The Importance of Lone Pair Electron Delocalization in the cis–trans Isomers of 1,2-Dibromoethenes

The subtle experimental energy difference between isomers of 1,2-dibromoethenes (0.090 ± 0.240 kcal mol−1) has been confirmed by high level MO calculations: the cis isomer is more stable by 0.04 kcal mol−1 than the trans isomer at the MP4(STDQ)/6-311++G(3df,3dp)//MP2/6-311++G(3df,3dp) level. It was strongly suggested that the most important origin of the cis-effect is the stabilizing hyperconjugative (charge-transfer) interaction of Br lone pairs with the πC=C* antibonding orbital.