1H/15N NMR chemical shielding, dipolar15N,2H coupling and hydrogen bond geometry correlations in a novel series of hydrogen-bonded acid-base complexes of collidine with carboxylic acids

Abstract

A novel series of hydrogen-bonded solid 1 : 1 acid–base complexes of 15N-labeled 2,4,6-trimethylpyridine (collidine) with carboxylic acids and their hydrogen bond deuterated analogs were synthesized and studied by 1H magic angle spinning (MAS) and 15N cross-polarization NMR with and without MAS. Not only zwitterionic complexes with the H-bond proton closer to nitrogen than to oxygen but also molecular complexes have been observed, where the proton is located closer to oxygen. For these complexes, the isotropic 1H and 15N chemical shifts and the 15N chemical shielding tensor elements were measured (the latter by lineshape simulation of the static powder spectra) as a function of the hydrogen bond geometry. For the deuterated analogs 1H/2H isotope effects on the isotropic 15N chemical shifts were obtained under MAS conditions. Lineshape simulations of the static 15N powder spectra revealed the dipolar 2H,15N couplings and hence the corresponding distances. The results reveal several hydrogen bond geometry–NMR parameter correlations which are analyzed in terms of the valence bond order model. (1) The collidine and apparently other pyridines isotropic 15N chemical shifts depend in a characteristic way on the nitrogen–hydrogen distance. This correlation can be used in the future to evaluate hydrogen bond geometries and solid-state acidities in more complicated systems. (2) A correlation of the 1H with the 15N isotropic chemical shifts is observed which corresponds to the well-known hydrogen bond geometry correlation indicating a strong decrease of the A···B distance in an AHB hydrogen bond when the proton is shifted to the hydrogen bond center. This contraction is associated with a low-field 1H NMR chemical shift. (3) The 15N chemical shift anisotropy principal tensor elements δt, δr and δ⟂ (tangential, radial and perpendicular with respect to the pyridine ring) exhibit a linear relation with the isotropic 15N chemical shifts. A crossing point of δt = δr is observed. Further correlations of the hydrogen bond geometry with the geometric H/D isotope effects on the 15N chemical shifts and with the pKa values of the associated acids are reported. Copyright © 2001 John Wiley & Sons, Ltd.