Natural bond orbital analysis of molecular interactions: Theoretical studies of binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3

The binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3 have been studied by ab initio molecular orbital theory and natural bond orbital (NBO) analysis. Most of the complexes involving N2, O2, F2, CO, and CO2 are found to have both hydrogen-bonded and non-hydrogen-bonded structures. The NBO analysis provides a consistent picture of the bonding in this entire family of complexes in terms of charge transfer (CT) interactions, showing the close correlation of these interactions with the van der Waals penetration distance and dissociation energy of the complex. Contrary to previous studies based on the Kitaura–Morokuma analysis, we find a clear theoretical distinction between H-bonded and non-H-bonded complexes based on the strength of CT interactions. Charge transfer is generally stronger in H-bonded than in non-H-bonded complexes. It plays an intermediate role in non-H-bonded CO2 complexes which have been studied experimentally. However, the internal rotation barrier (1 kcal mol−1) of the H2O⋅⋅⋅CO2 complex is found to be primarily of electrostatic origin with only a small (π-type) CT contribution. The role of electrostatic interactions, effect of electron correlation, and comparison of theory with experiment are also discussed.