AB-INITIO STUDY OF THE HYDROGEN-BONDED BRIDGING NH3-PYRIDONE COMPLEX

Ab initio calculations have been carried out at the MP2/6-31+G(d,p) level of theory to determine the equilibrium structure of the NH3:pyridone complex in the N-H and C=O region of pyridone. The NH3 molecule is both a proton donor and a proton acceptor in this complex, which is stabilized by nonlinear N-H...N and N-H...O hydrogen bonds. Computed and experimental rotational constants for NH3:pyridone and the barrier to rotation of the NH3 molecule are in good agreement. There is also good agreement between the computed and the experimental structure of the NH3:pyridone complex in the region of the N-H...N hydrogen bond in which NH3 is the proton acceptor molecule. However, differences are found between the experimental and the computed descriptions of the N-H...O hydrogen bond in which NH3 is the proton donor. In particular, the experimental structure has an N-H...O hydrogen bond which deviates from linearity to a much greater extent and a very long H...O distance. Sources of this discrepancy are discussed. The electronic binding energy of the NH3:pyridone complex at MP2/cc-pVTZ+// MP2/6-31+G(d,p) is 10.9 kcal/mol, leading to a room temperature binding enthalpy of 9.3 kcal/mol. The NH3: pyridone complex is approximately 1 kcal/mol less stable than the corresponding H2O:pyridone complex.