π-Conjugation in Y-Shaped Configuration. Does a Special Stability Exist?

The constrained HF method was applied to the estimation of conjugation of Y-shaped molecules. This method does not include the effect of the geometrical change, and gives a direct measurement of π-conjugation energy. We conclude that the magnitude of conjugation in a trimethylen–methane dianion is around that of a hexadienediide dianion, far less than that of a neutral cyclic aromatic compound (typically that of benzene). This is consistent with the previous conclusions obtained with the different energetic criteria. The physical meaning of Y-conjugation was interpreted in terms of relaxation of the kinetic-energy pressure of the π electrons.