MOLECULAR-ORBITAL THEORY OF THE HYDROGEN-BOND .21. 2-MONOSUBSTITUTED PYRIDINES AS PROTON ACCEPTORS

Ab initio SCF calculations with the STO-3G basis set have been performed to investigate substituent effects on the hydrogen bonding ability of 2-R-pyridines as proton-acceptor molecules, with the substituents including CH3, NH2, OH, F, C2H3, CHO, and CN. The equilibrium water-2-R-pyridine dimers generally have open structures stabilized by the formation of a linear hydrogen bond through the nitrogen lone pair of electrons, except for the water-2-aminopyridine dimer, which is cyclic, and the water-2-hydroxypyridine dimer, which has 2-hydroxypyridine as the proton donor. The relative orientation of the pyridine ring and the water molecule in an equilibrium dimer is determined by secondary substituent effects which include long-range interactions and the alignment of the dipole moment vectors of the proton-donor and proton-acceptor molecules. The hydrogen bonds in the water-2-R-pyridine dimers are weaker than in the water-pyridine dimer in all cases except the water-2-aminopyridine dimer. Primary substituent effects alter the electronic environment at the nitrogen, and are reflected in the n orbital energies of the pyridine bases. These energies are linearly related to the hydrogen-bond energies of a particular set of dimers in which the pyridine ring is perpendicular to the plane of the water molecule. Secondary substituent effects also influence the relative stabilities of the equilibrium water-2-R-pyridine dimers. © 1979, American Chemical Society. All rights reserved.