Some INDO/S parameterized calculations of nitrogen and oxygen screening constants

Abstract

The INDO/S parameterization scheme is employed within the framework of Pople's linear combination of gauge-dependent atomic orbital method to calculate some nitrogen and oxygen screening constants. The absolute screening and chemical shift data obtained are in closer agreement with experiment than those from previous semiempirical calculations. The chemical shifts are shown to arise almost entirely from changes in the local paramagnetic term. Small contributions arise from the nonlocal paramagnetic term for multiply bonded nuclei. With the exception of the carbonyl oxygen nuclei the paramagnetic term is not found to be dominated by transitions of a given symmetry type. Hence agreement between the observed chemical shift and the lowest excitation energy or estimates obtained from the average excitation energy approach is not expected for most of the nuclear environments included in this study.