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Crystal structures have been determined for trimethylboron, BMe3, and for a new polymorph of trimethylgallium, GaMe3; in addition, the crystal structure of trimethylthallium, TlMe3, has been redetermined. The BMe3 crystal structure represents a new structural type for the group 13 trimethyl derivatives in the solid state. In contrast to its heavier analogues, it consists of layers containing only very weakly interacting BMe3 molecules. GaMe3 forms a ladder-like pseudo-polymer via long gallium-to-methyl intermolecular interactions with Ga···C distances in the range 3.096(3)-3.226(4) Å. This is compared with a recently reported crystal structure of a polymorph, which, like InMe3 and TlMe3, is characterized by the formation of pseudo-tetramers. The effects of crystallization and secondary interactions have been analyzed by comparison with related crystallographic, gas-phase electron diffraction, and spectroscopic studies of these and other trimethyl derivatives of the group 13 elements. The energetic differences between polymorphs of BMe3, GaMe3, and InMe3 have been explored by plane wave DFT calculations. The energy differences between the BMe3-like layered structure and the InMe3-like pseudo-tetrameric structure are calculated to be -1.7, +3.6, and +10.4 kJ mol-1 for BMe3, GaMe3, and InMe3, respectively.
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