Fourier transform carbon-13 spin–lattice relaxation study of motion of cyclic hydrocarbons

Abstract

The molecular dynamics in solution for a series of cyclic and bicyclic hydrocarbon derivatives has been investigated by C-13 pulsed Fourier Transform nuclear magnetic resonance spin-lattice relaxation experiments. Molecular motion has been analysed in terms of an anisotropic rotational diffusion model, and an attempt has been made to assess the effects of structure on the observed rotational diffusion constants. Errors in the diffusion constants are discussed, and some of the other problems inherent in the use of these parameters to obtain structural information are outlined. Introduction Carbon-13 spin-lattice relaxation times (TI) have been widely used to investigate details of molecular motion in solution (I). A major contribution to TI of a carbon-13 nucleus in an organic molecule arises from dipolar interactions with protons (TIc1"). Under conditions of rapid molecular reorientation, and in the extreme narrowing limit of the I3C resonances signal, the TI'lc1 contribution is given by eq.[I], where T, is the correlation time for rotational motion and r,,, is the carbon-proton internuclear distance.

Carbon‐13 spin–lattice relaxation of tropane alkaloids. Anisotropic rotational motion of tropine and pseudotropine

“…Ref. 21). The most important observation is that the orientation angle a is such that the main axis of rotation passes through the centre of mass of the molecule, and either through the nitrogen atom or very near it (see above).…”