The Rotation Barrier of Ethane and Related Topics

It is possible to analyse the preceding results in terms of chemical concepts, provided that the ground and excited configurations of the perturbation calculation are built from localized orbitals equivalent to the standard molecular orbital. The various t3q es of excitations are indicated in Table 1. 

For both conformations, terms of type 1 give half of the total second-order correction at least, but their variation from one form to the other is completely balanced by the variation of the terms of type 2 and 3. Dispersion terms of t5 e 5 (t e 4 is negligible) decrease the barrier slightly however, their effect is very small (0.2 kcal/mol) because of the large distance between the CH3 groups and of the high symmetry of the molecule. For instance, in a rotation of 60°, the dispersion type interaction between two CH bonds varies by an amount equal to 0.3 kcal/mol. 

The reason of the failure of the pair functions is clarified by the following simple model 15) the physical situation in a rotation barrier problem is pretty well simulated by two hydrogen molecules, one being rotated with respect to the other (Fig. 2). 

On this model three types of calculation can be performed  

A pair-function calculation, where each electron pair is associated to a quasi-localized bond function built from Lowdin orthogonalized Is orbitals. This is equivalent to the treatment of ethane mentioned above 14)  

---