Conformational isomerism

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

On the Existence of Conformers of Cyclobutyl Monohalides. II. Temperature Dependence of the Infrared Spectra of Bromocyclobutane and Chlorocyclobutane

Abstract: The infrared spectra of bromocyclobutane and chlorocyclobutane vapor between 250 and 3100 cm−1 are reported as a function of temperature between 30° and 172°C. The spectra of the low‐temperature (−185°C) solids are also given. The data are described in terms of two conformers which are present in each halide. The energy difference between the two conformers of bromocyclobutane was measured to be about 1 kcal/mole. The conformers differ by their average dihedral angle: The more stable conformer is in a bent ring conformation (``equatorial''), the less stable one is in an essentially planar ring conformation. The sets of energy levels of the ring‐puckering motion of the two conformers are contiguous, there is no tunneling between the conformers. The conformations are sufficiently different as to lead to two widely separated carbon—halogen stretching fundamentals for each cyclobutyl halide. One stretching fundamental is based on the equatorial, the other (towards higher wavenumbers) is based on the planar ring conformation. The data were evaluated with the help of (1) calculations of the dipole moment as a function of the ring conformation, (2) computer calculations of vapor band envelopes including rotation—vibration interactions, (3) quantum‐mechanical computer calculations of the energy levels, probability distribution, transition moments, infrared intensities, and average dihedral angles of the ring‐puckering mode, and (4) some simple, qualitative considerations of the contributions of exchange interactions to the measured and calculated energy differences between the two conformers.

Calix[4]arene based monophosphites, identification of three conformations and their use in the rhodium-catalysed hydroformylation of 1-octene

Abstract: Eight monodentate phosphites (2–9) based on the calix[4]arene backbone were synthesised following two synthetic routes. Out of six conformations only three were actually formed under the applied reaction conditions. X-Ray analysis of two conformers (4 and 5) provided insight into the 3-dimensional structure of two of these conformations. The three conformations were characterised by 1H, 13C and 31P NMR spectroscopy. NMR experiments showed that several of the phosphites are flexible showing fluxional behaviour of the molecular backbone in solution, but no interconversion between the different conformers was observed. The conformation of the product in the phosphite synthesis is determined at the point where the phosphorus atom is linked to two hydroxyl groups of the calix[4]arene (phosphorus amidite). Such an intermediate phosphorus amidite (12) was isolated in the synthesis of 4 and 5. Phosphites 3–6, 8 and 9 were tested in rhodium-catalysed hydroformylation. Differences in rate can be correlated to the conformation of the ligand.

Structure and Rotational Isomerism of Ethylenediamine as Studied by Gas Electron Diffraction

Abstract: The structure and the rotational isomerism of ethylenediamine have been investigated by means of gas electron diffraction. Evidence has been given for the presence of one conformer (gauche) in the vapor phase (at 50–120°C); the N–C–C–N dihedral angle measured from the cis position is 64·0±4°, and the fraction of any other isomer, if present, is estimated to be less than 5%. A theoretical prediction based on the SCF-CNDO/2 method has essentially accounted for this finding. The rg distances and the angles based on the ra structure determined by a least-squares analysis on molecular intensities, with estimated limits of error, are as follows: C–C=1.545±0.008 A, C–N=1.469±0.004 A, ∠C–C–N=110.2±0.7°, C–H=1.109±0.01 A, ∠C–C–H=111·9±5°, and ∠H–C–H=112·7±8°.