Asymmetry of the Electrostatic Environment as the Origin of the Symmetry Breaking Effect of the Nitrate Ion in Aqueous Solution

Elucidating the mechanism of how vibrational modes areaffectedby intermolecular interactions is important for a better understandingof the nature of the former as probes of the latter. Here, such ananalysis is carried out for the N-O stretching modes of thenitrate ion interacting with water, with an emphasis on the symmetrybreaking effect. On the basis of theoretical calculations on the structural,vibrational, and electrostatic properties of molecular clusters andspectral simulations for an aqueous solution, a transparent view isdemonstrated on the mechanism that modulations of spatially localelectrostatic environment give rise to structural and spectroscopicsymmetry breaking effect. The electrostatic interaction model constructedhere is a seven-parameter model; the use of a single electrostaticparameter, such as the electric field on a single atomic site, isfound to be insufficient for quantitative evaluation. It is also shownthat the frequency modulations of the N-O stretching modesin aqueous solution occur on a time scale much shorter than 0.1 ps.