posted on 2007-12-27, 00:00authored byJoshua A. Plumley, Jeffrey D. Evanseck
The inherent difficulty in modeling the energetic character of the B−N dative bond has been investigated
utilizing density functional theory and ab initio methods. The underlying influence of basis set size and
functions, thermal corrections, and basis set superposition error (BSSE) on the predicted binding enthalpy of
ammonia borane (H3B−NH3) and four methyl-substituted ammonia trimethylboranes ((CH3)3B−N(CH3)nH3-n;
n = 0−3) has been evaluated and compared with experiment. HF, B3LYP, MPW1K, MP2, QCISD, and
QCISD(T) have been utilized with a wide range of Pople and correlation-consistent basis sets, totaling 336
levels of theory. MPW1K, B3LYP, and HF result in less BSSE and converge to binding enthalpies with
fewer basis functions than post-SCF techniques; however, the methods fail to model experimental binding
enthalpies and trends accurately, producing mean absolute deviations (MADs) of 5.1, 10.8, and 16.3 kcal/mol, respectively. Despite slow convergence, MP2, QCISD, and QCISD(T) using the 6-311++G(3df,2p)
basis set reproduce the experimental binding enthalpy trend and result in lower MADs of 2.2, 2.6, and 0.5
kcal/mol, respectively, when corrected for BSSE and a residual convergence error of ca. 1.3−1.6 kcal/mol.
Accuracy of the predicted binding enthalpy is linked to correct determination of the bond's dative character
given by charge-transfer frustration, QCTF = −(ΔQN + ΔQB). Frustration gauges the incompleteness of charge
transfer between the donor and the acceptor. The binding enthalpy across ammonia borane and methylated
complexes is correlated to its dative character (R2 = 0.91), where a more dative bond (less charge-transfer
frustration) results in a weaker binding enthalpy. However, a balance of electronic and steric factors must be
considered to explain trends in experimentally reported binding enthalpies. Dative bond descriptors, such as
bond ionicity and covalency are important in the accurate characterization of the dative bond. The B−N
dative bond in ammonia borane is 65% ionic, moderately strong (−27.5 ± 0.5 kcal/mol), and structurally
flexible on the donor side to relieve steric congestion.