Binding of metalloporphyrins to model nitrogen bases: collision-induced dissociation and ion–molecule reaction studies

Abstract

Binding of several volatile nitrogen bases to four metalloporphyrin cations was studied by collision-induced dissociation (CID) as well as ion–molecule reactions in a quadrupole ion trap mass spectrometer. Relative binding energy order was obtained from the CID data by comparing the stability of metalloporphyrin–amine complexes while accounting for the effects of the complex size. The efficiencies of ion–molecule association reactions were also used to compare binding in these systems. The results from these two approaches agree with each other. There appears to be no direct correlation between the proton affinity of model bases with their metalloporphyrin affinity. This discrepancy is rationalized in terms of steric hindrance and dipole moment effects.