Mass spectra of penicillin compounds were generated with the use of a thermospray mass spectrometer interface and deuterated solvents. Deuterium ion addition sites to the parent ion and the substituted ketene and thiazoline fragment ions were investigated with the use of molecular orbital calculations for computing the relative proton affinities for the various ion isomers. The energetics of the beta-lactam ring cleavage, which generates the fragment ions, were calculated, as well as the energetics of various C-H to C-D hydrogen-deuterium exchange processes, which generate the substituted ketene fragment ions. The calculations afforded the identification of the most probable structures for the generated ions.
Marshall M. Siegel and Norman B. Colthup, "Molecular Orbital Study of Reaction Processes in the Thermospray Mass Spectra of Deuterated Penicillins," Appl. Spectrosc. 43, 1215-1222 (1989)