Protonation sites and fragmentations of para-aminophenol

Structure and unimolecular reactivity of protonated para-aminophenol generated by the electrospray ionization were studied by means of mass spectrometry, IRMPD spectroscopy and DFT calculation. It is revealed that protonation of para-aminophenol takes place at the nitrogen atom. Under the conditions of the IRMPD experiment, thermodynamically favored elimination of the water molecule is observed, whereas under the CID experiment kinetically favored elimination of ammonia prevails. The IRMPD spectrum as well as all fragmentation pathways are rationalized based on DFT calculations.

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► Protonation of para-aminophenol takes place at the nitrogen atom as revealed by IRMPD spectroscopy.
► Fragmentation of para-aminophenol leads to the elimination of H2O in the thermodynamically preferred pathway, whereas NH3 is lost in the kinetically preferred process.
► The IRMPD spectrum as well as the fragmentation processes were rationalized based on DFT calculations.