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-Kα 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 π-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 π-bond character. This view was supported by the magnitudes of the topological bond orders and by the delocalization indices δ(ΩA, ΩB). An orbital decomposition of δ(ΩA, ΩB) demonstrated that there was significant π-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 π-delocalization, showing instead only limited σ-delocalization between nearest neighbors. Overall, the topological evidence and the atomic graphs of the oxygen atoms did not provide convincing evidence for π-delocalization involving the methoxy substituents.

本研究通过100 K下的Mo-Kα X射线衍射（Mo-Kα X-ray diffraction）实验，结合B3LYP/6-311++G(2d,2p)理论水平下的密度泛函理论（DFT）计算，对2,5-二甲氧基苯甲醛缩氨基硫脲(1)的电荷密度开展了实验与理论探究。研究采用分子中原子量子理论（QTAIM），考察了与苯环近乎共面的硫代酰胺侧链的π离域程度。沿键径得到的实验与理论椭圆率分布结果高度一致，表明侧链中的部分形式单键具备显著的π键特性。拓扑键级的数值与离域指数δ(ΩA, ΩB)均支持这一观点。对δ(ΩA, ΩB)的轨道分解分析证实，所有链间非氢化学键均存在显著的π键特征。与之相对，以链间键临界点为参考的源函数未能为π离域提供任何证据，仅体现出近邻原子间有限的σ离域行为。整体而言，拓扑分析证据与氧原子的原子拓扑图均未为甲氧基取代基参与的π离域提供令人信服的佐证。