Hybrid Meta-Generalized Gradient Functional Modeling of Boron-Nitrogen Coordinate Covalent Bonds.

Truhlar's new generation of hybrid meta-generalized gradient functionals has been evaluated in modeling the binding enthalpies of substituted B-N coordinate covalent bonds. The short-range exchange correlation (XC) energy of coordinate covalent bonding coupled with the medium-range XC energy of noncovalent interactions results in a particularly difficult case for density functional theory (DFT). In this study, M06, M06-2X, M05, M05-2X, MPWB1K, and MPW1B95 with the 6-311++G(3df,2p) basis set have been used to evaluate four methylated ammonia trimethylboranes, (CH3)3B-N(CH3)nH3-n (n = 0 to 3), along with H3B-NH3. The predicted binding enthalpies from the new functionals have been compared to experiment as well as previous DFT (B3LYP, MPW1K) and ab initio (HF, MP2, QCISD, and QCISD(T)) results. Previously, only MP2, QCISD, and QCISD(T) were found to model the experimental energetic trend accurately. The mean absolute deviation (MAD) from experimental binding enthalpies for M06-2X and M05-2X is 0.3 and 1.6 kcal/mol, respectively. M06-2X yields a lower MAD than more expensive ab initio methods (MP2 = 1.9 kcal/mol and QCISD = 2.3 kcal/mol) and a comparable MAD to QCISD(T) (MAD = 0.4 kcal/mol). M06-2X is shown to provide a balanced account of the short- and medium-range XC energies necessary to describe the binding enthalpy of coordinate covalent bonds accurately in sterically congested molecular systems.