Molecular structures, vibrational spectra and rotational barriers of C2H6, Si2H6, SiGeH6 and Ge2H6 - Experiment and theory in harmony

The staggered and eclipsed conformers of the title compounds were optimized at the HF, MP2, B3LYP, and CISD levels of theory utilizing polarized triple-zeta basis sets; CCSD(T) single points were computed for the CISD structures, All experimental geometrical parameters are reproduced well at the correlated levels, The rotational barriers are better described with larger basis sets, and require sophisticated electron correlation treatments (CISD or CCSD) for quantitative agreement with experiment. B3LYP rotational constants are generally somewhat too small due to the incomplete inclusion of higher-order perturbations, The vibrational frequencies computed using MP2 and B3LYP, agree well with experiment after the application of scaling factors of 0.945 and 0.98, respectively.