Electronic and Nuclear Contributions to Vibrational Stark Shifts of Hydroxyl Stretching Frequencies of Water Clusters

In spite of the importance of vibrational Stark effect (VSE) and many attempts, origin of VSE is still unclear in molecular level. Here, we studied on origin of VSE of hydroxyl stretching vibration in small water clusters (monomer, dimer, and tetramer) assuming that VSE can be separated by nuclear and electronic contribution. We calculated total Stark tuning rate (Δμ_tot) and its nuclear contribution (Δμ_geom) using the ab initio method, then the electronic contribution (Δμ_elec) was simply obtained by the difference, Δμ_tot - Δμ_geom. In all cases, the nuclear contribution has dominant contribution to VSE. The hydroxyl stretching mode with neighboring hydrogen acceptor showed larger Δμ_geom than that of dangling bonds. Furthermore, the calculated Δμ_geom became larger in larger cluster due to the hydrogen bond network. The comparison between Stark tuning rates including and excluding anharmonicity supports the importance of potential anharmonicity in VSE, as previously reported. Interestingly, a good linear relationship is observed between the hydroxyl stretch frequency (ν_geom) and hydroxyl bond length and also between the Stark tuning rate (Δμ_geom) and the change of hydroxyl bond length. Similarly, the electronic contribution of calculated frequencies and Stark tuning rate (Δμ_elec) showed a good linear relationship with atomic charge derived by electronic perturbation (Δq_elec) and change of that (Δ(Δq_elec)), respectively.