A reaction surface Hamiltonian treatment of the double proton transfer of formic acid dimer

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Abstract
The double proton transfer reaction of the isolated formic acid dimer has been investigated within the reaction surface Hamiltonian framework, using a newly calculated three‐dimensional ab initio potential energy surface. The symmetric (synchronous) proton movement, the asymmetric (asynchronous) proton movement and the relative motion of two formic acid molecules have been explicitly included in the calculation. The calculation gives a tunneling splitting of 0.004 cm−1, which is considerably smaller than a previous theoretical prediction (0.3 cm−1). An effective tunneling path has been calculated from the lowest vibrational eigenfunction of the reaction surface Hamiltonian, and the path deviates significantly from the minimum energy path on the potential energy surface. The new results are consistent with the conventional understanding of heavy–light–heavy mass combination reactions. The effective reaction path from this calculation reveals evidence of asymmetric proton movement. However, a synchronous double proton transfer is the major mode of reaction. Tunneling splittings for a few excited vibrational levels have also been calculated within the reaction surface Hamiltonian framework. Vibrational excitation of a large amplitude, heavy atom mode dramatically increases the tunneling splitting.