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A 1 H and 13 C NMR study of intramolecular rotations in syn ‐ and anti‐o ‐tolyl‐di‐ tert ‐butyl‐carbinols
Author(s) -
Tiffon Bernard,
Lomas John S.
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.1270220107
Subject(s) - intramolecular force , chemistry , rotation (mathematics) , conformational isomerism , relaxation (psychology) , carbon 13 nmr , force field (fiction) , crystallography , stereochemistry , nuclear magnetic resonance , molecule , physics , organic chemistry , psychology , social psychology , geometry , mathematics , quantum mechanics
The syn and anti rotamers of o ‐tolyl‐di‐ tert ‐butylcarbinol, 2a and 2b, respectively, have been studied by 1 H NMR at 200 MHz and by natural abundance 13 C NMR at 50 MHz. 1 H‐{ 1 H} NOE enhancement factors are consistent with the known structures and the calculated geometries of these compounds. The relaxation time, T 1 , of the 2‐Me protons in 2b is unexpectedly higher than that for 2a. The 13 C relaxation times of the 2‐Me and the quaternary carbon of the tert ‐butyl group are also both higher in 2b than in 2a, suggesting that the rotation of these groups is faster in the less stable isomer. The activation energies for t ‐Bu rotation, measured by 1 H DNMR, agree with this conclusion. Further confirmation is provided by theoretical calculation of the 2‐Me and t ‐Bu rotation barriers based on Allinger's MM2 force field. Comparison of measured ΔG‡ values from this work and from the literature with MM2‐calculated ΔH‡ values indicates that this force field systematically underestimates rotation barriers in open‐chain systems by a factor of approximately 0.64.