F number (chemistry)

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F number is a number used in the chemistry of polycyclic aromatic hydrocarbons (PAHs) as a descriptor of their molecular size. It was proposed by R.J. Hurtubise et al in 1977.[1]


From a PAH it can be calculated using the formula: F = B2 + C12 - R / 2,

B2 is the number of double bonds
C12 is the number of primary and secondary carbons
R is the number of non-aromatic rings.



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


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


  1. ^ J.F.Schabron, R.J.Hurtubise and H.F.Silver, Anal. Chem., 49, 2253 (1977)

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