McConnell equation

In physical chemistry, the McConnell equation gives the probability of an unpaired electron in an in aromatic radical compound (such as benzene radical anion ${\displaystyle C_{6}H_{6}^{-}}$) being on a particular atom. It relates this probability, known as the "spin density", to its proportional dependence on the hyperfine splitting constant.

The equation is

${\displaystyle a=Q\rho }$

where ${\textstyle a}$ is the hyperfine splitting constant, ${\textstyle \rho }$ is the spin density, and ${\textstyle Q}$ is an empirical constant that can range from 2.0 to 2.5 mT.