Cyclobutadiene is the smallest [n]-annulene ([4]-annulene), an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula C4H4. It is believed to be in equilibrium between a pair of rectangular/nonplanar ground states and a square, excited triplet state based upon theoretical calculations, and through spectroscopic and crystallographic investigation of substituted cyclobutadienes, in an argon matrix[citation needed] and inside carceplexes. Though it has alternating single and double bonds, it is predicted triplet, unstable and antiaromatic by Hückel's rule, because its ring has 4 π-electrons, and 4 is not twice an odd number. [Cyclobutadiene. Wikipedia]