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 (14)N nuclear quadrupole resonance spectra have been measured. The (14)N 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. (14)N nuclear quadrupole resonance is used to distinguish between these two possibilities. Rather large (14)N quadrupole coupling constants in the investigated cocrystals show that in these systems proton transfer O-H···N → O(-)···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 π-electron orbital. The analysis shows that the two effects are correlated.