Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

Abstract

Partially averaged dipolar couplings ͑also referred to as residual dipolar couplings͒ D ij can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche± conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and 13 C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method.

An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents

“…This choice is not unique, and to obtain a set which can be optimized in the procedure of fitting calculated to observed couplings is not obvious for substituted alkanes. For the molecule 1-chloro-2-bromo-ethane stable iterations were possible with e zz and e xx À e yy for the fragment containing the reference xyz frame, plus e Cl-Br directed along the Cl-Br direction [10]. A similar choice was tried for 1,2-dibromo,1,1-difluoroethane, that is e zz and e xx À e yy plus e Br-Br but without success: the rms error…”

“…However, adding an interaction parameter e 47 = e 58 , and reducing the other variables by one, does give a set which produces stable, convergent iterations in the fitting procedure. The values of D ij (vib) were obtained by assuming that only small-amplitude, harmonic vibrations contribute to the vibrational force field, and the latter, together with the vibrational frequencies, was calculated by the DFT method with the B3LYP/6-311+G* functional, as described in detail for 1-chloro-2-bromo-ethane [10].…”

“…The structure and conformational distribution will be determined by the same approach used for 1-chloro-2-bromoethane [10]. This involves analysing the spectra given by the isotopomers containing 13 C at natural abundance to yield values of T ij , which are assumed to be uncontaminated by J aniso ij , and so are equated to being the experimental, partially averaged dipolar couplings D ij (obs).…”

“…Such a method should be able to calculate the vibrational corrections to the couplings, and contain a theoretical model for how the orientational order varies with conformation. A procedure to do this has been developed and has been applied to a number of simple, flexible molecules chosen so as to test the various assumptions inherent to the method [5][6][7][8][9][10]. The test molecules contained only 1 H and 13 C as the magnetically active nuclei, and in this work attention is turned to flexible molecules which also contain 19 F nuclei, which introduces an extra complication.…”

Determination of the Dihedral Angle of the Monoanion of Succinic Acid in Aprotic Media

“…It is possible to go further to measure many of the other needed parameters for a three-conformer system but it is generally useful to assume values for some of the desired parameters. Usually, it is not practical to measure enough dipolar couplings to determine a geometry completely. , It is also found by a simple simulation that conformational preference and the dihedral angles can be obtained from dipolar couplings simultaneously; however, the accuracy of results obtained this way deteriorates quickly with the accuracy of the dipolar couplings. Chidichimo and co-workers have used dipolar couplings determined for ethylene glycol and for succinic acid and its anions in lyotropic solutions to estimate the dihedral angles of these entities by the artifice of assuming that the different conformational mixtures are effectively for practical purposes in aqueous solution consisting of 100% of a single conformer. , Thus, ethylene glycol, succinic acid, and monosuccinate were taken to be exclusively gauche, while the succinate dianion was taken to be only trans.…”