Does the McConnell-I Model Really Work? An ab Initio Study of the Magnetic Character of Some Intermolecular Contacts

Abstract

The performance of the first model proposed by McConnell to rationalize the nature of the magnetic interactions in purely organic molecular crystals (the McConnell-I model) has been analyzed using ab initio methods. As a first step, we analyzed the form of deriving the basic formula of the McConnell-I model from a general Heisenberg spin Hamiltonian. We reached the conclusion that it is not possible in general, although the two show some resemblance. When the same procedure is applied to the well known example of the [2.2]paracyclophanes, we found that the agreement with the experiment comes from the fortuitous cancellation of terms which is not likely to hold in general. We then studied the high-low spin energy difference for the H2NO dimer and the methyl-allyl radicals for different relative orientations of the two monomers using CASSCF calculations and the 6–31G(d) basis set. It is found that there are regions in which McConnell-I model does not reproduce the ab initio results. So, we can conclude that the McConnell-I does not work in general.