Electron Density Investigation of a Push–Pull Ethylene (C₁₄H₂₄N₂O₂⋅H₂O) by X‐ray Diffraction atT= 21 K

Abstract

The electron charge distribution in a strongly twisted push-pull ethylene [PPE, 3-(1,3-diisopropyl-2-imidazolidinylidene)-2,4-pentanedione] has been determined by low temperature (T = 21 K) single-crystal X-ray diffraction analysis. The derived electronic properties are consistent with a zwitterionic molecule, as indicated by a charge transfer of 0.82(16) e from the push to the pull moieties and a charge polarization of 0.29(7) e on the olefinic bond. A dipole moment of 12(3) D has been determined, which compares well with ab initio theoretical results in terms of both modulus and orientation. The second moments, which have also been obtained with good precision, characterize PPE as a highly quadrupolar molecule. The special electronic features of the molecule confer particular topological properties to the electron density distribution, as evidenced by comparison with "standard" organic molecules. The crystallographic asymmetric unit of the present system includes one water molecule, which is hydrogen bonded to PPE. Its topological properties have also been investigated, together with an analysis of the hydrogen bonds involved.

Synthesis of an Extremely Bent Acyclic Allene (A “Carbodicarbene”): A Strong Donor Ligand

“…The former option can be effected by steric congestion, as in the symmetrical compound C 1 (twist angle = 668; d C=C = 1.39 ), [11] whereas the latter is a result of p-bond polarization, as in C 2 (twist angle = 868; d C=C = 1.47 ). [12] The steric approach cannot be extended to allenes because the termini are too remote for significant interactions. Therefore, the only possible way to weaken the p bonds of allenes is by polarization, which can be accomplished by either a push-pull or a push-push substitution pattern.…”

Correlated MO Study of the Low-Barrier Intramolecular Motions in Donor−Acceptor Ethenes

“…Specifically, with turning the acceptor part of the molecule out of the π-symmetry plane, the donor part of the molecule becomes increasingly nonplanar as well. In fact, each heteroatom, having approximately the planar sp 2 configuration in the S 0 minimum, becomes more pyramidal, i.e., closer to sp 3 , in the CdC twisted configurations; see Figures 1,2,4,and 5, and the Supporting Information. The mentioned large-amplitude motions are significantly more pronounced for molecules without internal constraints, such as 4-6.…”

Ethyl (2-cyano-3-ethoxyacryloyl)carbamate: irreversibly thermal isomerization of a push-pull olefin

“…Because of more polar character, the transition state requires a greater degree of ordering of solvent molecules than the ground state, 9 leading to the negative entropy of activation. Rotational barriers in alkyl-substituted ethylenes are >55 kcal mol -1 , 10 while those of pushpull olefins, whose π-bonds are dramatically polarized, 11 are significantly lowered and shown in Table 2. 4a Rotational barrier (∆G ‡ = 24.4 kcal mol -1 ) of the isomerization of Z-2 to E-2 is much lower than those of the alkyl-substituted ethylenes, indicating that Z-2 is a push-pull olefin.…”