Polar solvent fluctuations drive proton transfer in hydrogen bonded complexes of carboxylic acid with pyridines: NMR, IR and ab initio MD study

Abstract

We study a series of intermolecular hydrogen-bonded 1 : 1 complexes formed by chloroacetic acid with 19 substituted pyridines and one aliphatic amine dissolved in CDCl at low temperature by H andC NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (OH-N) ones, while NMR spectra show the formation of short strong hydrogen bonds in intermediate cases. Analysis of C[double bond, length as m-dash]O stretching and asymmetric CO stretching bands in FTIR spectra reveal the presence of proton tautomerism. On the basis of these data, we construct the overall proton transfer pathway. In addition to that, we also study by use of ab initio molecular dynamics the complex formed by chloroacetic acid with 2-methylpyridine, surrounded by 71 CDCl molecules, revealing a dual-maximum distribution of hydrogen bond geometries in solution. The analysis of the calculated trajectory shows that the proton jumps between molecular and zwitterionic forms are indeed driven by dipole-dipole solvent-solute interactions, but the primary cause of the jumps is the formation/breaking of weak CHO bonds from solvent molecules to oxygen atoms of the carboxylate group.

Complexity in Acid–Base Titrations: Multimer Formation Between Phosphoric Acids and Imines

“…Solutions of Brønsted acids with bases have been intensively studied as model systems to understand the fundamentals of for example, strong hydrogen‐bonding, proton transfer pathways, Brønsted acid organocatalysis, or mechanisms of enzymatic activity . In general, upon association of an acid and a base, the proton is shared between the two molecules and the quantum nature of the light proton results in substantial delocalization of the proton between the acidic and the basic group .…”

“…This delocalization also limits the experimental and computational methods that are suitable to study such interactions. Experimentally, valuable insight into the location of the proton and as such the nature of the bond between the acids and bases has been obtained from nuclear magnetic resonance (NMR) experiments at reduced temperatures . For specific acids and bases the information obtained using NMR has been complemented by optical or vibrational spectroscopies .…”

“…Experimentally, valuable insight into the location of the proton and as such the nature of the bond between the acids and bases has been obtained from nuclear magnetic resonance (NMR) experiments at reduced temperatures . For specific acids and bases the information obtained using NMR has been complemented by optical or vibrational spectroscopies . To date many studies have focused on carboxylic acids and it has been concluded that the nature of this strong bond varies between a hydrogen‐bonded complex with the proton located in the proximity of the acid and an ion pair (full proton transfer), depending on the chemical nature of the acid and the base …”

“…For specific acids and bases the information obtained using NMR has been complemented by optical or vibrational spectroscopies . To date many studies have focused on carboxylic acids and it has been concluded that the nature of this strong bond varies between a hydrogen‐bonded complex with the proton located in the proximity of the acid and an ion pair (full proton transfer), depending on the chemical nature of the acid and the base …”

Triel–hydride triel bond between ZX3 (Z = B and Al; X = H and Me) and THMe3 (T = Si, Ge and Sn)

“…However, such proton transfer is different from that caused by a hydrogen bond. An alkaline H atom moves from a Lewis base to a Lewis acid in the ion‐pair triel–hydride interaction, while an acidic H atom transfers along the reverse direction in hydrogen bonding . Similarly, the triel–hydride interaction in THMe 3 ···AlH 3 is also stronger than the π triel bond in C 6 H 6 ···AlH 3 (11.5 kcal mol −1 ) and the single‐electron triel bond in CH 3 ···AlH 3 (8.23 kcal mol −1 ) .…”