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

• Published in: Physical chemistry chemical physics : PCCP 2017, Vol.19 (2), p.1010-1028
• Main Authors: Koeppe, B, Pylaeva, S A, Allolio, C, Sebastiani, D, Nibbering, E T J, Denisov, G S, Limbach, H-H, Tolstoy, P M
• Format: Article
• Language: English
• Subjects: Chemical bonds; Chloroacetic acid; Hydrogen bonds; Nuclear magnetic resonance; Pyridines; Solvents; Spectra; Stretching
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp06677a

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 CD Cl at low temperature by H and C NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (O H-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 CD Cl 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.