From quinoidal to diradical structure in substituted n-para-xylylene (n = 1, 2, 3) molecules. A Heisenberg Hamiltonian study

The evolution of the structure of terminally substituted n-para-xylylene molecules is adressed for n = 1, 2 and 3 central phenyl rings. Those compounds are quite difficult to describe from ab initio quantum chemical approaches since the competition between a diradical and a quinoidal structure induces multistabilities of MCSCF treatments. The problem is studied through a geometry-dependent Heisenberg Hamiltonian. It is found that the diradical contribution is essentially governed by the number of central phenyl rings and that the case n = 2 has an intermediate geometry resulting from a strong mixing of the two limiting forms. The effect of the side-delocalization and the rotation around the axial CC bond is also studied. The increase in diradical character of the singlet ground state is consistent with the decrease in energy difference between the singlet and triplet states. The results of the Heisenberg Hamiltonian are in agreement with experimental results.