Theoretical prediction of the spin-spin coupling constants between an axis and macrocycle of a rotaxane.

The indirect nuclear spin-spin coupling constants between nuclei belonging to the axis and to the macrocycle of three structurally related rotaxanes have been calculated by means of density functional theory. It has been shown that the through-space axis-macrocycle proton-proton coupling constants can be as large as 0.4-0.5 Hz and therefore of measurable values. The largest through-space axis-macrocycle carbon-proton and nitrogen-proton coupling constants are 0.2-0.3 Hz. Visualization of coupling pathways by means of the coupling energy density method indicates that the larger proton-proton couplings are indeed transmitted through the space between the coupled nuclei. Thus, it seems that measurement of such couplings should be possible and that indirect spin-spin couplings can be actually transmitted through-space, with no covalent or hydrogen bonds between the coupled nuclei.