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On the Origin of the Rotational Barrier in Trimethylene

A quantitative assessment of the barrier of rotation in singlet trimethylene is made by an energy partitioning analysis within the MINDO/2 approximation, including 3X3 configuration interaction between HOMO and LUMO. The dominating factors originate from (a) the nonbonding repulsion between 1,3-positioned atoms and (b) the weak attraction between the 1,5- positioned cis-hydrogens in the EE conformation. The energy difference between the EE and EF conformation is lowered by radical stabilizing substituents, such as a CH3 group (attached to the carbon atom bearing one unpaired electron), because one of the unpaired electrons is stabilized in the EF conformation.