Conformationally resolved structures of jet-cooled acetaminophen by UV–UV hole-burning spectroscopy

Abstract

The conformational structures of jet-cooled acetaminophen were investigated in the gas phase by resonant 2-photon ionization and UV–UV hole-burning spectroscopy. In contrast to the results from a previous study, two nearly isoenergetic conformers were distinctly found in a supersonic molecular beam expansion and positively identified as the cis and trans isomers of acetaminophen by UV–UV hole-burning spectroscopy. The 0–0 bands of the cis and trans isomers were found at 33518.7 and 33485.6 cm⁻¹, respectively. The vibronic bands of the two isomers are close-lying and/or partially overlapping due to the small energy difference (33 cm⁻¹) between the two 0–0 bands. As a consequence, the recorded resonant 2-photon ionization spectrum is highly congested in the low excitation energy region, which develops continuously into a featureless, broadened spectrum in the high energy region.