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Effect of molecular motion and solvent interactions on nitrogen‐15 relaxation in anilines
Author(s) -
Lambert Joseph B.,
Stec Daniel
Publication year - 1984
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270220506
Subject(s) - chemistry , solvent , aniline , aryl , amine gas treating , nitrogen , hydrogen bond , molecule , dipole , ion , solvent effects , relaxation (psychology) , ring (chemistry) , computational chemistry , photochemistry , medicinal chemistry , crystallography , organic chemistry , alkyl , psychology , social psychology
Dipolar relaxation of 15 N in anilines and anilinium ions is influenced by overall motion of the molecule, by rotation about the aryl–‐nitrogen bond, by inversion of the aniline nitrogen and by interactions of the NH 2 or NH 3 + group with the solvent. These factors are assessed by comparison of the 13 C and 15 N dipolar relaxation times as a function of para ‐substitution on the aryl ring. In the anilines (solvent CDCl 3 ), electron withdrawal brings about faster relative motion of the amine side‐chain, contrary to expectation from consideration of CN rotation but in agreement with the effects from nitrogen inversion. The 15 N dipolar relaxation time correlates with the Hammett σ p . For the anilinium ions (solvent Me 2 SO‐ d 6 ), there is no correlation with σ p and no qualitative relationship with either CN rotation or N inversion. Nitrogen‐15 relaxation, corrected for overall motion as judged by ring 13 C relaxation, correlates with the inductive parameter σ I . Electron withdrawal through induction reduces hydrogen bonding and increases side‐chain mobility. For most of the anilines and for all of the anilinium ions, solvent interactions cause the nitrogen side‐chain to be less mobile than the aryl ring. Under these circumstances, the Woessner approach cannot be used to calculate barriers. The hydrogen bond donor properties of the anilines are reduced in the absence of electron‐donating substituents, and the first barriers to NH 2 rotation/inversion were calculated by this procedure: aniline in CDCl 3 3.5 kcal/mol, p ‐chloroaniline in CDCl 3 3.4 kcal/mol and p ‐nitroaniline in acetone 3.8 kcal/mol.