Ab initio calculations on the C-H…O hydrogen-bonded systems CH₄-H₂O, CH₃NH₂-H₂O and CH₂NH₃⁺-H₂O

Ab initio calculations are reported on the C-H ... O hydrogen bonded complexes CH4-H2O, CH3NH2-H2O and CH3NH3+-H2O. The C-H ... O contacts are found to be attractive interactions. The interaction energies, obtained with the DZP' basis set at the MP2 level of theory are -0.53, -0.61 and -9.3 kcal mol(-1), at equilibrium C ... O distances of 7.3, 7.4 and 6.2 bohr, for CH4-H2O, CH3NH2-H2O and CH3NH3+-H2O, respectively, i.e. there is a large shortening of R(CO) upon the introduction of the NH3+ group. This shortening is even more pronounced for an alternative geometry in which the O atom approaches the donor molecule at the CH3 side. Here R(CO) is shortened from 7.2 to 5.5 bohr by NH3+ substitution. The reason for these large shortenings is that the C ... O potential is characterized by small values of the exchange repulsion, especially for the CH3 approach, and the electrostatic attraction between -NH3+ and the OH2 dipole is sufficient to overcome this repulsion. These findings suggest that the short C ... O distances encountered in some organic solids are not so much the result of hydrogen-bonding interactions, but rather of the softness of the carbon atom.