CALCULATION OF NUCLEAR MAGNETIC SHIELDINGS .15. ELECTRON CORRELATION-EFFECTS

Nuclear magnetic shieldings were calculated with electron correlations through third order. The calculation was performed on the use of London's gauge invariant atomic orbitals (GIAO) and the finite field many-body perturbation theory (FF-MBPT), in which the two kinds of perturbation fields, i.e., the external magnetic field and the field due to the nuclear magnetic moment, were introduced to get the perturbed one-electron states. The Hartree-Fock (HF) values and the second- and third-order Moller-Plesset (MP) correlation corrections of the nuclear magnetic shieldings were calculated in (i) four first-row hydrides HF, H2O, NH3, and CH4; and (ii) three linear molecules with a multiple bond N-2, CO, and HCN. The calculations showed that the post-HF correlations are important and much improve the calculated shielding values, and that the second-order corrections are positive, but the third-order ones are negative. Furthermore, the rovibrational corrections to the shielding constants in the HF molecule at 300 K Were estimated to be -9.9 ppm for F and -0.66 ppm for H.