Abstract
Non-bonded interaction and PPP calculations using various refinements were accomplished on cis- and trans-conformations of 5,5′-bis-isoxazole, to study the problem of internal rotation of this molecule. For comparison purposes, 2,2′-bipyridyl and biphenyl were also treated by the same procedure. Results are discussed together with those of previous researches on biphenyl. With the simple PPP approximations, values of ΔEcis-trans of –30 and +5.5 kcal/mol were found for bis-isoxazole and bipyridyl, respectively, while the introduction of “penetration” integrals gave values of +10 and +20 kcal/mol, respectively. The strong dependence of these results upon the different approximations indicates that this approach, which had been satisfactory for biphenyl, cannot describe more complex problems arising in the study of internal rotation.
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