Trimethylenemethane

Trimethylenemethane (often abbreviated TMM) is a chemical compound with formula C
4H
6. It is a neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, it can be viewed as an isobutylene molecule C
4H
8 with two hydrogen atoms removed from the terminal methyl groups.

Trimethylenemethane was first conjectured to be an intermediate in some chemical reactions. It was observed in 1966 by Paul Dowd at very low temperature.

The unusual electronic structure of Trimethylenemethane was recognized by Moffit and Coulson in 1948. It is a neutral four-carbon molecule containing four pi bonds. When trapped in a solid matrix at about 90 K, the six hydrogen atoms of the molecule are undistinguishable by spectroscopy. Thus, it must be described either as zwitterion, or as the simplest conjugated hydrocarbon that cannot be given a Kekulé structure. It can be described as the superposition of three states:

In 1966 Paul Dowd determined with electron spin resonance that this molecule has a triplet ground state (3
A
2'/3
B
2), and is therefore a diradical in the stricter sense of the term. Calculations predict that in this state the molecule is planar with three-fold rotational symmetry, with approximate bond lengths 1.40 Å (C–C) and 1.08 Å (C–H). The H–C–H angle in each methylene is about 121 degrees.

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