Hydrogen exchange in formic acid dimer: tunnelling above the barrier.

The Raman spectrum of formic acid dimer in the OH stretch fundamental range is analysed in terms of a fully coupled vibrational Hamiltonian for the seven most important in-plane degrees of freedom involved in the hydrogen exchange process. The Raman spectrum calculated with potential and polarizability functions obtained from density functional theory reproduces the observed band structure. Wavepacket calculations reveal a remarkably slow hydrogen exchange on the timescale of several ps even at excess energies in the range of 10-15 kJ mol(-1). The highly non-statistical process is shown to proceed via adiabatic tunnelling even if excess energy is deposited directly into the reaction coordinate.