High-resolution gas phase measurements of 1H NMR spectra at 400 MHz and atmospheric pressure of seven small hydrocarbons are presented. The developed new method and the experimental set-up are described. Ab initio GIAO MO calculations of 1H and 13C NMR absolute shieldings on the HF, MP2 and B3LYP levels using 25 standard gaussian basis sets are reported for these hydrocarbons, based on experimental re distances. The measured gas phase 1H chemical shifts have been converted to an absolute σ0 shielding scale by use of the literature shielding of methane. These and gas phase 13C literature values have been transferred with literature ZPV data to estimated σeexp shieldings which are used to evaluate the basis set dependence of the calculated σe shieldings utilizing linear least squares regressions. Exponential extrapolations of Dunning basis set calculations allow the determination of basis set limits for 1H and 13C shieldings. 1H and 13C chemical shifts have been derived from the HF calculated shieldings with shieldings of TMS which has been geometry optimized and GIAO calculated in each basis. Standard deviations (esd) as low as 0.09 ppm for 1H and 0.76 ppm for 13C calculations have been obtained.The statistically best basis set for simultaneous calculation of 1H and 13C absolute shieldings or relative shifts is 6-311G* within the HF and B3LYP methods. Aiming for highest accuracy and precision, 1H and 13C have to be treated separately. In this case, best results are obtained using MP2/6-311G** or higher for 1H shieldings and MP2/cc-pVTZ for 13C shieldings.
Vinyl or isopropenyl substituents can be used to indicate anisotropy effects in the surroundings of benzenoid hydrocarbons by experiments together with APUDI model and ab initio GIAO MO calculations from the difference in geminal proton splittings of the olefinic substituents. Geometry optimizations as a function of the torsional angle between substituents and aromatic planes were performed in two polarized basis sets for the HF, B3LYP and MP2 methods. Calculated splittings range between À0.70 and 0.48 ppm. Comparison with experimental 1 H NMR shifts does not lead precisely to the determination of the experimentally apparent effective torsional angle.
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 1 H 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 13 C 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 1 H NMR independent behavior of these parameters. The effect of variations of angles on 1 H and 13 C 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.
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