Electronic Interactions and Their Influence on the Conformational Stability of trans-2-Halocyclopentanol

Conformational analysis of small molecules: NMR and quantum mechanics calculations

This review deals with conformational analysis in small organic molecules, and describes the stereoelectronic interactions responsible for conformational stability. Conformational analysis is usually performed using NMR spectroscopy through measurement of coupling constants at room or low temperature in different solvents to determine the populations of conformers in solution. Quantum mechanical calculations are used to address the interactions responsible for conformer stability. The conformational analysis of a large number of small molecules is described, using coupling constant measurements in different solvents and at low temperature, as well as recent applications of through-space and through-hydrogen bond coupling constants JFH as tools for the conformational analysis of fluorinated molecules. Besides NMR parameters, stereoelectronic interactions such as conjugative, hyperconjugative, steric and intramolecular hydrogen bond interactions involved in conformational preferences are discussed.

NMR spectroscopy and theoretical calculations in the conformational analysis of 1-methylpyrrolidin-2-one 3-halo-derivatives

This study reports the results of ab initio and density functional theory (DFT) electronic structure calculations as well as (3)J(HH) experimental and calculated coupling constant data obtained in the investigation of the conformational equilibrium of 3-halo-derivatives of 1-methylpyrrolidin-2-one. The five-membered ring assumes an envelope conformation owing to the plane of formation of the O═C-N-R bond, with C4 forming the "envelope lid". When the conformation changes, the "lid" alternates between positions above and below the amide plane. The α-carbonyl halogen assumes two positions: a pseudo-axial and a pseudo-equatorial. In the gaseous phase, the calculations indicate that the pseudo-axial conformer is more stable and preferable going down the halogen family. Natural bond orbital analysis showed that electronic delocalization is significant only for the iodo derivative. In the other derivatives, the electrostatic repulsion between oxygen and the halogen determines the conformational equilibrium. When the solvated molecule was taken into account, the pseudo-equatorial conformer population increased with the relative permittivity of the solvent. This variation was strong in the fluoro derivative, and the preference was inverted. In the chlorine derivative, the two populations became closer in methanol and acetonitrile. In the bromine and iodine derivatives, the percentage of pseudo-equatorial conformer increased only slightly owing to the dipole moment of the conformation: the pseudo-equatorial conformation has a greater dipole moment and thus is stable in media with high relative permittivity.

1H NMR spectra of ethane-1,2-diol and other vicinal diols in benzene: GIAO/DFT shift calculations

The proton NMR spectra of several 1,2-diols in benzene have been analysed so as to associate each magnetically nonequivalent proton with its chemical shift. The shifts and coupling constants of the OH and methylene protons of ethane-1,2-diol have been determined in a wide range of solvents. The conformer distribution and the proton NMR shifts of these 1,2-diols in benzene have been computed on the basis of density functional theory. The solvent is included using the integral-equation-formalism polarizable continuum model implemented in Gaussian 09. Relative Gibbs energies for all stable conformers are calculated at the Perdew, Burke and Enzerhof (PBE)0/6-311 + G(d,p) level, and shifts are calculated using the gauge-including atomic orbital method with the PBE0/6-311 + G(d,p) geometry but using the cc-pVTZ basis set. Previous calculations on ethane-1,2-diol and propane-1,2-diol have been corrected and extended. New calculations on tert-butylethane-1,2-diol, phenylethane-1,2-diol, butane-2,3-diols (dl and meso) and cyclohexane-1,2-diols (cis and trans) are presented. Overall, the computed NMR shifts are in good agreement with experimental values for the OH protons but remain systematically high for CH protons. Some results based on the Gaussian 03 solvation model are included for comparison.