Electron reorganization in chemical reactions. structural changes from the analysis of bond order profiles

The applicability of bond order profiles as a new tool for the interpretation and the visualization of electron reorganization during chemical reactions is examined, with emphasis on (i) the sensitivity to the particular choice of the definition of bond order, (ii) the sensitivity to the quality of the computational methods used, comparing HF and post-HF (MP2, SC and CASSCF) levels of theory, and (iii) the possibility of a mechanistic interpretation of bond order profiles in terms of the Hammond postulate. The systems studied are the dissociation of molecular hydrogen, the conrotatory ring opening of cyclobutene to cis-butadiene, and the s + s Diels-Alder cycloaddition reaction of butadiene and ethene.