Study of the torsional potential energies of 2-methylpropane, n-butane, and 2-methylbutane with high-level ab initio calculations

An accurate representation of the torsional potential energies of alkanes and molecules containing alkane sequences, e.g. polyolefins, is essential for reliable force field modeling and simulation of these molecules. Therefore the torsional potentials of 2-methylpropane, n-butane, and 2-methylbutane as three major building blocks of polyolefins were studied at approximately the CCSD(T)/cc-pVQZ level. From previous studies it appears that calculations in the larger basis set and more extensive electron correlation treatments is more influential on the cis barrier in n-butane and transition state than other conformers. Larger basis set and level of calculations in this study show that the trans-gauche barrier in 2-methylbutane and rotational energy barrier in 2-methylpropane improves 10-13%, but the trans-gauche energy difference in n-butane and 2-methylbutane do not change significantly. The gauche energy, the trans-gauche barrier, and the cis barrier in n-butane were 0.628, 3.303, and 5.460 kcal/mol, respectively. These values were in good agreement with previous calculations from quantum mechanics. The calculated torsional energy barrier in 2-methylpropane, 3.373 kcal/mol, agreed satisfactorily with previous experimental measurement. For 2-methylbutane the greater gauche energy and trans-gauche barrier, 0.789 and 5.159 kcal/mol, indicated that the gauche conformer in n-butane is more stable than 2-methylbutane. On the other hand, from smaller barrier between two trans conformer in 2-methylbutane, 2.925 kcal/mol, compared with the trans-gauche barrier it is observed that the trans conformer has been the determiner conformer in the conformational characteristic and properties. Finally, approximate representations of the torsional potential term in empirical force fields of the calculated data are expressed as cosine series expansions.