Structure, configuration, conformation and quantification of the push–pull effect of 2-alkylidene-4-thiazolidinones and 2-alkylidene-4,5-fused bicyclic thiazolidine derivatives

Structures of a series of push–pull 2-alkylidene-4-thiazolidinones and 2-alkylidene-4,5-fused bicyclic thiazolidine derivatives were optimized at the B3LYP/6-31G(d) level of theory in the gas phase and discussed with respect to configurational and conformational stability. Employing the GIAO method, 13C NMR chemical shifts of the C-2, C-2′, C-4 and C-5 atoms were calculated at the same level of theory in the gas phase and with inclusion of solvent, and compared with experimental data. Push–pull effect of all compounds was quantified by means of the quotient π∗/π, length of the partial double bond, 13C NMR chemical shift difference (ΔδCdouble bond; length as m-dashC) and 1H NMR chemical shifts of olefinic protons. The effect of bromine on donating and accepting ability of other substituents of the push–pull Cdouble bond; length as m-dashC double bond is discussed, too.