PREDICTION OF NITROGEN AND OXYGEN NMR CHEMICAL-SHIFTS IN ORGANIC-COMPOUNDS BY DENSITY-FUNCTIONAL THEORY

Sum-over-states density functional perturbation theory (SOS-DFPT) and Hartree-Fock (HF) theory based on the ''individual gauge for localized orbitals'' (IGLO) scheme were used to calculate C-13,N-14/N-15, and O -17 NMR chemical shifts of 23 molecules. Employing the (11s7p2d/6s2p)[ 7s6p2d/4s2p] basis set, SOS-DFPT chemical shifts were obtained that ar e clearly better than HF- or UDFT-based chemical shifts, as is reflect ed by mean absolute deviations of 2.8 (SOS-DFPT), 7.6 (UDFT), and 5.6 ppm (HF) for C-13 chemical shifts, 11.8, 22.1, and 100.5 ppm for N-14/ N-15 chemical shifts, and 36.4, 57.2, and 45.7 ppm for O-17 chemical s hifts with regard to experimental values. By estimating appropriate ga s phase values for N-14/N-15 chemical shifts from measured solvent eff ects, average errors of SOS-DFPT chemical shifts are reduced to 3 ppm, which is close to uncertainties in experimental values. SOS-DFPT is t he method of choice for predicting reasonably accurate NMR chemical sh ifts at relatively low computational cost even in cases where correlat ion corrections are large. However, further improvements of SOS-DFPT a re necessary to calculate more reliable O-17 shift values of conjugate d pi-systems.