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8686567 
Journal Article 
Application of Electron Spin Resonance Spectroscopy to Problems of Structure and Conformation. XV. Conformational Analysis of Semidiones Derived from Cyclobutane, Cyclopentane, Cyclohexane, and Cycloheptane 
Russell, GA; Underwood, GR; Lini, DC 
1967 
Yes 
Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 
89 
25 
6636-6644 
English 
The effect of temperature on the esr spectra of a number of medium-sized cycloalkanesemidiones has been investigated. The effect of temperature on the population of the equatorial (more stable) and axial conformations of the methylcyclohexanesemidiones yields enthalpy differences of 0.7 and 1.4 kcal/mole (ΔS ≅ 0) for the 3- and 4-methyl substituents. The treatment is extended to di- and trialkylated cyclohexanesemidiones. Cyclohexanesemidione and several symmetrically substituted derivatives yielded coalescence temperatures and demonstrated line-broadening effects above and below the coalescence temperature. Coalescence temperatures occurat conformational lifetimes of -∼10-7 sec. Cyclohexanesemidione undergoes the interconversion of halfchair conformations with an enthalpy of activation of 4.0 kcal/mole, entropy of activation +1 eu. 3,3-Dimethyl- or 4,4-dimethylcyclohexanesemidiones have similar enthalpies of activation (3.9, 3.2 kcal/mole) but less favorable entropies of activation (-6, -8 eu). 3,3,5,5-Tetramethylcyclohexanesemidionehas an enthalpy of activation of only 2.6 kcal/mole but the entropy of activation for ring inversion is - 8 eu. The treatment has been extended to N-methylpiperidine-3,4-semidione which is much more conformationally stable than cyclohexanesemidione (coalescence temperature ∼+60 and -85°, respectively) and to cycloheptanesemidione for which a coalescence temperature could not be observed because of conformational stability. 1967, American Chemical Society. All rights reserved.