Quantum energy flow and the kinetics of water shuttling between hydrogen bonding sites on trans-formanilide.

A potential energy surface for trans-formanilide (TFA)-H2O is calculated and applied to study energy flow in the complex as well as the kinetics of water shuttling between hydrogen bonding sites on TFA. In addition to the previously identified H2O-TFA(C[Double Bond]O) and H2O-TFA(NH) minima, with the water monomer bound to the C[Double Bond]O and NH groups, respectively, the new surface reveals a second local minimum with the water bound to the C[Double Bond]O group, and which lies energetically 310 cm(-1) above the previously identified H2O-TFA(C[Double Bond]O) global minimum. On this surface, the energy barrier for water shuttling from H2O-TFA(C[Double Bond]O) global minimum to H2O-TFA(N-H) is 984 cm(-1), consistent with the experimental bounds of 796 and 988 cm(-1) [J. R. Clarkson et al. Science 307, 1443 (2005)]. The ergodicity threshold of TFA is calculated to be 1450 cm(-1); for the TFA-H2O complex, the coupling to the water molecule is found to lower the ergodicity threshold to below the isomerization barrier. Energy transfer between the activated complex and the vibrational modes of TFA is calculated to be sufficiently rapid that the Rice-Ramsperger-Kassel-Marcus (RRKM) theory does not overestimate the rate of water shuttling. The possibility that the rate constant for water shuttling is higher than the RRKM theory estimate is discussed in light of the relatively high energy of the ergodicity threshold calculated for TFA.