Development of molecular fragment interaction method for designing organic ferromagnets

The design and preparation of organic ferromagnets are important and challenging from both experimental and theoretical viewpoints. We propose a molecular fragment interaction method for predicting the high-spin stability of conjugated hydrocarbon radicals without quantum chemistry calculations, which comprise three steps: (1) division of the system into fragments and numbering of the carbon atoms, (2) construction of a secular equation, and (3) solving of the secular equation. If there are two or more nonbonding molecular orbitals (NBMOs) and the smallest number of bonds between two carbon atoms with unpaired electrons is an even number, then the conjugated hydrocarbon radical has a high-spin ground state. This method can also be used to compare the high-spin stability between different conjugated hydrocarbon radicals. If a conjugated hydrocarbon radical has a high-spin ground state, then its derivative has a high-spin ground state as long as the degeneracy of the NBMOs is kept. This method is useful for the rapid design of organic ferromagnets.