text: Numerous attempts have been made to quantify aromaticity with respect to the observed ring current. One method is called diamagnetic susceptibility exaltation Λ defined as the difference between the measured magnetic susceptibility of a compound and a calculated value based on group additivity tables. Large negative values are aromatic, for example, benzene (Λ = &minus;13.4). Values close to zero are non-aromatic, for example, borazine (Λ = &minus;1.7) and cyclohexane (Λ = 1.1). Large positive values are antiaromatic, for example, cyclobutadiene (Λ = +18).
  Another measurable quantity is the chemical shift of lithium ions Li in complexes of lithium with aromatic structures because lithium tends to bond as a π-coordinate complex to the face of the aromatic rings. Thus the lithium atom in cyclopentadienyl lithium (CpLi) has a chemical shift of &minus;8.6 ppm (aromatic) and its CpLi complex  a shift of &minus;13.1.
  Both methods suffer from the disadvantage that values depend on ring size.