Comparison of the Standard 6-31G and Binning-Curtiss Basis Sets for Third Row Elements

Ab initio calculations were carried out for isogyric reactions involving the third row elements, Ga, Ge, As, Se, and Br. Geometries of all the reactants and products were optimized at the HF, MP2, and B3LYP levels of theory using the 6-31G(d) and 6-31G(d,p) basis sets. For molecules containing third row elements geometries, frequencies and thermodynamic properties were calculated using both the standard 6-31G and the Binning-Curtiss (BC6-31G) basis sets. In order to determine the performance of these basis sets, the calculated thermodynamic properties were compared to G3MP2 values and where possible to experimental values. Geometries and frequencies calculated with the standard 6-31G and the BC6-31G basis sets were found to differ significantly. Frequencies calculated with the standard 6-31G basis set were generally in better agreement with the experimental values (MAD=40.1 cm<sup>-1</sup> at B3LYP/6-31G(d,p) and 94.2 cm<sup>-1</sup> at MP2/6-31G(d,p) for unscaled frequencies and 29.6 cm<sup>-1</sup> and 24.4 cm<sup>-1</sup>, respectively, for scaled frequencies). For all the reactions investigated, the thermodynamic properties calculated with the standard 6-31G basis set were found to consistently be in better agreement with the G3MP2 and the available experimental results. However, the BC6-31G basis set performs poorly for the reactions involving both second and third row elements. Since, in general, the standard 6-31G basis set performs well for all the reactions, we recommend that the standard 6-31G basis set be used for calculations involving third row elements. Using G3MP2 enthalpies of reaction and available experimental heats of formation (Δ<i>H</i><sub>f</sub>), previously unknown Δ<i>H</i><sub>f</sub> for CH<sub>3</sub>SeH, SiH<sub>3</sub>SeH, CH<sub>3</sub>AsH<sub>2</sub>, SiH<sub>3</sub>AsH<sub>2</sub>, CH<sub>3</sub>GeH<sub>3</sub>, and SiH<sub>3</sub>GeH<sub>3</sub> were found to be 18.3, 18.0, 38.4, 82.4, 41.9, and 117.4 kJ mol<sup>-1</sup>, respectively.