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

The NMR shielding constants and the spin-spin coupling constants of acetyle ne in C2H2-H+ and C2H2-He complexes are calculated as functions of the inte rmolecular 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 o f He and H+ are opposite for all the NMR parameters of acetylene. The R-3 a nd 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 lar ge for the C-13 shielding, smaller for the coupling constants, and negligib le for the H-1 shielding. (C) 1999 Elsevier Science B.V. All rights reserve d.