Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland.
J Phys Chem A. 2011 Dec 29;115(51):14708-16. doi: 10.1021/jp209712s. Epub 2011 Dec 2.
In the present study the natural orbitals for chemical valence (NOCVs) combined with the energy decomposition scheme (ETS) were used to characterize bonding in various clusters of ammonia borane (borazane): dimer D, trimer TR, tetramer TE, and the crystal based models: nonamer N and tetrakaidecamer TD. ETS-NOCV results have shown that shortening of the B-N bond (by ~0.1 Å) in ammonia borane crystal (as compared to isolated borazane molecule) is related to the enhancement of donation (by 6.5 kcal/mol) and electrostatic (by 11.3 kcal/mol) contributions. This, in turn, is caused solely by the electrostatic dipole-dipole interaction between ammonia borane units; dihydrogen bonding, BH···HN, formed between borazane units exhibits no direct impact on B-N bond contraction. On the other hand, formation of dihydrogen bonding appeared to be very important in the total stabilization of single borazane unit, namely, ETS-based data indicated that it leads to significant electronic stabilization ΔE(orb) = -17.5 kcal/mol, which is only slightly less important than the electrostatic term, ΔE(elstat) = -19.4 kcal/mol. Thus, both factors contribute to relatively high melting point of the borazane crystal. Deformation density contributions (Δρ(i)) obtained from NOCVs allowed to conclude that dihydrogen bonding is primarily based on outflow of electron density from B-H bonding orbitals to the empty σ*(N-H) (charge transfer component). Equally important is the covalent contribution resulting from the shift of the electron density from hydrogen atoms of both NH and BH groups to the interatomic regions of NH···HB. Quantitatively, averaged electronic strength of dihydrogen bond per one BH···HN link varies from 1.95 kcal/mol (for the crystal structure model, N), 2.47 kcal/mol (for trimer TR), through 2.65 kcal/mol (for tetramer TE), up to 3.95 kcal/mol (for dimer D).
在本研究中，自然轨道化学价（NOCVs）结合能量分解方案（ETS）被用于描述氨硼烷（硼烷）的各种簇：二聚体 D、三聚体 TR、四聚体 TE 以及晶体基模型：九聚体 N 和二十四聚体 TD 中的键合。ETS-NOCV 结果表明，氨硼烷晶体中 B-N 键的缩短（相对于孤立的硼烷分子约为 0.1 Å）与供体（增加 6.5 kcal/mol）和静电（增加 11.3 kcal/mol）贡献的增强有关。这反过来又仅由氨硼烷单元之间的静电偶极-偶极相互作用引起；硼烷单元之间形成的氢键，BH···HN，对 B-N 键的收缩没有直接影响。另一方面，形成氢键对于单个硼烷单元的总稳定化非常重要，即 ETS 数据表明，它导致显著的电子稳定化ΔE(orb)=-17.5 kcal/mol，这仅略低于静电项ΔE(elstat)=-19.4 kcal/mol。因此，这两个因素都有助于硼烷晶体较高的熔点。从 NOCVs 获得的变形密度贡献（Δρ(i)）可以得出结论，氢键主要基于从 B-H 成键轨道到空的σ*(N-H)（电荷转移分量）的电子密度流出。同样重要的是来自于 NH 和 BH 基团的氢原子的电子密度向 NH···HB 原子间区域的转移的共价贡献。定量地，每一个 BH···HN 键的氢键的平均电子强度从 1.95 kcal/mol（对于晶体结构模型 N）变化到 2.47 kcal/mol（对于三聚体 TR），通过 2.65 kcal/mol（对于四聚体 TE），直至 3.95 kcal/mol（对于二聚体 D）。
