The simplest b 2 + ion: determining its structure from its energetics by a direct comparison of the threshold collision-induced dissociation of protonated oxazolone and diketopiperazine

Energy-resolved collision-induced dissociation of b2 + ions derived from protonated GGG and GAG are compared with that of protonated diketopiperazine (H+DKP). Absolute dissociation cross sections are obtained using a guided ion beam tandem mass spectrometer and analyzed using statistical methods. The fragmentation pattern of the b2 + ion from H+GGG differs significantly from that of H+DKP, although decarbonylation (forming the a2 + ion) is the lowest energy pathway found for both species. The threshold measured for b2 + decarbonylation differs appreciably from that of H+DKP, but both processes have onsets that are consistent with those obtained from quantum chemical calculations, positively identifying the b2 + ion as the protonated 2-aminomethyl-5-oxazolone, H+AMOx. The threshold for the b2 + ion derived from H+GAG has a slightly lower threshold, again consistent with theory. Examination of the higher energy dissociation pathways observed shows that these generally involve subsequent dissociation of the primary a2 + product ion. As a consequence, statistical analysis of these pathways for threshold information is difficult and ultimately provides little agreement with theory. This disagreement is rationalized on the basis of considerable kinetic energy release that has been observed previously by Harrison for these subsequent reactions. The present results are also favorably compared with previous results obtained by Siu and coworkers, although a number of refinements in the analysis and mechanisms are suggested.