Conformational energy profiles of n-butane, ethyl methyl ether and ethyl methyl sulfide - Comparison of electron correlation correction procedures

The relative energies of the two minima and the two transition states on the torsional potential of the title compounds were obtained by ab initio molecular orbital calculations with electron correlation energy correction by the Moller-Plesset methods (MP2, MP3 and MP4(SDTQ)), coupled cluster calculations (CCD, ST4CCD, CCSD and CCSD(T)) and configuration interaction energy calculations (CISD, QCISD and QCISD(T)) using the 6-31G* basis set. The calculated trans-gauche energy differences and the rotational barrier heights by the MP2 and CISD methods are slightly different from those with other methods. The calculated relative energies with other electron correlation correction methods are close to each other and close to the experimental values.