35Cl pure quadrupole resonance in acetylated glycopyranosyl chlorides. Relation between chlorine nuclear quadrupole coupling constant and molecular conformation

Abstract

Nuclear pure quadrupole resonance of the chlorine atom bound to carbon atom C1 in several hexo- and pentopyranosyl derivatives has been detected. Observed resonance frequencies are distributed over two groups, centered, respectively, at 35.535 and 35.123 MHz. The two groups can be related to the two possible positions, axial and equatorial, of the chlorine atom bound to the anomeric carbon. The interpretation is supported by general rules of conformational analysis and by conclusions derived from the study of the equilibrium between conformers in solution. An all electron, ``ab initio'' calculation of molecular wavefunction, carried out on a simplified model (2-chlorotetrahydropyran) similar to the molecules under study, yields electronic densities on each atomic orbital, especially those of chlorine, as well as bond indexes for both axial and equatorial conformations. Values of the chlorine electric field gradient, as deduced from the computed populations with the help of the Townes and Dailey theory, are consistent with the variation of the experimental frequencies between axial and equatorial compounds.