Molecular orbital theory of the hydrogen bond. X. Monosubstituted carbonyls as proton acceptors

A b initio SCF calculations with a minimal STO−3G basis set have been performed to determine equilibrium structures and energies for a series of mixed dimers formed from water and substituted carbonyl compounds. These dimers may be represented by the general formula HOH⋅⋅⋅OCHR, where R may be CH3, NH2, OH, F, CHO, and C2H3. For each dimer in this series except water−formic acid, two equilibrium structures have been found, in which the substituent R is ’’cis’’ or ’’trans’’ to the proton donor water molecule with respect to the carbonyl CO bond. Except for a cyclic equilibrium water−formamide dimer, all equilibrium dimers have open−chain trans structures consistent with the General Hybridization Model. There is only a small variation in the hydrogen bond energies of equilibrium dimers HOH⋅⋅⋅OCHR, except for the water−formamide dimers which are significantly more stable than all others in this series. The structures and energies of the equilibrium dimers HOH⋅⋅⋅OCHR have been analyzed in terms of the effect of the substituent R on the proton accepting ability of the substituted carbonyl compound RCHO.