Cryogenic Messenger-IR Ion Spectroscopy Study of Phenol & Aniline Molecular Ions and of the common Fragment Ion [C5H6][rad]+ formed by EI-MS

Abstract

Molecular ions of phenol and aniline as well as the fragment ion at m/z 66 formed by electron ionization are examined with cryogenic messenger-IR ion spectroscopy and theory. According to the recorded IR spectra aromatic ground state molecular ions of phenol and aniline are clearly identified. Similar to earlier reports our new set of spectroscopic and computational results supports the assumption that tautomeric molecular ions of phenol and aniline are only short-lived intermediates in the reaction path to the ultimate CO loss reaction product of phenol as well as the HNC loss product of aniline. Even the present cryogenic messenger-IR ion spectroscopy approach failed to collect spectroscopic evidence of the intermediate presence of these elusive tautomers of phenol and aniline molecular ions. However, our study clearly shows that the electron ionization mass spectrometry dissociation product ion of phenol and aniline at m/z 66 is the cyclopentadiene radical cation [C5H6]+ for the presence of which convincing spectroscopic evidence could be collected proving the structure assignment and identification. All messenger-IR ion spectroscopy experiments were conducted on a cryogenic 22-pole ion trap with the use of neon atoms for tagging. The MS instrument was coupled to a free electron laser delivering wavelength tunable IR radiation for spectroscopy. Harmonic and anharmonic force field calculations complement the experiments and support band assignments of the recorded IR spectra and general data interpretation.