IR plus vacuum ultraviolet spectroscopy of neutral and ionic organic acid molecules and clusters: Acetic acid

  • Hu Y
  • Fu H
  • Bernstein E
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Infrared IR vibrational spectroscopy of acetic acid (A) neutral and ionic monomers and clusters, employing vacuum ultraviolet VUV, 10.5 eV single photon ionization of supersonically expanded and cooled acetic acid samples, is presented and discussed. Molecular and cluster species are identified by time of flight mass spectroscopy: the major mass features observed are AnH+ n =1�9, ACOOH+ VUV ionization without IR radiation present, and A+ with both IR and VUV radiation present. The intense feature ACOOH+ arises from the cleavage of A2 at the -CC bond to generate ACOOH++CH3 following ionization. The vibrational spectrum of monomeric acetic acid 2500�7500 cm-1 is measured by nonresonant ionization detected infrared NRID-IR spectroscopy. The fundamentals and overtones of the CH and OH stretches and some combination bands are identified in the spectrum. Mass selected IR spectra of neutral and cationic acetic acid clusters are measured in the 2500�3800 cm-1 range employing nonresonant ionization dip-IR and IR photodissociation IRPD spectroscopies, respectively. Characteristic bands observed at approximately 2500�2900 cm-1 for the cyclic ring dimer are identified and tentatively assigned. For large neutral acetic acid clusters Ann2, spectra display only hydrogen bonded OH stretch features, while the CH modes 2500�2900 cm-1 do not change with cluster size n. The IRPD spectra of protonated cationic acetic acid clusters AnH+ n=1�7 exhibit a blueshift of the free OH stretch with increasing n. These bands finally disappear for n6, and one broad and weak band due to hydrogen bonded OH stretch vibrations at approximately 3350 cm-1 is detected. These results indicate that at least one OH group is not involved in the hydrogen bonding network for the smaller n5 AnH+ species. The disappearance of the free OH stretch feature at n6 suggests that closed cyclic structures form for AnH+ for the larger clusters n6.

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