This reaction mechanism was proposed by Roberts and Kimball in 1937. With it they explained the observed stereospecific "trans"-additions in brominations of maleic acid and fumaric acid. Maleic acid with a "cis"-double bond forms the dibromide as a mixture of enantiomers:  while the "trans"-isomer fumaric acid forms a single "meso" compound:  The reaction is even stereospecific in alkenes with two bulky "tert"-butyl groups in a "cis" position as in the compound "cis"-di-"tert"-butylethylene. Despite the steric repulsion present in the chloronium ion, the only product formed is the "anti"-adduct. In an alternative reaction scheme depicted below the reactive intermediate is a β-bromocarbocation or β-bromocarbonium ion with one of the carbon atoms a genuine carbocation. For reactions taking place through this mechanism no stereospecificity is expected and indeed not found. Roberts and Kimball in 1937 already accounted for the fact that brominations with the maleate ion resulted in "cis"-addition driven by repulsion between the negatively charged carboxylic acid anions being stronger than halonium ion formation. In alkenes such as anetholes and stilbenes the substituents are able to stabilize the carbocation by donating electrons at the expense of the halonium ion. Halonium ions can be identified by means of NMR spectroscopy. In 1967 the group of George A. Olah obtained NMR spectra of tetramethylethylenebromonium ions by dissolving 2,3-dibromo-2,3-dimethylbutane in magic acid at −60 °C. The spectrum for the corresponding fluorine compound on the other hand was consistent with a rapidly equilibrating pair of β-fluorocarbocations.