On the Role of Nonbonded Interactions in Vibrational Energy Relaxation of Cyanide in Water

The vibrationally excited cyanide ion (CN–) in H2O or D2O relaxes back to the ground state within several tens of picoseconds. Pump–probe infrared spectroscopy has determined relaxation times of T1 = 28 ± 7 and 71 ± 3 ps in H2O and D2O, respectively. Atomistic simulations of this process using nonequilibrium molecular dynamics simulations allow determination of whether it is possible at all to describe such a process, what level of accuracy in the force fields is required, and whether the information can be used to understand the molecular mechanisms underlying vibrational relaxation. It is found that, by using the best electrostatic models investigated, absolute relaxation times can be described rather more qualitatively (T1H2O = 19 ps and T1D2O = 34 ps) whereas the relative change in going from water to deuterated water is more quantitatively captured (factor of 2 vs 2.5 from experiment). However, moderate adjustment of the van der Waals ranges by less than 20% (for NVT) and 7.5% (for NVE), respectively, leads to almost quantitative agreement with experiment. Analysis of the energy redistribution establishes that the major pathway for CN– relaxation in H2O or D2O proceeds through coupling to the water-bending plus libration mode.