Vibrational Symmetry Breaking of NO3 - in Aqueous Solution: NO Asymmetric Stretch Frequency Distribution and Mean Splitting
Abstract
We apply a solute-solvent approach to a theoretical study of vibrational symmetry breaking in aqueous NO3
-
solution. Experimental infrared and Raman spectra have shown that the NO asymmetric stretches, which are
degenerate for the isolated anion, are split by 35-60 cm-1 in dilute solution. As an initial step to calculating
the spectra, we have computed the distribution of energies, or the “static spectrum”, and the resulting mean
splitting of the two NO asymmetric stretch eigenstates in an aqueous milieu. These have been obtained in a
two-mode treatment that considers only the NO asymmetric stretch mode pair as well as a full six-mode
treatment. In both sets of calculations, six eigenstates, namely, the ground state, the two NO asymmetric
stretch fundamentals, and its three overtones, were determined to suffice for converged energy distributions
and mean splittings. The couplings between these six states are driven by the solvent forces on the anion’s
modes, which were extracted from molecular dynamics simulations. The solvent forces on the two central
modes were found to give rise to a majority of the computed mean splitting of 21.7 cm-1. The distribution
of NO asymmmetric stretch excitation energies with these two modes alone was found to have a
Maxwell-Boltzmann shape. The solvent forces on the in-plane bends were found to modestly reduce the
splitting size and slightly alter the width of the parent distribution. The symmetric stretch force was found to
have no effect on the splitting but instead resulted in a widening on the distribution shape. The force gradients
were found to have a weak effect on both the eigenvalue distribution and the mean splitting.