Self‐consistent reaction field calculations on the proton transfer in ammonia‐formic acid systems as a model for hydrogen bonding in amino acids in solution

The reaction field (RF) model of solvent effects, implemented within the SCF‐CNDO/2 scheme of calculation, has been applied to analyze the proton transfer in the NH3…HCOOH system in the presence of several polarizable media. The aim of such a study was to characterize the tatutomeric 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 NO 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 cor

13C-NMR and theoretical studies of internal rotation in methylated anilines

The conformational properties of ten ring-methylated N-methyl- and N,N-dimethylanilines have been studied using 13C-NMR chemical shifts and spin-lattice relaxation times in CDCl3, and semi-empirical (AM1) quantum-chemical calculations. The experimental results indicate that, like aryl methyl ethers, N-methylanilines prefer conformations in which the N-methyl carbon lies near the ring plane. Ortho-substitution in these compounds, while forcing the N-methyl group to adopt an anti orientation with regard to the ortho substituent, does not induce any important changes from the vantage point of the electron donor ability of the amine function and therefore does not affect the N-methyl 13C chemical shifts or spin-lattice relaxation times to any appreciable extent. The preferred conformations of ortho-unsubstituted N,N-dimethylanilines leave the N-methyl carbon atoms oscillating on either side of the ring plane, but the conformational space of these compounds is strongly limited by ortho-met

Quantum-Chemieal Modeling of Cateeholamine Storage Including Continuum Solvent Effeets

A model for catecholamine storage in vesicles is analyzed within the ,CRI CNDO/2 approach inc1uding conlinuum solvenl crfcels. The model considers lhe approach of calionic norepinephrine (NE) 10 a positively charged guanidinium moiely. lon-pair formalion is found for ¡he whole range of dielectrie eonslanls. [ven though slablc slales of II-bonded parlnns are found for Jarge diclcctric constants, this proecss is ruled out to occur bccause it involvcs 100 high cnergies. 11 appears that the medium's polarily is determinant in lowering the energy barrier between the ion-pair complex and the separatcd parlners. Thus, as the mediul11 dielectric constanl increases. ¡he equilibrium between the two statcs is enhanced. IntroductionThis work has received financial support from FONDECYT (Projects l1ll-1988 and 915-89)

Basicity and solvent effects on hydrogen bonding in NR3 ⋯ HCOOH (R = H, CH3) model systems

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 of HB may be related to the proton affinity (PA) difference between the carboxylate anion and the methyl-amine, ΔPA = PA(HCOO-) - PA(NR3). The changes in the driving potential ΔPA 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. © 1999 John Wiley & Sons, Inc