Nguyen Vinh Son, Matus Myrna H, Grant Daniel J, Nguyen Minh Tho, Dixon David A
Department of Chemistry, University of Leuven, B-3001 Leuven, Belgium.
J Phys Chem A. 2007 Sep 13;111(36):8844-56. doi: 10.1021/jp0732007. Epub 2007 Aug 17.
High-level electronic structure calculations have been used to map out the relevant portions of the potential energy surfaces for the release of H2 from dimers of ammonia borane, BH3NH3 (AB). Using the correlation-consistent aug-cc-pVTZ basis set at the second-order perturbation MP2 level, geometries of stationary points were optimized. Relative energies were computed at these points using coupled-cluster CCSD(T) theory with the correlation-consistent basis sets at least up to the aug-cc-pVTZ level and in some cases extrapolated to the complete basis set limit. The results show that there are a number of possible dimers involving different types of hydrogen-bonded interactions. The most stable gaseous phase (AB)2 dimer results from a head-to-tail cyclic conformation and is stabilized by 14.0 kcal/mol with respect to two AB monomers. (AB)2 can generate one or two H2 molecules via several direct pathways with energy barriers ranging from 44 to 50 kcal/mol. The diammoniate of diborane ion pair isomer, [BH4-][NH3BH2NH3+] (DADB), is 10.6 kcal/mol less stable than (AB)2 and can be formed from two AB monomers by overcoming an energy barrier of approximately 26 kcal/mol. DADB can also be generated from successive additions of two NH3 molecules to B2H6 and from condensation of AB with separated BH3 and NH3 molecules. The pathway for H2 elimination from DADB is characterized by a smaller energy barrier of 20.1 kcal/mol. The alternative ion pair [NH4+][BH3NH2BH3-] is calculated to be 16.4 kcal/mol above (AB)2 and undergoes H2 release with an energy barrier of 17.7 kcal/mol. H2 elimination from both ion pair isomers yields the chain BH3NH2BH2NH3 as product. Our results suggest that the neutral dimer will play a minor role in the release of H2 from ammonia borane, with a dominant role from the ion pairs as observed experimentally in ionic liquids and the solid state.
高水平电子结构计算已被用于描绘氨硼烷（BH₃NH₃，简称AB）二聚体释放H₂过程中势能面的相关部分。在二阶微扰MP2水平上使用相关一致的aug-cc-pVTZ基组，对驻点的几何结构进行了优化。在这些点上使用耦合簇CCSD(T)理论，并至少使用到aug-cc-pVTZ水平的相关一致基组，在某些情况下外推到完全基组极限来计算相对能量。结果表明，存在许多涉及不同类型氢键相互作用的可能二聚体。最稳定的气相(AB)₂二聚体由头对尾的环状构象形成，相对于两个AB单体稳定14.0千卡/摩尔。(AB)₂可通过几种直接途径生成一个或两个H₂分子，能垒范围为44至50千卡/摩尔。二硼烷离子对异构体的二氨合物[BH₄⁻][NH₃BH₂NH₃⁺]（DADB）比(AB)₂稳定性低10.6千卡/摩尔，可通过克服约26千卡/摩尔的能垒由两个AB单体形成。DADB也可由两个NH₃分子连续加成到B₂H₆以及AB与分离的BH₃和NH₃分子缩合形成。从DADB消除H₂的途径的特征是能垒较小，为20.1千卡/摩尔。另一种离子对[NH₄⁺][BH₃NH₂BH₃⁻]经计算比(AB)₂高16.4千卡/摩尔，其H₂释放的能垒为17.7千卡/摩尔。两种离子对异构体消除H₂均产生链状产物BH₃NH₂BH₂NH₃。我们的结果表明，中性二聚体在氨硼烷释放H₂过程中起次要作用，而离子对起主导作用，这与在离子液体和固态中的实验观察结果一致。
