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.