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Dynamic proton magnetic resonance studies on complex spin systems. Non‐mutual three‐spin exchange in four 1‐substituted cyclohexanes‐2,2,3,3,4,4,5,5‐ d 8 and mutual four‐spin exchange in cyclohexane‐1,1,2,2,3,3,4,4‐ d 8
Author(s) -
Höfner Dieter,
Lesko Susan A.,
Binsch Gerhard
Publication year - 1978
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.1270110405
Subject(s) - pseudorotation , cyclohexanes , chemistry , substituent , cyclohexane , ring (chemistry) , deuterium , computational chemistry , proton , stereochemistry , physics , atomic physics , organic chemistry , quantum mechanics
The intrinsic advantage proffered by coupled three‐proton spin systems in the quantitative study of conformational equilibria and equilibrations, as elaborated and exemplified in the preceding publication, is exploited for the determination of precise thermodynamic and dynamic parameters for octadeuteriocyclohexanes containing a cyano, chloro, bromo or deuteriomethoxy substituent and hydrogen atoms on the substituted and on one contiguous carbon. Dynamic quantities for the ring reversal of cyclohexane‐1,1,2,2,3,3,4,4‐ d 8 , obtained by bandshape analysis of the deuterium‐decoupled proton NMR spectra broadened by mutual four‐spin exchange, are also reported. The measured standard entropy differences between the axial and equatorial conformational isomers of the monosubstituted cyclohexanes show that, in addition to the rotational entropy, there must be a significant contribution from the vibrational partition functions. The experimental activation entropies for the cyclohexane chair‐to‐twist‐boat conversion are at variance with conclusions drawn from naive symmetry considerations, but agree with the more sophisticated treatment of Pickett and Strauss; the data establish that there is virtually free pseudorotation in the transition states of the ring reversal for cyclohexane itself as well as for the monosubstituted derivatives studied in this investigation.