Acetamide as the model of the peptide bond: Nonadiabatic photodynamical simulations in the gas phase and in the argon matrix

The short-time photodynamics of acetamide in the gas phase and in the Ar-matrix starting from the first excited singlet S1 (n–π*) and S2 (π–π*) states was explored by the direct trajectory surface hopping method based on multiconfigurational ab initio calculations. For the n–π* state effect of embedding into Ar matrix employing hybrid QM/MM approach on the lifetime and dissociation paths was also discussed. The calculations show that nonadiabatic deactivation from the S1 to the ground state occurs within 785 fs via the crossing seam related to the CN bond stretching MXS. The major products in the gas phase are NH2 + CH3CO radicals which partially undergo further dissociation of the CH3CO or recombination to the parent molecule. The embedding of the molecule in the Ar matrix considerably suppresses the CN dissociation and prevents dissociation of the CH3CO radical.