Местоположение издательства:Sankt-Peterburg, Russia (Federation)
Первая страница:35
Последняя страница:35
Аннотация:Spin-spin coupling constants (SSCC) are the key NMR parameters for structure determination nowadays. However, direct measurement of these parameters is difficult in many cases due to peaks overlapping, short relaxation times and/or second order effects (see [1-2]). Theoretical description for SSCC needs also to be developed in practical aspects. We showed recently [3], that dynamic behavior of many important molecular systems can be described in terms of vibrations with large amplitude. Accurate structure studies of saturated four- and five-membered cycles imply solving specific problem of quantitative description of dynamic processes with very low barriers in them. Here we present application of few new effective techniques for extraction information on the dynamic structure [3, 4] via the high precision analysis of NMR multiplets and theoretical description of the NMR parameters.
We developed a practical method for evaluation of the parameters of conformational dynamics in terms of vibrations with large amplitude. The method based on: (i) the results of complete analysis of high resolution NMR spectra, (ii) ab’initio calculations of a reaction path and surfaces of potential energy and spin-spin coupling constants, (iii) a numerical solution of corresponding vibration problem and (iv) refinement for the parameters of the energy surface based on the best fit of experimental and calculated spin-spin couplings.
As a starting point, the undistorted potential energy surface (PES) of inner rotation for the compounds studied was built by applying the scanning technique to skeletal dihedral angles. This allows us to get a trial “reaction path” for the pseudorotation process. Conformational dependencies for spin-spin coupling constants (SSCC) for principal points on the reaction path were calculated using FP DFT technique. 1H NMR spectra were recorded for a series of solvents and “Bruker AV-600” spectrometer at room temperature, and were treated using total lineshape analysis technique (program VALISA) which allows to get very accurate estimates of experimental SSCC values. Finally, the reverse spectral problem was solved to adjust experimental and calculated data and build up the “true” potential of pseudorotation. We developed REVIBR program, which solves numerically corresponding vibration problem and models the dynamic averaging using the technique of convolution of the spin-spin coupling surfaces using the whole set of vibration energies and eigenvectors (normally, 200 lowest ones). Convolution criterion used in REVIBR program allows to get calculated SSCC for given temperature. Nonlinear optimization (Levenberg-Marquart techniques) of the estimated parameters for the “true” pseudorotation PES (modeling difference of ground states of main conformers E and heights for the conformational barriers E≠) used to get best fit of experimental and calculated SSCC values.
Advantages of the technique developed demonstrated on a series of monosubstituted cyclobutanes, trans-1,2-dihalocyclopentanes, tetrahydrofuran, tetrahydro-thiophene, tetrahydrothiophene-1-oxide, pyrrolidine, proline and ribonucleosides. The data obtained shows, that the pseudorotation process in every four- and five-membered system under study is carrying out by the mechanism with high amplitude of vibration. Major conformations of tetrahydrofuran and terahydrothiophene are twists 4Т5 and 5Т4, for pyrrolidine – envelope Е1 with equatorial NH-bond, for terahydrothiophene-1-oxyde – envelopes Е3 and with axial oxygen and for proline – envelope Е5 with axial СООН-group. Method used also for characterization of internal rotation in acyclic systems: styrene, substituted cis- and trans-azobenzenes, cinnamic aldehyde as well as in natural endogenic hormones noradrenaline and adrenaline.