Critical Evaluation of the Bond Order Concept with Application on Thermal Hydrogen Shifts

摘要

Since Coulson introduced the concept of bond order into the molecular orbital theory its significance has been proven as an important tool for the discussion and calculation of bond strength and bond length.On studying simple pi-conjugated odd- and even-membered carbon-hydrogen systems in an open and cyclic geometry,it could be shown that the use of the concept of bond order with respect to bond length in geometric representations for ground or transition states must be handled with caution.This has been concluded from a theoretical study of Okajima and Imafuku on chlorine and hydrogen thermal shifts in (cyclo)pentadiene and cycloheptatriene compounds.The results of their calculations and our previous calculations on the [1,5]-H shift in cis-1,3-pentadiene following a suprafacial transition state compared with a shift of the migrating hydrogen in the plane of the carbon skeleton ask for a critical evaluation.The impact on the result of the pi systems under investigation and on the results of others will be disclosed.