Ab initio molecular-orbital study of hydrogen-bonded complexes of carbonyl aliphatic compounds and hydrogen fluoride

Ab initio molecular-orbital calculations at the HF/6-31G(d,p) level were used to investigate the hydrogen bonding between open-chain aliphatic carbonyl compounds and hydrogen fluoride. The carbonyl compounds studied are H2CO, HCOOH, HCOSH, HCOOCH3, HCONH2, HCONO2, HCOCN, HCOF, HCOCl, HCOCH3, HCOCF3, CH3COOH, CH3COSH, CH3COOCH3, CH3CONH2, CH3CONO2, CH3COCN, CH3COF, CH3COCl, CH3COCH3, and CH3COCF3. Geometry optimization and vibrational-frequency and infrared-intensity calculations at the optimized geometry were performed for isolated and hydrogen-bonded systems. The estimated energies of hydrogen-bond formation were corrected for zero-point vibrational energy and basis-set superposition error. In agreement with expectations a linear relation (R = 0.994) between the energy of hydrogen-bond formation, Delta E, and the H-F stretching frequency shift, Delta nu(H-F), was obtained for the systems studied. A linear dependency was also found between Delta E and the change of H-F bond length, Delta r(H-F) (R 0.994). Effective bond charges of the hydrogen bond, delta(O ... H) for the series of compounds studied were evaluated from the theoretically derived dipole-moment derivatives. A satisfactory correlation (R = 0.938) between delta(O ... H) and the energy of hydrogen bond formation, Delta E, was obtained. The attempt to explain the differences in hydrogen-bond strength, Delta E, with the variation of the carbonyl oxygen atomic charges in the isolated molecules showed that neither Mulliken nor CHELPG and MK atomic charges can be successfully used for the purpose. However, the molecular electrostatic potential at the carbonyl oxygen in the isolated molecules, Phi(0), correlates in a quite satisfactory manner (R = 0.979) with the energy of hydrogen-bond formation for the entire series of compounds. This result appears quite significant in view of its importance for understanding the mechanisms of intermolecular interactions leading to hydrogen bonding.