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15 N NMR studies of guanidines. I—hydrogen bonding and electronic effects in conformationally flexible guanidines
Author(s) -
Bedford Geoffrey R.,
Taylor Peter J.,
Webb Graham A.
Publication year - 1995
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.1260330510
Subject(s) - chemistry , heteroatom , intramolecular force , substituent , hydrogen bond , ring (chemistry) , resonance (particle physics) , solvent , nitrogen , nuclear magnetic resonance spectroscopy , guanidine , acceptor , proton nmr , crystallography , stereochemistry , photochemistry , molecule , organic chemistry , condensed matter physics , physics , particle physics
Hydrogen bonding and electron distribution in some heterocyclic guanidines were studied by 15 N NMR spectroscopy, using other substituted guanidines as points of reference. Solvent effects were used to distinguish the 15 N NMR signals for their two sp 2 nitrogen atoms, since ring nitrogen forms an intramolecular hydrogen bond and so is much less sensitive to solvent than is the guanidine unit. The effect of substituent is discussed in terms of competition between through‐resonance and the more localized resonance of the π‐inductive effect. These are favoured, respectively, by oxygen (nitro, acyl) and nitrogen (cyano) acceptor groups, whereas heterocycles, also nitrogen acceptors, occupy an intermediate position. Among the latter class, there are indications that the balance between these forms of resonance may be influenced by competitive conjugation with ring π‐donor heteroatoms.