MOLECULAR-ORBITAL STUDIES OF C-H...O H-BONDED COMPLEXES

Fully optimized ab initio and semiempirical molecular orbital calculations are reported on complexes containing C-H...O interactions that are prototypes of interactions commonly found in crystals. The ab initio calculations were performed both at the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) levels using the 6-31G(d,p) and D95++(d,p) basis sets. The semiempirical calculations used the AMI, PM3, and SAM1 methods. The complexes considered are those of acetylene or hydrogen cyanide with water, formaldehyde, and ozone. The interaction energies, geometries, and vibrations are presented with corrections for zero-point vibration energy (ZPVE), basis set superposition error (BSSE), and enthalpy at 298 K, where appropriate. The fully corrected H-bonding interactions (kcal/mol) at the MP2/D95++(d,p) level are -3.79, -2.74, and -1.15 for HCN and -2.19, -1.15, and -0.49 for HCCH for interactions with H2O, H2C=O, and 03, respectively. The potential surfaces were calculated to be rather flat. In particular, the energetic differences between planar and nonplanar complexes with H2O and symmetric and unsymmetric three-center H bonds in complexes with 03 are insignificant. The fact that BSSE can influence the shape of the potential surface and, consequently, the ZPVE is demonstrated. The unscaled MP2 calculated vibrations agree reasonably well with experimentally derived harmonics, while the HF vibrations were about 12% too high. Agreement of AM1 and the best ab initio calculations was generally good with respect to both energetics and structure. SAM1 consistently predicted stronger complexes with shorter H-bonding interactions, while PM3 was erratic. The semiempirical vibrations were 5-10% too high and had much larger standard deviations than the ab initio results.