Hypothetical chemical compound

A hypothetical chemical compound is a chemical compound that has been conceived of, but is not known to have been synthesized, observed, or isolated (identified or shown to exist).

Some hypothetical compounds cannot form at all, due to steric effects (e.g. tetra-tert-butylmethane, C(C(CH3)3)4, chlorine heptafluoride, ClF7, or bromine heptafluoride, BrF7) or bond stress (e.g. tetrahedrane C4H4). Others might turn out to be highly unstable, decomposing, isomerizing, polymerizing, rearranging, or disproportionating. Some are thought to exist only briefly as reactive intermediates or in vacuum. Some have no known pathway for synthesis (e.g. hypercubane).

Some compounds of radioactive elements have never been synthesized due to their radioactive decay and short half-lives (e.g. francium hydroxide FrOH, radon hexafluoride RnF6, astatine heptafluoride AtF7, polonium(II) fluoride PoF2).

Some "parent compounds" have not been or cannot be isolated, even though stable structural analogs with substituents have been discovered or synthesized (e.g. borole C4H4BH). Hypothetical compounds are often predicted or expected from known compounds, such as a families of salts for which the "parent acid" is not a stable molecule, or in which salts form with some cations but not others. Examples of such "phantom acids" are disulfurous acid HO\sS(\dO)\sS(\dO)2\sOH and sulfurous acid O\dS(\sOH)2, whose salts are stable.

Hypothetical compounds are used in some thought experiments.

Some compounds long regarded as hypothetical have later been isolated. Ethylene dione was suggested in 1913 and observed spectroscopically in 2015. Another stable compound, potassium trichromate, has been produced in a small scale and is known to be a very powerful oxidising agent. Sodium trichromate and sodium and potassium tetrachromate have been hypothesized but are yet to be synthesized.

Other compounds were once thought to have already been produced, but are now regarded as hypothetical chemical compounds unlikely to ever be produced, such as polywater, oxygen tetrafluoride OF4, chromium hexafluoride CrF6 and osmium octafluoride OsF8.

Other examples of hypothetical compounds are xenon octafluoride XeF8, pentazole N5H (all nitrogen analog of azole), hexazine N6 (all nitrogen analog of benzene), octaazacubane N8 (all nitrogen analog of cubane), cyclotrioxidane O3, nitrogen pentafluoride NF5, tetrafluoroammonium fluoride [NF4]+F−.

Despite the fact that rhenium heptahydride ReH7 has not been isolated, its salt potassium nonahydridorhenate(VII) is stable.

Prediction
Stability and other properties can be predicted using energy calculations and computational chemistry.

"[Using] the Born–Haber cycle to estimate ... the heat of formation ... can be used to determine whether a hypothetical compound is stable." However, "a negative formation enthalpy does not automatically imply the existence of a hypothetical compound." The method predicts that NaCl is stable but NeCl is not. It predicted XePtF6 based on the stability of O2PtF6.