Determination of Ground State Spin Multiplicity of Basic non-Kekule Hydrocarbons by Measuring Paramagnetic Susceptibility.

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2,3-Dimethylenecyclohexa-1,4-diyl (1) is a disjoint diradical and the ground-state multiplicity is still in controversy. By means of ESR spectroscopy and magnetic measurement, the ground-state multiplicity of the title compound was studied. From orthophtalic acid, 5,6-dimethylene-2,3-diazabicyclo[2,2,2, ]oct-2-ene was synthesized according to the method of W.R.Roth. The photolysis was carried out in MTHF matrix by using light from high-pressure mercury lamp to give 1. The photoproduct depended on the wavelenght of irradiation. The light of 312.6 nm gave the target molecule and the light of 302.2 nm gave another product. The magnetic property of 1 was investigated by means of direct measurement of the magnetization. It was found that the singlet and triplet state of 1 was almost degenerate. This result is a crucial determination of the problem where there has been a long controversy between theory and experiment.