Ab initio benchmark study of (2-pyridone)2, a strongly bound doubly hydrogen-bonded dimer

The 2-pyridone dimer, (2PY)(2), has two antiparallel N-H...O H-bonds analogous to nucleobase dimers. The gas-phase rotational constants and all six intermolecular vibrational frequencies of (2PY)(2) have been previously measured, providing benchmarks for theory. The structure, rotational constants, vibrational frequencies, and binding and dissociation energies of (2PY)(2) were calculated at the correlated level using second-order Moller-Plesset perturbation theory (MP2) with medium to very large basis sets. The MP2 binding energy limit was extrapolated to the complete basis set (CBS) as D-e,D-CBS = -22.62 +/- 0.07 kcal/mol. Higher order correlation energy contributions to D-e at the CCSD(T) level are destabilizing (+0.77 kcal/mol). This implies that (2PY)(2) is the most strongly bound doubly hydrogen-bonded dimer known so far. The Hartree-Fock contribution to D-e,D-CBS is only approximate to65%. Several medium-size basis sets yield MP2 D-e's within +/-5% of the CBS value, as well as structure, rotational constants, and intermolecular vibrations in good agreement with experiment. The PW91 density functional method also shows very good performance with regard to all properties calculated, comparable to MP2. The results imply that correlated methods combined with carefully chosen medium-size basis sets may give near-quantitative results for the structures, binding energies, and intermolecular vibrational frequencies of nucleic acid base dimers.