Restricted Rotation. Part-IV : Determination of Barrier to Rotation Around Some Crowded C-C and C-N Bonds by Dynamic Nuclear Magnetic Resonance

Department of Chemistry, Indian Institute of Technology, Kharagpur-721 302

Temperature dependence of the nmr spectra of a few typical methyl 2-aryl-6- oxocycloher-1-enyl-acetates (I)-(VII) has been reinvestigated. The free energy, enthalpy and entropy of activation for the restricted rotation about the arylcyclohexe­none bond have been determined by line shape analysis of the spectra of the side chain methylene protons using approximate equations. The free energy of activation for conformational inversion of the molecule (VII) now found to be appreciably higher than that reported earlier although the result is still to be rationalised. The barrier energy around C-N bond in two 1-aryl-1,4^-dihydro-4,4.6-trimethyl-2-thiopyrimidones (XIII) and (XV) has also been measured by dynamic nmr taking advantage of chemical nonequivalence of the geminal methyl groups and is found to be quite high (\(ca\) 98 kJ mol^-1).