A quantitative analysis of the importance of π-non-bonded interactions in double rotors

Abstract

The effects of the π interactions upon the methyl rotational barrier in double rotors CH₃–X–CH₃ is investigated in terms of a procedure which allows rotational barriers to be computed also in the absence of the π interactions under examination. It is found that when X is a π acceptor such as CO, the change in the methyl rotational barrier going from the single to the double rotor is controlled by the π non-bonded interactions. On the other hand, when X is a π donor such as O, both π and σ conjugative interactions combine to give an increase in the rotational barrier.