F number is a correlation number used in the analysis of polycyclic aromatic hydrocarbons (PAHs) as a descriptor of their hydrophobicity and molecular size.^[1] It was proposed by Robert Hurtubise and co-workers in 1977.^[2]

Calculation

The F number is calculated using the formula:

${\displaystyle F=B_{2}+C_{12}-{\frac {1}{2))R}$

where:

B₂ is the number of double bonds

C₁₂ is the number of primary carbon and secondary carbon atoms

R is the number of non-aromatic rings.

Example

For fluorene, there are 6 apparent double bonds (three pi bonds in each side benzene-like ring); the central ring has one secondary carbon and is non-aromatic. Therefore:

${\displaystyle F=6+1-{\frac {1}{2))1=6.5}$

Correlation

It has been found that the F number linearly correlates with the log k' value (logarithm of the retention factor) in aqueous reversed-phase liquid chromatography. This relationship can be used to understand the significance of different aspects of molecular architecture on their separation using different stationary phases.^[3] This size analysis is complementary to the length-to-breadth (L/B) ratio, which classifies molecules according to their "rodlike" or "squarelike" shape.^[4]