MOLECULAR-ORBITAL STUDIES OF THE NITROMETHANE-AMMONIA COMPLEX - AN UNUSUALLY STRONG C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-N HYDROGEN-BOND

Ab initio molecular orbital calculations using various basis sets up to D95++(d,p) with full geometry optimization at the second order Moller-Plesset (MP2) level have been performed on several possible geometries of the 1:1 complex of nitromethane and ammonia. The complex is stabilized by 6.38 kcal/mol at MP2/D95++(d,p). After application of the counterpoise correction (CP) for the basis set superposition error (BSSE), the stabilization becomes 4.40 kcal/mol. Corrections for zero-point vibrational energy (ZPVE) and other vibrational corrections lower the stabilization by 1.38 to give a stabilization enthalpy of 3.02 kcal/mol. As the combination of CP and ZPVE is known to overcorrect, the interaction energy should be somewhat greater than this value. The preferred geometry involves a C-H...N and two N-H...O interactions. The CH...N interaction can be characterized as an H bond, while the N-H...O interactions seem more characteristic of electrostatic interactions. Rotation of the nitromethane to break the N-H...O interactions lowers the stabilization energy by only 0.99 kcal/mol before consideration of ZPVE (which would increase the relative stabilization of this rotated structure). Two other likely geometries, such as that optimizing the individual H bonds between the each of nitro oxygens and two of the ammonia H's, or that with a three center O...H...O bond between the nitro and the ammonia, are shown to be less stable. The former is predicted to be a transition state. Semiempirical calculations were performed for comparison. While the AM1 results agree with the ab initio calculations, the PM3 and SAM1 results do not.