Theoretical determination of the indirect nuclear spin–spin coupling tensors, nuclear screening tensors, and magnetic susceptibilities of multiply bonded systems via the equations-of-motion method

In this paper we document the possible performances of the equations-of-motion method for predicting second-order magnetic properties (indirect nuclear spin–spin coupling tensors, nuclear screening tensors, and magnetic susceptibilities). Nonempirical calculations for a series of molecules containing conjugated or cumulated multiple bonds and most of them suffering from UHF triplet instability are reported. In spite of the limitation of the 631G** basis set used, the results fit reasonably well with the main features of the experimental data in all cases but some geminal J’s and the σ’s in diacetylene. Inclusion of the (2p-2h) interactions brings the theoretical results to a closer agreement with observation. Effects of electron correlation at (1p-1h) and (1p-1h) +(2p-2h) levels of approximation are analyzed by comparison with corresponding results obtained by standard CHF procedures. They are found to be of remarkable importance for the paramagnetic component of σ and χ and for the Fermi-contact contribution to the J’s, especially of geminal-type, but not for the noncontact terms of J.