Twisting and planarization in push–pull ethylenesElectronic supplementary information (ESI) available: tables of X-ray crystallographic bond lengths and angles of compounds 8–19 (including selected torsional angles and hydrogen bonds), X-ray crystal structures of compounds 10, 11, 13, 16, and 18 (Figs. S1–S5) and packing diagrams for 17·H2O and 19·H2O (Figs. S6–S7). See http://www.rsc.org/suppdata/p2/b1/b109624a/

Abstract

As determined by X-ray crystallography, Meldrum's acid derivatives 8–19 feature dihedral angles around the central CC double bonds between 3 and 83°. Hydrogen bonds between substituents RHN and the carbonyl groups favour near-planarity. Sterically demanding substituents favour large dihedral angles and zwitterionic structures as in formula 20. AM1 calculations of the structures are in excellent agreement with the experimental X-ray data, provided a dielectric field is incorporated (e = 40). This can be ascribed to the highly polar (zwitterionic) nature of the molecules. It is further predicted that all these molecules, including those that are stabilised in a planar form by intramolecular hydrogen bonds, undergo rapid rotation about the central CC bonds at room temperature. DFT calculations incorporating a dielectric field model (PCM) are in excellent agreement with the near-perpendicular arrangement of the alkene moiety in 19.