Dreux Katelyn M, McNamara Louis E, Kelly John T, Wright Ashley M, Hammer Nathan I, Tschumper Gregory S
Department of Chemistry and Biochemistry, University of Mississippi, University , Mississippi 38677-1848, United States.
J Phys Chem A. 2017 Aug 10;121(31):5884-5893. doi: 10.1021/acs.jpca.7b03509. Epub 2017 Jul 11.
Although ammonia borane is isoelectronic with ethane and they have similar structures, BHNH exhibits rather atypical bonding compared to that in CHCH. The central bond in ammonia borane is actually a coordinate covalent or dative bond rather than the conventional covalent C-C bond in ethane where each atom donates one electron. In addition, strong intermolecular dihydrogen bonds can form between two or more ammonia borane molecules compared to the relatively weak dispersion forces between ethane molecules. As a result, ammonia borane's physical properties are very sensitive to the environment. For example, gas-phase and solid-state ammonia borane have very different BN bond lengths and BN stretching frequencies, which led to much debate in the literature. It has been demonstrated that the use of cluster models based on experimental crystal structures led to better agreement between theory and experiment. Here, we employ a variety of cluster models to track how the interaction energies, bond lengths, and vibrational normal modes evolve with the size and structural characteristics of the clusters. The M06-2X/6-311++G(2df,2pd) level of theory was selected for this analysis on the basis of favorable comparison with CCSD(T)/aug-cc-pVTZ data for the ammonia borane monomer and dimer. Fourteen unique fully optimized molecular cluster geometries, (BHNH), and nine crystal models, (BHNH), were used to elucidate how the local environment impacts ammonia borane's physical properties. Computational results for the BN stretching frequencies are also compared directly to the Raman spectrum of solid ammonia borane at 77 K using Raman under liquid nitrogen spectroscopy (RUNS). A strong linear correlation was found to exist between the BN bond length and stretching frequency, from an isolated monomer to the most distorted BHNH unit in a cluster or crystal structure model. Excellent agreement was seen between the frequencies computed for the largest crystal model and the RUNS experimental spectra (typically within a few wavenumbers).
尽管氨硼烷与乙烷等电子且结构相似，但与乙烷中的键合相比，硼氮氢（BHNH）表现出相当不典型的键合。氨硼烷中的中心键实际上是一个配位共价键或授受键，而不是乙烷中传统的共价碳 - 碳键，乙烷中每个原子贡献一个电子。此外，与乙烷分子之间相对较弱的色散力相比，两个或更多氨硼烷分子之间可以形成强分子间双氢键。因此，氨硼烷的物理性质对环境非常敏感。例如，气相和固态氨硼烷具有非常不同的硼氮键长度和硼氮伸缩频率，这在文献中引发了很多争论。已经证明，基于实验晶体结构使用簇模型能使理论与实验之间达成更好的一致性。在这里，我们采用各种簇模型来追踪相互作用能、键长和振动简正模式如何随簇的大小和结构特征而演变。基于与氨硼烷单体和二聚体的耦合簇单双激发组态相互作用方法（CCSD(T)）/aug-cc-pVTZ数据的良好比较，选择M06-2X/6-311++G(2df,2pd)理论水平进行此分析。使用14个独特的完全优化分子簇几何结构（BHNH）和9个晶体模型（BHNH）来阐明局部环境如何影响氨硼烷的物理性质。还使用液氮光谱下的拉曼光谱（RUNS）将硼氮伸缩频率的计算结果直接与77 K下固态氨硼烷的拉曼光谱进行比较。从孤立单体到簇或晶体结构模型中最扭曲的硼氮氢（BHNH）单元，发现硼氮键长度与伸缩频率之间存在很强线性相关性。在最大晶体模型计算的频率与RUNS实验光谱之间观察到了极好的一致性（通常在几个波数范围内）。
