Ab initio/GIAO-CCSD(T) study of the effects of protonation on borane-ammonia (H3BNH3) and related complexes

Structures of neutral borane-ammonia complex H3BNH31 and its protonated form H4BNH3+2 were calculated using ab initio method at the MP2/cc-pVTZ and MP2/cc-pVQZ levels. At the MP2/cc-pVQZ level the BN bond distance of 1 was found to be 1.646 Å. This is close to the experimentally measured gas phase BN distance (r0) of 1.672 Å (microwave spectroscopy). At the same MP2/cc-pVQZ level, the BN bond distance of 2 was found to be only 1. 577 Å. This is very close to the X-ray determined bond distance of 1.564 Å. Protonation of 1 was found to be highly exothermic by 192.9 kcal/mol. The 11B NMR chemical shifts of 1 and 2 were also calculated by the correlated GIAO-CCSD(T) method. Both δ11B values of 1 and 2 were calculated to be close to the experimental value of H3BNH31. For comparison, structures and 11B NMR chemical shifts of the neutral complexes H3BX (X = PH33, OH25, SH27 and CO 9) and their corresponding BH protonated structures were also computed.