Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Theoretical investigations of conformational aspects of polymorphism. Part 1: o-acetamidobenzamide

Abstract: o-Acetamidobenzamide crystallises in two polymorphs designated α and β. In the α-polymorph an intramolecular hydrogen bond is present, which is not found in the conformation adopted in the β-polymorph. The geometries of the molecules found in the crystal structures have been studied in detail using molecular mechanics, semi empirical and ab initio quantum chemistry techniques. Conformational energy differences have been evaluated, and gas phase potential energy surfaces generated to explore the conformational freedom of o-acetamidobenzamide.It has been found that both observed solid state conformations occur close to, but not actually at, minima on the calculated gas phase potential energy surfaces. Conformational energy differences of 8–10 kcal mol–1 have been found between the conformers found in the crystals, depending on the method used for the calculation. Based on lattice energy calculations for both polymorphs it would appear that the less stable conformation is not compensated for by an equivalent stabilisation of the crystal lattice. The total (intra- and inter-molecular) energy difference between polymorphs is greater than that conventionally accepted.The difficulties of using theoretical tools in conformational studies and the implications of our findings for ab initio crystal structure prediction are discussed.

Ab initio conformational study of caffeic acid

Abstract: A complete conformational analysis of caffeic acid, a phenolic derivative with well known antioxidant properties, was carried out by ab initio calculations, at the density funtional theory (DFT) level. Fourteen different conformers were obtained, the most stable ones being planar, as the conformational preferences of this molecule were found to be mainly determined by the stabilising effect of π-electron delocalisation. Harmonic vibrational frequencies, as well as potential energy profiles for rotation around several bonds within the molecule, were also calculated.

Ylidyl-dihalogenphosphane – Strukturbilder einer sich anbahnenden Dissoziation

Abstract: Ylidyl-dihalophosphanes Provide Structural Snapshots on Their Way to Dissociation The reaction of phosphonium ylides with phosphorus trihalides has been studied for the synthesis of ylidyl-dihalophos-phanes (= dihalophosphanyl ylides) Ph3PCRPX23, X = Cl, and 9, X = Br. Compounds 3, R = aryl, are readily prepared from the phosphonium bromides [Ph3PCH2R]Br, compounds 3, R = alkyl, SiMe3 or PCl2, and 9 are obtained from silylylides Ph3PCRSiMe3, compound 3, R = PPh3+ results from the addition of PCl3 to the hexaphenylcarbodiphosphorane. A (β-morpholinovinyl)dichlorophosphane 12 has also been prepared. Ylides 3 are oxidized by sulfur and selenium and are converted to ylidyl-chlorophosphenium (= chloro-phosphaalkenyl-phosphonium) salts [Ph3PCRPCl]AlCl410. In the 31P-NMR spectra of 3 and 9 the geminal coupling 2JPP indicates the phosphorus lone pair to be synperiplanar to the phosphonio group. In one case the P(III)C rotation barrier has been estimated from VT-31P-NMR spectra. By X-ray crystallography the structures of 3, R = Me, 2,6-Cl2C6H4, 4-NO2C6H5, PCl2, of 9, R = Me (two molecules), SiMe3, of an ylidyl-selenophosphonyl dichloride (11b), and of 12 have been analyzed. They provide representatives for the full range of rotation from the symmetric conformer with two equal PX bonds to the conformer with one PX bond perpendicular to the PCP plane and with this bond being extremely elongated. Thus, they map out the pathway to PX bond breaking. On this way the initial charge transfer from the ylidic carbon to the antibonding PX orbital ends up in a π donation and PX dissociation.