Carbonyles tres decales par rapport a l'eclipse dans les aldehydes et les cetones aliphatiques satures: analyse conformationnelle theorique

The theoretical conformational analysis of two series of molecules HCOR, MeCOR is treated. Spectroscopic data 15] have indicated the existence of non-eclipsed conformations which become more important with increasing steric hindrance of the group R.All the methods used (semi-empirical quantum methods, P.C.I.L.O., I.N.D.O., C.N.D.O./2, and the Liquori empirical potential method) lead to the following stable conformations: with the carbonyl group exclusively eclipsed for slightly substituted R (R = Me, Et, tBu); with the carbonyl eclipsed and also staggered (? ? 60°) for moderately substituted compounds (MeCOPr, MeCOiBu, MeCOCiPr(Me)₂) (low rotational barriers permitting equilibria between eclipsed and staggered conformations); with the carbonyl exclusively staggered (? ? 60°) for more hindered compounds (MeCOCH₂tBu, MeCOCH(tBu)Me, MeCOC(Me),tBu, MeCOCH(tBu)₂).Contradictory results are obtained for certain compounds (iPrCOMe, tBuCH₂CHO for example) : the calculated changeover from eclipsed to staggered conformations differentiates the semi-empirical methods from the empirical potential method used.