Methylene (compound)

Methylene (IUPAC name: Methylidene, also called carbene or methene) is an organic compound with the chemical formula CH2 (also written [CH2]). It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct. Methylene is the simplest carbene. It is usually detected only at very low temperatures, or as a short-lived intermediate in chemical reactions. The trivial name carbene is the preferred IUPAC name. The systematic names methylidene and dihydridocarbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. Methylidene is viewed as methane with two hydrogen atoms removed. By default, this name pays no regard to the radicality of the methylene. Although in a context where the radicality is considered, it can also name the non-radical excited state, whereas the radical ground state with two unpaired electrons is named methanediyl. Methylene is also used as the trivial name for the substituent groups methanediyl (>CH2), and methylidene (=CH2). Methylene has an electron affinity of 0.65 eV. Using the technique of flash photolysis with the compound diazomethane, Gerhard Herzberg and Jack Shoosmith were the first to produce and spectroscopically characterize the methylene molecule. In their work they obtained the ultraviolet spectrum of gas phase methylene at around 141.5 nm. Their analysis of the spectrum lead them to the conclusion that the ground electronic state was an electronic triplet state and that the equilibrium structure was either linear, or else it had a large bond angle of about 140°. It turns out that the latter is correct. The reactions of methylene were also studied around 1960, by infrared spectroscopy in frozen gas matrix isolation experiments. Methylene can be prepared, under suitable conditions, by decomposition of compounds with a methylidene or methanediyl group, such as ketene (ethenone) (CH2=CO), diazomethane (linear CH2=N2), diazirine (cyclic [-CH2-N=N-]) and diiodomethane (I-CH2-I).