Photoinduced dynamics of formic acid monomers and dimers: the role of the double hydrogen bond.

Nonadiabatic dynamics in the framework of time-dependent density functional theory was used to simulate gas-phase relaxation dynamics of pairs of conformations of formic acid monomers (cis and trans FAM) and dimers (acyclic aFAD and cyclic cFAD). In the early phase of the excited state dynamics, elongation of the C═O bond and pyramidalization of the carbon atom is observed in both FAM and FAD. Subsequently, the photodynamics of FAM is shown to be dominated by fragmentation processes occurring mostly in the excited state and resulting in HCO and OH radicals. In only a few cases does the dissociation take place from the vibrationally excited ground electronic state, whereby CO and H(2)O are the major reaction products. In the dimers, single proton transfer triggers ultrafast relaxation to the ground electronic state. In the single hydrogen bonded dimer about half of the trajectories dissociate into electronically excited monomers, whereas this potentially destructive dissociation is effectively suppressed in the double hydrogen bonded dimer. Upon relaxation to the ground electronic state, separation of FAD into monomers takes place, but without their further fragmentation on the time scale of the simulation. We conclude that the crucial difference between the FAM and FAD photodynamics is that the latter is dominated by nondestructive radiationless deactivation pathways during which a key protective role is assumed by the single (aFAD) or double (cFAD) intermonomer hydrogen bonds.