Quantification of the push–pull Effect in disubstituted alkynes – Application of occupation quotients π*/π and 13C chemical shift differences ΔδCC

Structures, 13C chemical shifts, and the occupation quotients of anti-bonding π* and bonding π orbitals of the CC triple bond along a series of push–pull alkynes (p)XC6H4C(O)CCNHC6H4Y(p) (X,Y = H, Me, OMe, NMe2, NO2, COMe, COOMe, F, Cl, Br) were computed at the DFT level (B3LYP/6-311G**) of theory. Both the stereochemistry (cis/trans-isomers) by steric twist and the push–pull character by both 13C chemical shift differences (ΔδCC) and the occupation quotient (π*CC/πCC) were studied; the latter two parameters can be readily employed to precisely quantify the push–pull effect in alkynes.