Another factor in determining the barrier is λ associated with structural changes required for the energy transfer. Ranging from 24.6 to 38.4 kcal/mol, the molecules in Group A generally require higher λ values than those in Group B that show λ values of in the range of 16.7 to 17.1 kcal/mol. To better understand this trend, HQ (λ = 24.6 kcal/mol) and Dan (λ = 16.8 kcal/mol) were chosen to represent molecules in Group A and Group B, respectively, as depicted in Figure .