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1 H/ 15 N NMR chemical shielding, dipolar 15 N, 2 H coupling and hydrogen bond geometry correlations in a novel series of hydrogen‐bonded acid–base complexes of collidine with carboxylic acids
Author(s) -
Lorente Phillipe,
Shenderovich Ilja G.,
Golubev Nikolai S.,
Denisov Gleb S.,
Buntkowsky Gerd,
Limbach HansHeinrich
Publication year - 2001
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.946
Subject(s) - chemistry , chemical shift , hydrogen bond , molecular geometry , bond order , deuterium nmr , proton nmr , hydrogen , isotropy , geometry , computational chemistry , bond length , nuclear magnetic resonance spectroscopy , crystallography , molecule , stereochemistry , organic chemistry , crystal structure , physics , mathematics , quantum mechanics
A novel series of hydrogen‐bonded solid 1 : 1 acid–base complexes of 15 N‐labeled 2,4,6‐trimethylpyridine (collidine) with carboxylic acids and their hydrogen bond deuterated analogs were synthesized and studied by 1 H magic angle spinning (MAS) and 15 N 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 1 H and 15 N chemical shifts and the 15 N 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 1 H/ 2 H isotope effects on the isotropic 15 N chemical shifts were obtained under MAS conditions. Lineshape simulations of the static 15 N powder spectra revealed the dipolar 2 H, 15 N 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 15 N 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 1 H with the 15 N 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 1 H NMR chemical shift. (3) The 15 N chemical shift anisotropy principal tensor elements δ t , δ r and δ ⟂ (tangential, radial and perpendicular with respect to the pyridine ring) exhibit a linear relation with the isotropic 15 N chemical shifts. A crossing point of δ t = δ r is observed. Further correlations of the hydrogen bond geometry with the geometric H/D isotope effects on the 15 N chemical shifts and with the p K a values of the associated acids are reported. Copyright © 2001 John Wiley & Sons, Ltd.