Conformational structures of jet-cooled acetaminophen-water clusters: a gas phase spectroscopic and computational study.

Phys Chem Chem Phys

Department of Chemistry (BK21+) and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea.

Published: February 2017

Jet-cooled acetaminophen (AAP)-water clusters, AAP-(HO), were investigated by mass-selected resonant two-photon ionization (R2PI), ultraviolet-ultraviolet hole-burning (UV-UV HB), infrared-dip (IR-dip), and infrared-ultraviolet hole-burning (IR-UV HB) spectroscopy. Each syn- and anti-AAP rotamer has three distinctive binding sites (-OH, >CO, and >NH) for a water molecule, thus 6 different AAP-(HO) conformers are expected to exist in the molecular beam. The origin bands of the AAP(OH)-(HO) and AAP(CO)-(HO) conformers (including their syn- and anti-conformers) in the R2PI spectrum are shifted to red and blue compared to those of the AAP monomer, respectively. These frequency shifts upon complexation between a water molecule and a specific binding site of AAP are also predicted by theoretical calculations. The spectral assignments of the origin bands in the R2PI spectra and the IR vibrational bands in the IR-dip spectra of the four lowest-energy conformers of AAP-(HO), [syn- and anti-AAP(OH)-(HO) and syn- and anti-AAP(CO)-(HO)], are aided by ab initio and time-dependent density functional theory (TDDFT) calculations. Further investigation of the IR-dip spectra has revealed a hydrogen-bonded NH stretching mode, supporting the presence of the syn-AAP(NH)-(HO) conformer. Moreover, by employing IR-UV HB spectroscopy, we have reconfirmed the existence of the syn-AAP(NH)-(HO) conformer, which happened to be buried underneath the broad background contributed by the AAP(OH)-(HO) conformers. These observations have led us to conclude that all of the possible conformers of AAP-(HO) have been found in this study.

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http://dx.doi.org/10.1039/c6cp06863dDOI Listing

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