A quantum mechanical "jack in the box": rapid rearrangement of a tetrahedryl-tetrahedrane via heavy atom tunneling.

A bridged tetrahedryl-tetrahedrane ("TT") was, theoretically, recognized as the holder of the chemical record for the shortest C-C single bond in a stable molecule. However, owing to its strained nature, this molecule is prone to rearrange to a carbene via ring contraction. Although TT was predicted to be stable to rearrangement at very low temperatures, our calculations find that, due to carbon quantum mechanical tunneling, even at 0 K it will immediately rearrange, with a half-life of only 4 ms. TT provides an excellent example of why tunneling effect should be considered in the stability analysis of a theoretically conceived molecule, even at cryogenic temperatures.