Substituent effects on benzdiyne: A density functional theory study

Density functional theory (DFT) studies were performed to investigate the effect of substituents on the properties of benzdiyne derivatives. Twelve substituted benzdiyneS-C6X2, where X = F, Cl, Br, Me, CF3, CN, OH, NO2, NH2, OMe, NMe2, and Ph-were considered along with the unsubstituted 1,4-benzdiyne. The structures, vibrational frequencies, and IR intensities of these benzdiynes were studied with a popular three-parameter hybrid density functional (B3LYP) combined with the split-valence 6-31G(d) basis set and Dunning's correlation-consistent polarized triple-zeta (cc-pVTZ) basis set. The relative stabilities of the substituted benzdiynes were studied with the help of reaction energies of isodesmic reactions, which showed that the electron-withdrawing groups destabilized the benzdiynes more than they did the corresponding benzenes, whereas the electron-donating groups stabilized the benzdiynes more than they did their benzene counterparts. Correlation analyses revealed that field/inductive effects played a more important role than did resonance effects. The changes in atomic charges and spin populations due to the substituents were also studied. The asymmetric v(CdropC) stretching modes obtained were close to the 1500-cm(-1) mark. Reinvestigation of the experimental results supported these results; a weak IR band at 1486 cm(-1) was assigned to this asymmetric stretching mode in C-6(CF3)(2) F. Some other benzdiynes also had large IR intensity values for their asymmetric v(CdropC) vibrational modes due to the coupling with other vibrational modes. Heats of formation for the substituted benzdiynes were obtained from the reaction energies calculated at the B3LYP/cc-pVTZ level of theory.