Solvent effects on NMR spectrum of acetylene calculated by ab initio methods

Abstract

The NMR spectrum of acetylene in the gas phase and in the solution was calculated on the SCF and CASSCF level. The calculated NMR parameters were studied in relation to the active space dimension and the basis set. Solvent-induced changes of acetylene coupling constants were calculated by the reaction field method; for shielding constants the supermolecular method was also used. The shielding and coupling constants calculated for the isolated molecule showed a strong electron correlation dependence as contrasted with the changes caused by the presence of a dielectric medium. Comparison with the experimental values confirmed an exceptional magnitude of the 1J CC solvent-induced changes in acetylene. The coupling constants changes calculated by the reaction field method are consistent with the experimental values; the shielding constants changes revealed a large discrepancy. On this basis we conclude that the shielding constants in solution are affected mainly by the short-range specific interactions while for the coupling constants the long-range electrostatic interactions also contribute significantly.