Exploring the G3 method in the study of rotational barrier of some simple molecules

摘要

The G3 method was used to determine the internal rotation barriers of some simple molecules (H_2O_2, H_2S_2, N _2H_4, CH_3OH, CH_3NH_2, and C_2H_6). The barriers were accurate with deviations lower than 0.5 kcal mol ~(-1) with respect to the experimental data and comparable with other methods like CCSD(T)/aug-cc-pVTZ results. The G3 components showed that the MP4/6-31G(d) energy is quantitatively improved according to: δE_(G3Large) > ΔE_(2df), _p > δE_+ > ΔE_(QCI). However, the relative energies showed that molecules containing lone pairs in neighbor atoms presented high barriers, which were particularly sensitive to polarization and diffuse functions. The G3Large and QCI effects showed no qualitative or quantitative relative contribution. Molecules containing lone pairs in one atom or only bond pairs presented low barriers and were sensitive not only to the polarization and diffuse effects but also depending on the improvement of the basis set through the G3Large correction. The QCI effect showed no qualitative or quantitative relevant contribution in any of the cases studied.