Odelius Michael, Folkestad Sarai Dery, Saisopa Thanit, Rattanachai Yuttakarn, Sailuam Wutthigrai, Yuzawa Hayato, Kosugi Nobuhiro, Paul Alexander C, Koch Henrik, Céolin Denis
Department of Physics, Stockholm University, 10691 Stockholm, Sweden.
Department of Chemistry, Norwegian University of Science and Technology, NTNU, 7491 Trondheim, Norway.
J Phys Chem Lett. 2025 Apr 10;16(14):3411-3419. doi: 10.1021/acs.jpclett.4c03625. Epub 2025 Mar 27.
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 NH 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.
氨水溶液的氮 K 边 X 射线吸收（XA）光谱显示主边缘存在分裂，通过理论建模表明这与氢键中的对称性破缺有关。NH 的 XA 主边缘由一对简并的芯激发到扩展分子轨道形成。在水溶液中，这些与周围水分子的轨道形成反键混合物。尽管对畸变的光谱响应很复杂，但我们表明芯激发的简并性通过氢键供体（NH···O）中的不对称性而消除。通过在定义明确的簇模型中的系统对称性破缺以及对氨水溶液模拟 XA 光谱的分子动力学采样，建立了水合壳层中的不对称性与氮 K 边 XA 光谱主边缘分裂之间的定量关系。这一发现表明，XA 光谱也是生物分子中官能团周围溶剂化不对称性的灵敏探针。
