The influence of electrostatic and dispersion interactions on the NMR parameters of acetylene Original Research Article

The NMR shielding constants and the spin–spin coupling constants of acetylene in C2H2–H+ and C2H2–He complexes are calculated as functions of the intermolecular geometry, using the supermolecular method on CAS SCF level. For the obtained surfaces a fit resulting from the perturbation theory of weak interactions is applied. The signs of the changes induced by the presence of He and H+ are opposite for all the NMR parameters of acetylene. The R−3 and R−2 radial dependence is prevalent for the shielding constants in C2H2–H+, while for the shielding constants in C2H2–He the dispersion interactions and the influence of He shielding anisotropy are of similar magnitude. The changes in the coupling constants are dominated by the FC term. BSSE is large for the 13C shielding, smaller for the coupling constants, and negligible for the 1H shielding.