Structural Insight into ESI+-Generated Ions of Oxazolochlorin Derivatives Using Cryogenic Infrared Ion Spectroscopy

Oxazolochlorins are synthetic porphyrinoids that are distinguished from regular tetrapyrrolic pigments by the presence of an oxazoline moiety in place of a pyrrole. They are amenable to electrospray ionization mass spectrometric analysis. Ionization can take place via protonation, the loss of an alkoxy side chain forming an α-oxacarbocation, or in the case of oxazolochlorinato metal complexes by (formal) oxidation of the metal center. Structural aspects of the ions were studied by using cryogenic ion vibrational spectroscopy. Comparison of the vibrational band patterns generated using density-functional theory calculations with the experimental spectra indicates that it is possible to distinguish specific protonation sites as well as oxazolochlorin constitutional macrocycle isomers. Information is also obtained regarding the nature of the alkoxy side chains on the protonated oxazolochlorins. Lastly, we explore how centrally coordinated metal ions (Ni2+ and Ag2+, both square planar coordinated divalent metals) influence the vibrational band patterns displayed by the macrocycles. This study provides a proof of concept that cryogenic infrared ion spectroscopy is suitable to expand our structural understanding of the ions formed by porphyrinoids under electrospray conditions.