Karplus equation for³J_HH spin-spin couplings with unusual³J(180°) < ³J(0°) relationship

Vicinal ³J_HH coupling constants for monosubstituted ethane molecules present the unusual relationship ³J_HH (180°) < ³J_HH (180°) when the substituent contains bonding and antibonding orbitals with strong hyperconjugative interactions involving bond and antibond orbitals of the ethane fragment. This anomalous behavior is studied as a function of the substituent rotation for three model systems (propanal, thiopropanal, and 1-butene) at the B3LYP/TZVP level. The consistency of this level of theory to study this problem is previously established using different ab initio methods and larger basis sets. The origin of the unusual ³J_HH(180°) - ³J_HH(180°) relationship is attributed to simultaneous σ/π hyperconjugative interactions ^σC _α-H_α → π^*C _-c=x, and ^σC_α-C_β → π^*C_c-x. These interactions depend on the substituent rotation and their effects are different for ³J _HH(180°) than for ³J_HH(180°). The modelization carried out shows an increase of those effects as the substituent changes from weaker (CH=CH₂) to stronger (CH=S) electron acceptor π^*C=X.