The connection between specific features in the water X-ray absorption spectrum and X-ray emission spectrum (XES) and the local H-bond coordination is studied based on structures obtained from path-integral molecular dynamics simulations using either the opt-PBE-vdW density functional or the MB-pol force field. Computing the XES spectrum using all molecules in a snapshot results in only one peak in the lone-pair (1b₁) region, while the experiment shows two peaks separated by 0.8-0.9 eV. Different H-bond configurations were classified based on the local structure index (LSI) and a geometrical H-bond cone criterion. We find that tetrahedrally coordinated molecules characterized by high LSI values and two strong donated and two strong accepted H-bonds contribute to the low energy 1b₁ emission peak and to the post-edge region in absorption. Molecules with the asymmetric H-bond environment with one strong accepted H-bond and one strong donated H-bond and low LSI values give rise to the high energy 1b₁ peak in the emission spectrum and mainly contribute to the pre-edge and main-edge in the absorption spectrum. The 1b₁ peak splitting can be increased to 0.62 eV by imposing constraints on the H-bond length, i.e., for very tetrahedral structures short H-bonds (less than 2.68 Å) and for very asymmetric structures elongated H-bonds (longer than 2.8 Å). Such structures are present, but underrepresented, in the simulations which give more of an average of the two extremes.

中文翻译：

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X射线发射吸收光谱学与水中局部H键环境的关系

基于使用opt-PBE-vdW的路径积分分子动力学模拟获得的结构，研究了水X射线吸收光谱和X射线发射光谱（XES）的特定特征与局部H键配位之间的联系。密度泛函或MB-pol力场。使用快照中的所有分子计算XES光谱只会在孤对（1 b ₁）区域中产生一个峰，而实验显示两个峰之间的距离为0.8-0.9 eV。基于局部结构指数（LSI）和几何H键锥标准，对不同的H键构型进行了分类。我们发现具有高LSI值和两个强捐赠和两个强氢键的四面体配位分子有助于降低能量1 b_1个发射峰并吸收到后边缘区域。具有不对称氢键环境的分子，其中一种强接受氢键，一种强捐赠氢键且LSI值低，在发射光谱中产生高能量1 b ₁峰，并且主要对前缘和主峰做出贡献。吸收光谱的边缘。通过对H键长度施加限制，可以将1 b ₁峰分裂增加到0.62 eV，即，对于非常四面体的结构，短H键（小于2.68Å）和对于非常不对称的结构，细长的H键（大于2.8Å）。这样的结构在模拟中存在，但代表性不足，它给出了两个极端的平均值。