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to form sheets parallel to a crystallographic plane, with the alkyl substituents on the nitrogen atoms essentially perpendicular to the sheets.Along any chain, acfiacent NH+---0 hydrogen bonds are separated by about 6.5 A. The zwitterion dipole moment vectors are inclined at alternating angles of +9 and -9°to the chain axis, with |9| ~20°. For such a lattice, electrostatic calculations in the point-dipole approximation20 have shown that dipole-dipole interactions are energetically very important within a given sheet, but parallel sheets are held so far apart by the alkyl groups that electrostatic interactions between them are negligible.The present experimental results imply that, within such a two-dimensional lattice of dipoles at low temperature, the proton in a given hydrogen bond is subject to a slightly asymmetric,

ABSTRACT:The effect of the basicity of methyl-amines on hydrogen bonding HB with HCOOH is examined in both gas and solution phases. In the gas phase, the strength Ž . of HB may be related to the proton affinity PA difference between the carboxylate anion Ž y . Ž . and the methyl-amine, ⌬PA s PA HCOO y PA NR . The changes in the driving 3 potential ⌬PA are explained on the basis of electronic substituent effects. The electronic substituent effects are rationalized in terms of local reactivity indices such as the Fukui function and the local hardness and softness at the basic center. A simple model is then proposed to explain the enhancement HB in the solution phase. The HB pattern in the solution phase is changed by electrostatic and nonelectrostatic solvation of the zwitterionic and neutral species in equilibrium.

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The reaction field (RF) model of solvent effects, implemented within the SCF-CND0/2 scheme of calculation, has been applied to analyze the proton transfer in the NH, . . . HCOOH system in the presence of several polarizable media. The aim of such a study was to characterize the tautomeric equilibrium between the neutral and zwiterionic forms of H-bonded amino acids in aprotic solvents. Qualitative results concerning the energetics of this equilbrium show the stabilization of two different H-bonded complexes, corresponding to two separate minima in the free energy surface. These well known double minima potentials are found to be dependent on both the intermolecular N-0 distance and the strength of the reaction field. The behavior of this model is qualitatively consistent with experimental observations of nitrogen-substituted amino acids in solution: both show, for low values of the dielectric constant, tautomeric equilibria where the H-bonded complexes appear to be more stable than the corresponding monomeric forms. The charge transfer process associated with the proton migration along the H-bond is also discussed. It is found that the amount of charge transferred increases with the N-0 distance and with the RF strength. In order to test the general approach and compare it with previous work, calculations on the real monomeric systems glycine, p-alanine, and y-amino butyric acid was also performed.