Head-Gordon Teresa, Johnson Margaret E
University of California at San Francisco/University of California at Berkeley Joint Graduate Group in Bioengineering, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2006 May 23;103(21):7973-7. doi: 10.1073/pnas.0510593103. Epub 2006 May 12.
It has been suggested, based on x-ray absorption spectroscopy (XAS) experiments on liquid water [Wernet, Ph., et al. (2004) Science 304, 995-999], that a condensed-phase water molecule's asymmetric electron density results in only two hydrogen bonds per water molecule on average. The larger implication of the XAS interpretation is that the conventional view of liquid water being a tetrahedrally coordinated random network is now replaced by a structural organization that instead strongly favors hydrogen-bonded water chains or large rings embedded in a weakly hydrogen-bonded disordered network. This work reports that the asymmetry of the hydrogen density exhibited in the XAS experiments agrees with reported x-ray scattering structure factors and intensities for Q > 6.5 A(-1). However, the assumption that the asymmetry in the hydrogen electron density does not fluctuate and is persistent in all local molecular liquid water environments is inconsistent with longer-ranged tetrahedral network signatures present in experimental x-ray scattering intensity and structure factor data for Q < 6.5 A(-1).
基于对液态水的X射线吸收光谱（XAS）实验[韦尔内特，Ph.等人（2004年）《科学》304卷，995 - 999页]，有人提出，凝聚相水分子的不对称电子密度导致每个水分子平均只有两个氢键。XAS解释的更大影响是，液态水是四面体配位随机网络的传统观点现在被一种结构组织所取代，这种结构组织反而强烈倾向于氢键连接的水链或嵌入弱氢键无序网络中的大环。这项工作报告称，XAS实验中显示的氢密度不对称与Q > 6.5 Å⁻¹时报道的X射线散射结构因子和强度一致。然而，氢电子密度的不对称不波动且在所有局部分子液态水环境中持续存在这一假设，与Q < 6.5 Å⁻¹的实验X射线散射强度和结构因子数据中存在的长程四面体网络特征不一致。
