The physical meaning of Lewis acid and base affinities are defined as the negative of the enthalphy for formation of an adduct with a reference base and acid, respectively: \[B~+~A_{reference}~ ⇌ ~B-A~~~~~~~- \Delta H~=~\sf{Lewis~basicity~of~B~towards~A} \nonumber \] \[B_{reference}~+~A~ ⇌ ~B-A~~~~~~- \Delta H~=~\sf{Lewis~acidity~of~A~towards~B} \nonumber \] Two common scales of Lewis base affinity involve the use of SbCl 5 and BF 3 as the reference base. Specifically, In the Guttman donor-acceptor scale, Lewis bases' donor numbers are equal to the negative enthalpy change for formation of the base's SBCl 5 adduct in dilute 1,2-dichloroethane (EDC*): \[Base(EDC~solution) ~+~SbCl_5(EDC~solution)~\rightarrow~Base-SbCl_5(EDC~solution)~~~~~~- \Delta H~=~\sf{Guttman~Donor~Number} \nonumber \] The BF 3 affinity scale has largely supplanted the Guttman scale owing to the ease with which its results may be correlated with computational data (it is easier to calculate energies for adducts of BF 3 than of SbF 5 ). In the BF 3 affinity scale, Lewis bases' BF 3 affinities are equal to the negative enthalpy change for formation of a solution phase adduct between the base and gaseous BF 3 . The reaction is: \[Base(solution) ~+~BF_3(g)~\rightarrow~Base-BF_3(solution)~~~~~~- \Delta H~=~\sf{BF_3 ~Affinity}~ \nonumber \] BF 3 affinity values for a wide range of organic bases were compiled by Laurence, Graton, and Gal in reference 2 and salient trends in this data will be discussed in subsequent sections. References Laurence, C.; Graton, J.; Gal, J.-F., An Overview of Lewis Basicity and Affinity Scales. Journal of Chemical Education 2011, 88 (12), 1651-1657. Laurence, C.; Gal, J.-F. o., Lewis basicity and affinity scales: data and measurement. John Wiley: Chichester, West Sussex, U.K., 2010. Notes * EDC stands for ethylene dichloride, the common name of 1,2-dichloroethane.