Through-space spin-spin coupling in acetylenic systems. Ab initio and DFT calculations

We have investigated, by means of ab initio and DFT calculations, the magnitude of through-space spin-spin couplings (J(CH) and J(HH)) in CH/pi bonded van der Waals dimers involving acetylene, and in a structurally related covalent compound (4-ethynylphenanthrene). Within regions where the interaction is stabilizing J(HH) couplings are very small (< 0.1 Hz) for all complexes. In the acetylene-methane complex J(CH) is also very small, whereas in the acetylene-benzene complex and the acetylene dimer it shows a relatively large dependence on the tilt angle from the T-shaped arrangement, for which the smallest values are calculated, to a parallel slipped arrangement where J(CH) is ca. 0.5 Hz. The relative magnitude of through-space couplings in 4-ethynylphenanthrene and a tilted acetylene-benzene dimer, featuring the same internuclear arrangement as in model dimers, are compared.