynamics of N,N -dimethylformamide at 225 nm and 245 nm.

N,N -Dimethylformamide, (CH 3 ) 2 NCHO, is the simplest tertiary amide and a model compound for investigating the photofragmentation of peptide bonds. We report the results of a velocity-map imaging study into the photodissociation dynamics of DMF following excitation at 225 nm and 245 nm. Excitation at either wavelength generates a variety of products, with the primary dissociation pathways involving cleavage of either the N-CO amide bond or an N-CH 3 bond. Excitation at 225 nm is predominantly to the S 2 2 1 A'' state via a parallel transition, with dissociation of the amide bond occurring either on this state or on a lower singlet surface following internal conversion. The topographies of all of the potential energy surfaces involved result in dissociation from a range of planar (apart from the methyl-group hydrogen atoms) and non-planar molecular geometries. Dissociation from planar geometries leads to little product internal excitation, correspondingly high photofragment velocities, and near-limiting values of the recoil-anisotropy parameter β . Dissociation from non-planar geometries leads to significant product internal excitation, with correspondingly lower photofragment velocities and breakdown of the axial recoil approximation to give reduced values of β . Excitation at 245 nm involves the same excited-state surfaces, but at the longer wavelength the S 2 state can only be reached from non-equilibrium geometries of the ground state, leading to a reduction in the recoil anisotropy parameter relative to excitation at 225 nm. The potential energy curves associated with cleavage of the N-CH 3 bond are less well characterised. However, the pathway is characterised by an isotropic angular distribution and a TKER distribution peaking at low energies, both of which can be rationalised in terms of the molecular geometry and the orientation of the transition dipole involved in the excitation step.