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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 De,CBS = −22.62 ± 0.07 kcal/mol. Higher order correlation energy contributions to De 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 De,CBS is only ≈65%. Several medium-size basis sets yield MP2 De's within ±5% of the CBS value, as well as structure, rotational constants, and intermolecular vib...