NCI Study of the Influence of Steric Hindrance on the Conformational Isomers of Propane, Butane, and Ethane Derivatives

The role of the steric clash in the rotational energy barrier in CX3CX3 (X=H, CH3, F), propane, and butane compounds was studied using the NCI method and QTAIM approach. We found that the values of the repulsive density integral of NCI agrees with the traditional chemical steric model, as it increases with the size of the substituent. However, not in every case studied here, was the steric clash identified as the origin of the energy barrier. In the ethane case we note the absence of repulsive and attractive NCI across the full spectrum of the dihedral angles studied, whereas only in the propane and butane cases are the results consistent with the classical picture of the steric clash being the origin of the energy barrier. The energy profile for 2,2,3,3-tetramethylbutane shows two energy barriers of 8.7 kcal/mol and 0.96 kcal/mol, their origin was analysed in terms of the alkyl-alkyl interactions.