THEORETICAL-ANALYSIS OF INTERACTIONS AFFECTING (1)J(CH) NMR COUPLINGSIN AN SP(3) HYBRIDIZED CARBON-ATOM .1. THE EXOANOMERIC EFFECT IN 3-METHOXY-1,2,4,5-TETROXANE

It is studied how much and under which conditions the anomeric effect affects a (1)J(CH) coupling constant. 3-Methoxy-1,2,4,5-tetroxane and related compounds are chosen as model systems. With the polarization p ropagator formalism the (1)J((CH)-H-3) coupling is decomposed into con tributions originated in different occupied and vacant localized molec ular orbitals representing, respectively, bonds or lone pairs, and ant ibonding orbitals. It is found that, for a highly polar C-H bond, the anomeric effect increases the s character of its antibonding orbital, giving a large (1)J(CH) coupling. Under these circumstances, the value of this coupling departs notably from the commonly accepted proportio nality between one-bond J(CH) couplings and the s character of its C-H bond.