Wang Huan, Agmon Noam
The Fritz Haber Research Center, Institute of Chemistry, The Hebrew University of Jerusalem , Jerusalem 91904, Israel.
J Phys Chem B. 2015 Feb 12;119(6):2658-67. doi: 10.1021/jp509004j. Epub 2015 Jan 9.
Protonated water clusters that are hydrogen-bonded to a neutral benzene molecule are a reductionist model for protons at hydrophobic surfaces, which are of fundamental importance in biological energy transduction processes. Of particular interest is the protonated water dimer ("Zundel ion") on benzene, whose gas-phase messenger IR spectrum has been previously interpreted in terms of an asymmetric binding of the protonated water dimer to the benzene ring through a single water molecule. This "standing Eigen" isomer has a hydronium core. We have found an alternative "crouching Zundel" isomer, which attaches to the benzene ring symmetrically via both of its water molecules. When Ar-tagged, it has an IR spectrum in much better agreement with experiment than the standing Eigen isomer, particularly at the lower frequencies. These conclusions are based on static harmonic (and anharmonic) normal-mode analysis using density functional theory with various (dispersion corrected) functionals and particularly on dynamic anharmonic spectra obtained from the dipole autocorrelation functions from classical ab initio molecular dynamics with the BLYP, PBE, and B3LYP functionals. Possible implications to protons on water/organic-phase interfaces are discussed.
与中性苯分子形成氢键的质子化水簇是疏水表面质子的简化模型，这在生物能量转换过程中至关重要。特别令人感兴趣的是苯上的质子化水二聚体（“祖德尔离子”），其气相信使红外光谱先前已根据质子化水二聚体通过单个水分子与苯环的不对称结合来解释。这种“直立的本征”异构体具有水合氢离子核心。我们发现了一种替代的“蹲伏的祖德尔”异构体，它通过其两个水分子对称地附着在苯环上。当用氩标记时，它的红外光谱与实验结果的吻合度比直立的本征异构体好得多，特别是在较低频率下。这些结论基于使用各种（色散校正）泛函的密度泛函理论进行的静态谐波（和非谐波）正常模式分析，尤其基于从具有BLYP、PBE和B3LYP泛函的经典从头算分子动力学的偶极自相关函数获得的动态非谐波光谱。文中还讨论了对水/有机相界面上质子的可能影响。
