Experimental Gas Phase ¹H NMR Spectra and Basis Set Dependence of ab initio GIAOMO Calculations of ¹H and ¹³C NMR Absolute Shieldings and Chemical Shifts of Small Hydrocarbons

Abstract

Abstract High-resolution gas phase measurements of 1 H 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 1 H and 13 C 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 1 H chemical shifts have been converted to an absolute σ 0 shielding scale by use of the literature shielding of methane. These and gas phase 13 C literature values have been transferred with literature ZPV data to estimated σ e exp shieldings which are used to evaluate the basis set dependence of the calculated σ e shieldings utilizing linear least squares regressions. Exponential extrapolations of Dunning basis set calculations allow the determination of basis set limits for 1 H and 13 C shieldings. 1 H and 13 C 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 1 H and 0.76 ppm for 13 C calculations have been obtained. The statistically best basis set for simultaneous calculation of 1 H and 13 C absolute shieldings or relative shifts is 6-311G* within the HF and B3LYP methods. Aiming for highest accuracy and precision, 1 H and 13 C have to be treated separately. In this case, best results are obtained using MP2/6-311G** or higher for 1 H shieldings and MP2/cc-pVTZ for 13 C shieldings.