Influence of Systematic Variations of Molecular Geometries of Small Hydrocarbons on ab initio HF/6-31G* and 6-311G* GIAO Calculated 1H and 13C NMR Chemical Shifts

GIAO HF calculations with symmetry retaining variations of selected CH or CC distances in a large range but constant retention of other geometric parameters have been performed for 10 molecules using the 6-31G* basis set (set A) and for a larger set of 18 molecules 1 to 18 in the 6-311G* basis set (set B). The graphical representations for calculated 1H NMR shifts in dependence on CH or CC bond lengths variations are mostly linear or slightly curved and show no extreme values in the range studied. The slopes are generally positive, i. e. an elongation of distances leads to low field shifts because of reduction of the density of shielding electrons in the varied bonds. The slopes of regression lines may be classified for CH bonds according to hybridization and with the kind of substitution. Calculated 13C NMR chemical shifts are dependent for connected C atoms on variations of CC as well as on CH bond distances. The graphs are mostly curved, also positive and in magnitude dependent on the types of bonds. Two dimensional plots of simultaneous variations of CH and CC bonds show for 1H NMR independent behavior of these parameters. The effect of variations of angles on 1H and 13C shifts was studied only for a few molecules and shows curved graphs with rather small slopes. The determined slopes of linear regressions may be used to estimate zero-point vibrational corrections in close agreement to directly calculated values.