Introduction: Historical Remarks

Abstract

The research on molecular magnetism may be traced back to the 19th century when M. Faraday studied the magnetic susceptibility of a variety of organic molecular substances [1]. Almost all of these substances were found, however, to be non-magnetic except for an effect which was later called molecular dia- magnetism. In 1927, shortly after quantum mechanics had been established, W. Heitler and F. London [2] had a monumental success in applying this new theory to the hydrogen molecule, the simplest molecule in nature. Following that model, W. Heisenberg [3] gave a general conception of an exchange inter- action. 1 Faraday’s results were explained by a quantum mechanical rule based on the Pauli principle stating that any molecular orbital in stable molecules, i.e., non-radicals, is necessarily occupied by spin-up and -down electrons. The main problem in those days was to understand the molecular diamagnetism induced by a shielding current flowing along an aromatic ring in response to an external magnetic field [4].