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

A novel series of hydrogen-bonded solid 1:1 acid-base complexes of N-15-labeled 2,4,6-trimethylpyridine (collidine) with carboxylic acids and their hydrogen bond deuterated analogs were synthesized and studied by H-1 magic angle spinning (MAS) and N-15 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 H-1 and N-15 chemical shifts and the N-15 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 H-1/H-2 isotope effects on the isotropic N-15 chemical shifts were obtained under MAS conditions. Lineshape simulations of the static N-15 powder spectra revealed the dipolar H-2, N-15 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 N-15 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 H-1 with the N-15 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 H-1 NMR chemical shift. (3) The N-15 chemical shift anisotropy principal tensor elements delta (t), delta (r) and delta (tangential, radial and perpendicular with respect to the pyridine ring) exhibit a linear relation with the isotropic N-15 chemical shifts. A crossing point of delta (t) = delta (r) is observed. Further correlations of the hydrogen bond geometry with the geometric H/D isotope effects on the N-15 chemical shifts and with the pK(a) values of the associated acids are reported. Copyright (C) 2001 John Wiley & Sons, Ltd.