Distinction of Push,pull effect and steric hindrance in disubstituted alkynes

The push,pull effect in two series of disubstituted alkynes was studied at the DFT level [B3LYP/6-311G(d)] by application of the 13C chemical shift differences (GIAO) between the alkyne carbon atoms (ΔδCtriple bond; length of mdashC), the charge difference between these carbons (ΔqCtriple bond; length of mdashC), the occupation quotient (NBO) of anti-bonding π∗, and bonding π orbitals (π∗Ctriple bond; length of mdashC/πCtriple bond; length of mdashC) and the bond length (dCtriple bond; length of mdashC) of the Ctriple bond; length of mdashC triple bond. The linear dependence of dCtriple bond; length of mdashC versus π∗Ctriple bond; length of mdashC/πCtriple bond; length of mdashC quantifies changes in the push,pull effect while deviations from the latter correlation indicate and ascertain quantitatively to what extent steric hindrance restricts the strain-less conjugation of the Ctriple bond; length of mdashC triple bond π-orbitals in the disubstituted alkynes.