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

Abstract

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 (ΔδC{triple bond, long}C), the charge difference between these carbons (ΔqC{triple bond, long}C), the occupation quotient (NBO) of anti-bonding π*, and bonding π orbitals (π*C{triple bond, long}C/πC{triple bond, long}C) and the bond length (dC{triple bond, long}C) of the C{triple bond, long}C triple bond. The linear dependence of dC{triple bond, long}C versus π*C{triple bond, long}C/πC{triple bond, long}C 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 C{triple bond, long}C triple bond π-orbitals in the disubstituted alkynes. © 2009 Elsevier Ltd. All rights reserved.