Nuclear magnetic resonance chemical shifts using optimized geometries

Publisher Website
Google Scholar
Abstract
Isotropic chemical shifts and shift anisotropies for carbon, nitrogen, oxygen, and fluorine in first-row-atom molecules have been calculated in the perturbed Hartree–Fock gauge including atomic orbital scheme for both experimental and optimized molecular structures using a 6-311G* basis for heavy atoms and a scaled 4-31G basis for hydrogen. Structure optimization leads to the expected shortening of bond lengths, which is accompanied by an increase in the isotropic chemical shifts. The increased shifts show much improved agreement with gas phase experimental values for nitrogen and oxygen, while the results for carbon are only mildly affected and remain good; shift anisotropies for all species tend to decrease in magnitude and also generally improve. Fluorine is anomalous, its increasing shifts upon structure optimization moving further away from experiment at this level of basis set. The trend in the optimized isotropic shifts is explained in terms of the general tendency for atoms in the right-hand portion of the first row of the periodic table to have negative shift derivatives with bond extension.