F number (chemistry)

F number is a number used in chemistry of polycyclic aromatic hydrocarbons (PAHs) as a descriptor of their molecular size. It was proposed by Hurtubise.

From a PAH it can be calculated using formula: F = B2 + C12 - R / 2, where B2 is a number of double bonds, C12 is a number of primary and secondary carbons and R is a number of non-aromatic rings.

For fluorene is F number: F = 6 + 1 - 0.5 = 6.5.

It has been found that F number linearly correlates with log k' value (logarithm of retention factor) in aqueous reversed-phase liquid chromatography.