Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
J Am Chem Soc. 2012 Sep 19;134(37):15430-5. doi: 10.1021/ja305500x. Epub 2012 Sep 6.
The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D(0)) of (D(2)O)(2). The value obtained, 1244 ± 10 cm(-1) (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm(-1) (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H(2)O)(2). In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H(2)O)(2) and (D(2)O)(2) were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.
水的氢键主要由成对的二聚体相互作用主导，本文报道了振动激发后水二聚体的预解离。速度图成像被用于实验测定（D2O）2 的离解能（D0）。得到的值为 1244±10 cm-1（14.88±0.12 kJ/mol），与计算值 1244±5 cm-1（14.88±0.06 kJ/mol）非常吻合。这一理论与实验之间的良好一致性与最近对（H2O）2 获得的结果相当。此外，在分别激发氢键 OH 和 OD 伸缩基频后，获得了（H2O）2 和（D2O）2 振动预解离后水碎片的成对相关的 rovibrational 态分布。使用精确的全维势能面的准经典轨迹计算与实验相符，并有助于阐明实验。实验和理论都发现氢键断裂时主要激发了碎片的弯曲模式。也观察到一个较小的通道，其中两个碎片都处于基态且具有高度的转动激发。理论计算揭示了在供体和受体亚基中弯曲激发的概率相等，这是供体和受体作用互换的结果。与主要通道相关的转动分布，其中一个水碎片具有一个弯曲量子，而较小的通道中两个水碎片都处于基态振动，与实验相符。
