Geometric isotope effect on the N2H7+ cation and N2H5- anion by ab initio path integral molecular dynamics simulation
Geometric isotope effect on the N2H7+ cation and N2H5- anion by ab initio path integral molecular dynamics simulation
复制标题
DOI：
10.1002/cphc.200700570
复制
发表时间：
2008-02-22
期刊：
影响因子：
2.9
通讯作者：
Tachikawa, Masanori
中科院分区：
文献类型：
--
作者：
Ishibashi, Hiroaki;Hayashi, Aiko;Tachikawa, Masanori
在氢键中，由于共享氢的氘化，原子位置存在小的位移，这被称为几何同位素效应（GIE）。已知这种效应存在于氢键铁电晶体系统中，其中涉及氢键阳离子，例如磷酸二氢钾[1]，其导致通过氘核取代的晶格膨胀。另一方面，GIE在氢键分子团簇中并不为人所知。虽然气相红外光谱的最新进展可以识别出可以归属于氢键质子的尖锐光谱特征，[2]但如何推断几何结构的细微变化并不明显。在我们最近的工作中，使用从头算路径积分分子动力学（PIMD）模拟，[3]我们研究了气相分子团簇中氢键的GIE，并专注于带正电荷和负电荷的水二聚体，H5 O2+和H3 O2+。有趣的是，它被发现这两个物种之间的GIE是不同的，在H5 O2+，但更长的H3 O2氘取代氧分离变短。这种行为归因于氢键的量子效应的变化，无论共享的氢是在单阱或双阱势面上。[3，4]理论上，GIE与两个因素有关：核量子效应和势非谐性。首先，GIE来自氢原子核（质子或氘核）的量子力学效应，因为只要原子核服从经典统计力学（在玻恩-奥本海默（BO）近似下，原子间势能对于同位素异构体是相同的），关于同位素异构体构型的热分布就必须完全相同。换句话说，如果氢核被假定为经典粒子，GIE将消失。第二，GIE的出现是非谐效应的结果;如果BO势能面近似为谐的，即使氢核被量子力学处理，它也会消失。因此，可以说GIE是量子效应和非简谐效应共同作用的结果。本文采用从头算PIMD模拟技术研究了带正电荷和负电荷的氨二聚体N2 H7+和N2 H5+中氢键的GIE是否存在。有许多关于质子化氨二聚体（N2 H7+阳离子）的报道。从振动光谱中可以看出，键合的氢原子位于两个氮原子的中心，作为NH_2H_2N [5]，而从迄今为止的所有从头计算分子轨道（MO）计算中，都报道了平衡结构向一侧移动，作为NH_2H_2N··· N。[6]实验结果和理论结果之间的这种矛盾可能源于核量子效应，这在这些理论计算中是缺失的。事实上，最近基于一维波包动力学的计算[7]，其中核量子效应仅在质子转移坐标中考虑，通过包括零点能量再现了对称结构。然而，正如我们在这里所展示的，多维效应对于定量分析H/D GIE是必不可少的。在另一种方法中，Gao et al. [8]最近，通过结合半经验量子力学/分子力学（QM/MM）方法（其中QM区域基于半经验方案）和路径积分方法，研究了质子化液氨，并表明核量子效应在质子化氨中确实是重要的。质子化和中性液体。
In hydrogen bonds, there is a small displacement of atomic positions due to deuterization of shared hydrogens, which is referred to as the geometric isotope effect (GIE). This effect is known to be present in hydrogen-bonded ferroelectric crystal systems in which a hydrogen-bonded cation is involved, for example, in potassium dihydrogen phosphate,[1] which results in crystal-lattice expansion by deuteron substitution. On the other hand, the GIE is not well known in hydrogen-bonded molecular clusters. Although recent advances in gas-phase infrared spectroscopy can identify sharp spectral features that can be assigned to the hydrogen-bonded proton,[2] it is not obvious how a subtle change in geometry can be deduced. In our recent work using ab initio path integral molecular dynamics (PIMD) simulation,[3] we investigated the GIE of the hydrogen bonds in the gas-phase molecular clusters, and focused on positively and negatively charged water dimers, H5O2+ and H3O2 Ā. Interestingly, it is found that the GIE is different between these two species in that the oxygen separation becomes shorter in H5O2+ but longer in H3O2 Ā by deuteron substitution. This behavior is ascribed to the change in the quantum effect of hydrogen bonds, whether the shared hydrogen is on a single-or double-well potential surface.[3, 4] Theoretically, the GIE is related to two factors: the nuclear quantum effect and potential anharmonicity. First, the GIE comes from the quantum mechanical effect of hydrogen nuclei (proton or deuteron), because the thermal distribution with respect to the configuration of isotopomers must be exactly the same as long as the nuclei are subject to classical statistical mechanics (within the Born–Oppenheimer (BO) approximation, where the interatomic potential energy is identical for isotopomers). In other words, the GIE will disappear if the hydrogen nucleus is assumed to be a classical particle. Second, the GIE arises as a consequence of the anharmonic effect; it will disappear if the BO potential energy surface is approximated to be harmonic, even if the hydrogen nucleus is treated quantum mechanically. Therefore, it can be said that the GIE arises by the combination of quantum and anharmonic effects.Herein, we use the ab initio PIMD simulation technique to address whether the GIE exists for the hydrogen bonds in positively and negatively charged ammonia dimers, N2H7+ and N2H5 Ā. There are a number of reports on the protonated ammonia dimer, the N2H7+ cation. From vibrational spectroscopy it is suggested that the bonded hydrogen atom locates in the center of two nitrogen atoms as NĀHĀN,[5] while from all the ab initio molecular orbital (MO) calculations so far it is reported that the equilibrium structure is shifted to one side as NĀH··· N.[6] Such a contradiction between experimental and theoretical results may stem from the nuclear quantum effect, which is missing in these theoretical calculations. In fact, a recent calculation based on one-dimensional wave-packet dynamics,[7] in which the nuclear quantum effect is considered just in the proton-transfer coordinate, has reproduced the symmetrized structure by including the zero-point energy. However, as we show here, the multidimensional effect is indispensable to analyze the H/D GIE quantitatively. In another approach, Gao et al.[8] have recently studied protonated liquid ammonia by the combination of a semiempirical quantum mechanics/molecular mechanics (QM/MM) method (in which the QM region is based on a semiempirical scheme) and a path integral method, and showed that the nuclear quantum effect is indeed important in protonated ammonia. Protonated and neutral liquid …