Trifluorooxonium
The trifluorooxonium cation is a hypothetical positively charged polyatomic ion with chemical formula . It is structurally equivalent to the hydronium ion where the hydrogen atoms surrounding the central oxygen atom have been replaced by fluorine, and is isoelectronic with nitrogen trifluoride. This cation would be an example of oxygen in the unprecedented +4 oxidation state. The cation was shown to be vibrationally stable at all levels of theory applied (HF, MP2, CCSD(T)). was proposed to possess a pyramidal structure with an O–F bond length of 1.395 Å and F–O–F bond angles of 104.2° (CCSD(T) level  of theory). The detachment energy of the cation was calculated to be +110.1 kcal mol. However, the low-temperature reaction of , and under UV irradiation, besides unreacted starting materials only yielded the dioxygenyl salt . The oxidation of with salts also failed to produce evidence for the title cation. The formation of the hypothetical salt was calculated to be about thermoneutral, but slightly unfavorable with (g) + (g) + (g) → (s) = +10.5 kcal mol.