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

✍ Scribed by Phillipe Lorente; Ilja G. Shenderovich; Nikolai S. Golubev; Gleb S. Denisov; Gerd Buntkowsky; Hans-Heinrich Limbach

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 394 KB
• Volume: 39
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.946

✦ Synopsis

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 d t , d r and d ? (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 d t = d 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 pK a values of the associated acids are reported.