Nuclear Quadrupole Resonance Investigation of Hydrogen Bonding in Some Cocrystals of 2,3,5,6-Tetramethylpyrazine and Carboxylic Acids

Cocrystals of 2,3,5,6-tetramethylpyrazine and several carboxylic acids have been prepared, and the complete N-14 nuclear quadrupole resonance spectra have been measured. The N-14 nuclear quadrupole resonance spectra have been used to check whether the cocrystals are indeed formed and to investigate the hydrogen bonding scheme of 2,3,5,6-tetramethylpyrazine molecules. Since a 2,3,5,6-tetramethylpyrazine molecule has two hydrogen bond acceptors, it may form either 1:1 or 1:2 cocrystals with carbocylic acids. N-14 nuclear quadrupole resonance is used to distinguish between these two possibilities. Rather large N-14 quadrupole coupling constants in the investigated cocrystals show that in these systems proton transfer O-H center dot center dot center dot N -> O-center dot center dot center dot H-N+ does not occur. The quadrupole coupling tensor in 2,3,5,6-tetramethylpyrazine cocrystals has been analyzed in terms of the deformation of the electron lone pair orbital and population of the pi-electron orbital. The analysis shows that the two effects are correlated.