From the isolated molecule to oligomers and the crystal:: A static density functional theory and Car-Parrinello molecular dynamics study of geometry and potential function modifications of the short intramolecular hydrogen bond in picolinic acid N-oxide

The difference between the calculated metric parameters that are characteristic of the hydrogen bond (H-bond) in the isolated title compound and ones obtained from diffractions on crystals are unusually large (similar to0.1Angstrom) even if high levels of theory are used. The probable origin of this discrepancy lies in strong intermolecular interactions that are investigated here using two models. The first one is the periodic density functional theory (DFT) with plane-wave basis sets and relativistic pseudopotentials (Car-Parrinello molecular dynamics, CPMD 3.5 program). An isolated molecule was also treated by this approach to facilitate the comparison with the local basis set model. The metric parameters calculated by the periodic model are in acceptable agreement with diffraction data, except for the O-H bond length. However, accounting for the quantum nature of the proton brought further improvement. The second model consisted of clusters of 17 molecules up to n = 3. With increase in the cluster size, the calculated geometry approached the experimental one. One-dimensional proton potential functions were calculated with the cluster and the periodic models. The effects of aggregation are markedly reflected in the shapes of the respective functions in that the increasing number of aggregated molecules tends to flatten the potential. The O-H stretching frequencies calculated from the potentials move to lower values with increasing aggregation. The frequency calculated for the crystal phase (1407 cm(-1)) is in accord with the estimated center of the broad absorption appearing in the crystal spectrum. To obtain insight into the origin of the effects of molecular aggregation, we applied the natural bond orbital analysis [Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899-926]. The results suggest that the energy of association that is in the range of 3-5 kcal/mol originates in mutually induced charges.