Symmetry Breaking around Aqueous Ammonia Revealed in Nitrogen K-edge X-ray Absorption

Abstract

Nitrogen K-edge X-ray absorption (XA) spectroscopy of aqueous ammonia reveals a splitting in the main-edge, which through theoretical modeling is shown to be related to symmetry breaking in hydrogen bonding. The XA main-edge of NH₃ is formed by a pair of degenerate core-excitations into extended molecular orbitals. In aqueous solution, these form an antibonding mixture with orbitals of the surrounding water molecules. Although the spectral response to distortions is complex, we show that the degeneracy of the core-excitations is lifted by asymmetry in hydrogen bond donation (NH···O). A quantitative relation between asymmetry in the hydration shell and splitting in the main-edge of the nitrogen K-edge XA spectrum is established from systematic symmetry breaking in well-defined cluster models and through molecular dynamics sampling of simulated XA spectra of aqueous ammonia. The finding indicates that XA spectroscopy is a sensitive probe of asymmetry in solvation also around functional groups in biomolecules.