Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, USA.
J Chem Phys. 2012 Nov 14;137(18):184301. doi: 10.1063/1.4764044.
With a view toward a better understanding of changes in the peak position and shape of the first absorption band of water with condensation or temperature, results from electronic structure calculations using high level wavefunction based and time-dependent density functional methods are reported for water pentamers. Excitation energies, oscillator strengths, and redistributions of electron density are determined for the quasitetrahedral water pentamer in its C(2v) equilibrium geometry and for many pentamer configurations sampled from molecular simulation of liquid water. Excitations associated with surface molecules are removed in order to focus on those states associated with the central molecule, which are the most representative of the liquid environment. The effect of hydrogen bonding on the lowest excited state associated with the central molecule is studied by adding acceptor or donor hydrogen bonds to tetramer and trimer substructures of the C(2v) pentamer, and by sampling liquid-like configurations having increasing number of acceptor or donor hydrogen bonds of the central molecule. Our results provide clear evidence that the blueshift of excitation energies upon condensation is essentially determined by acceptor hydrogen bonds, and the magnitudes of these shifts are determined by the number of such, whereas donor hydrogen bonds do not induce significant shifts in excitation energies. This qualitatively different role of donor and acceptor hydrogen bonds is understood in terms of the different roles of the 1b(1) monomer molecular orbitals, which establishes an intimate connection between the valence hole and excitation energy shifts. Since the valence hole of the lowest excitation associated with the central molecule is found to be well localized in all liquid-like hydrogen bonding environments, with an average radius of gyration of ~1.6 Å that is much lower than the nearest neighbor O-O distance, a clear and unambiguous connection between hydrogen bonding environments and excitation energy shifts can be established. Based on these results, it is concluded that peak position of the first absorption band is mainly determined by the relative distribution of single and double acceptor hydrogen bonding environments, whereas the shape of the first absorption band is mainly determined by the relative distribution of acceptor and broken acceptor hydrogen bonding environments. The temperature dependence of the peak position and shape of the first absorption band can be readily understood in terms of changes to these relative populations.
为了更好地理解水在冷凝或温度变化时第一吸收带的峰值位置和形状的变化，我们报告了使用基于高水准波函数和含时密度泛函方法的电子结构计算结果，这些结果针对的是五聚水分子。我们确定了在 C(2v)平衡几何结构下准四面体形五聚水分子的激发能、振子强度和电子密度再分布，以及从液态水的分子模拟中采样的许多五聚体构型。去除了与表面分子相关的激发，以便集中研究与中心分子相关的那些状态，这些状态最能代表液态环境。通过向 C(2v)五聚体的四聚体和三聚体子结构添加受体或给体氢键，以及采样具有越来越多中心分子受体或给体氢键的液态样构型，研究了氢键对与中心分子相关的最低激发态的影响。我们的结果提供了明确的证据，表明冷凝时激发能的蓝移主要由受体氢键决定，这些蓝移的幅度由受体氢键的数量决定，而给体氢键不会引起激发能的显著蓝移。这种受体和给体氢键的不同作用可以用 1b(1)单体分子轨道的不同作用来理解，这就建立了价孔和激发能蓝移之间的紧密联系。由于与中心分子相关的最低激发的价孔在所有液态氢键环境中都被发现很好地定域化，平均转动半径为~1.6 Å，远低于最近邻的 O-O 距离，因此可以在氢键环境和激发能蓝移之间建立清晰而明确的联系。基于这些结果，可以得出结论，第一吸收带的峰值位置主要由单接受体和双接受体氢键环境的相对分布决定，而第一吸收带的形状主要由接受体和破坏的接受体氢键环境的相对分布决定。第一吸收带的峰值位置和形状的温度依赖性可以根据这些相对群体的变化来很好地理解。
