Evidence for Side-Chain π-Delocalization in a Planar Substituted Benzene: An Experimental and Theoretical Charge Density Study on 2,5-Dimethoxybenzaldehyde Thiosemicarbazone

The charge density in 2,5-dimethoxybenzaldehyde thiosemicarbazone (1) has been studied experimentally using Mo-K-alpha X-ray diffraction at 100 K, and by theory using DFT calculations at the B3LYP/6-311++G(2d,2p) level. The quantum theory of atoms in molecules (QTAIM) was used to investigate the extent of pi-delocalization in the thioamide side-chain, which is virtually coplanar with the benzene ring. The experimental and theoretical ellipticity profiles along the bond paths were in excellent agreement, and showed that some of the formal single bonds in the side-chain have significant pi-bond character. This view was supported by the magnitudes of the topological bond orders and by the delocalization indices delta(Omega(A), Omega(B)). An orbital decomposition of delta(Omega(A), Omega(B)) demonstrated that there was significant pi-character in all the interchain non-H chemical bonds. On the other hand, the source function referenced at the interchain bond critical points could not provide any evidence for pi-delocalization, showing instead only limited sigma-delocalization between nearest neighbors. Overall, the topological evidence and the atomic graphs of the oxygen atoms did not provide convincing evidence for pi-delocalization involving the methoxy substituents.