Hindered rotation in systems of the type R'YC(=X)NR2, where X,Y = 0 , O ; 0,s; S,O; S,S; R = CHI, CzH6; R' = CH3, Si(CH&, Sn(CH3)3, has been investigated by variable temperature nmr spectroscopy. Rotational parameters were calculated by the total line shape and intensity ratio methods. Effects of substituent groups X, Y, and R' on the free energy of activation are discussed in terms of their effect on the relative importance of the contributing resonance structures.lthough the synthetic aspects of the insertion of (14) M. T. Rogers and . I .

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