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Infrared (IR) spectroscopy based on vacuum-ultraviolet one-photon ionization detection was carried out to investigate geometric structures of neutral and cationic clusters of acetic acid: (CH3COOH)2, CH3COOH–CH3OH, and CH3COOH–H2O. All the neutral clusters have cyclic-type intermolecular structures, in which acetic acid and solvent molecules act as both hydrogen donors and acceptors, and two hydrogen-bonds are formed. On the other hand, (CH3COOH)2+ and (CH3COOH–CH3OH)+ form proton-transferred structures, where the acetic acid moiety donates the proton to the counter molecule. (CH3COOH–H2O)+ has a non-proton-transferred structure, where CH3COOH+ and H2O are hydrogen-bonded. The origin of these structural differences among the cluster cations is discussed with the relative sizes of the proton affinities of the cluster components and the potential energy curves along the proton-transfer coordinate. [ABSTRACT FROM AUTHOR]

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH4+...NH2 or CH3OH2+...OCH3, is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters. [ABSTRACT FROM AUTHOR]

Infrared predissociation spectroscopy of vacuum ultraviolet-pumped ion (IRPDS-VUV-PI) is performed on ammonia cluster cations (NH3)n+ (n=2–4) that are produced by VUV photoionization in supersonic jets. The structures of (NH3)2+ and (NH3)4+ are determined through the observation of infrared spectra and vibrational calculations based on ab initio calculations at the MP2/6-31G** and 6-31++G** levels. (NH3)2+ is found to be of the “hydrogen-transferred” form having the (H3N+–H...NH2) composition. In contrast, (NH3)4+ exhibits the “head-to-head” dimer cation (H3N...NH3)+ core structure, where the positive charge is shared between two ammonia molecules in the core, and two other molecules are hydrogen bonded onto the core. An unequivocal assignment of the infrared spectrum of (NH3)3+ has not been achieved, because the presence of two isomeric structures could be suggested by the observed spectrum and theoretical calculations. [ABSTRACT FROM AUTHOR]

The S[sub 1]-S[sub 0] electronic spectra, and the vibrational spectra of jet-cooled 2-pyridone (2PY) and its hydrogen bonded clusters, 2PY-H[sub 2]O and 2PY dimer, have been investigated by population labeling and various double-resonant vibrational spectroscopies. For bare 2PY, the S[sub 1]-S[sub 0] spectrum was measured by laser-induced fluorescence and population labeling spectroscopy. In addition, IR and Raman spectra of the NH stretching vibration were observed in S[sub 0] and S[sub 1]. The results led to the conclusion that 2PY has two close lying electronic states in the S[sub 1] region, whose structures are slightly different with respect to the NH group. It was also found that the NH stretching frequency becomes smaller in S[sub 1] than in S[sub 0], indicating that the NH bond strength of 2PY becomes weaker in S[sub 1]. The effect of the electronic excitation on the hydrogen bond strength has also been investigated by measuring the NH and OH stretching vibrations of the hydrogen bonded clusters in the two electronic states, and it was found that the hydrogen-bond strength is weaker in S[sub 1] than in S[sub 0]. For 2PY dimer, the IR and the Raman spectra of the NH stretching bands showed a clear intensity alternation, confirming its C[sub 2h] symmetric structure. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on the vibrational spectroscopy of the key functional vibrations of 2-pyridone and its hydrogen-bonded clusters with water, methanol, dimethylether, as well as its dimer, using infrared-ultraviolet and stimulated Raman-UV double resonance methods. Appropriateness of the ring-type hydrogen-bonded structure for the cluster with water or methanol.

Infrared (IR) spectroscopy of neutral and cationic dimers of tetrahydrofuran (THF) and tetrahydropyran (THP) is carried out to investigate isomerization reactions of these dimers following ionization. In the neutral state, both the THF and THP dimers tend to form the stacking structures bound essentially by dispersion. After ionization, both of these dimer cations form the proton transferred structures, in which the proton of the CαH bond is intermolecularly transferred and shared between the oxygen atoms. Along with the pseudorotation of the five-member ring, the barrierless proton transfer from CH occurs in the ionized THF dimer, and the dimer finally forms the proton transferred structure. In the case of the THP dimer cation, the barrierless intracluster proton transfer occurs with the conformational change of the proton donor site because the acidic CH site of the THP cation strongly depends on its conformation. These isomerization processes involving proton transfer from CH are discussed based on IR spectroscopy and theoretical reaction path searches. [ABSTRACT FROM AUTHOR]

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH(4) (+)...NH(2) or CH(3)OH(2) (+)...OCH(3), is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters.

Infrared predissociation spectroscopy of vacuum ultraviolet-pumped ion (IRPDS-VUV-PI) is performed on ammonia cluster cations (NH3)n+ (n=2-4) that are produced by VUV photoionization in supersonic jets. The structures of (NH3)2+ and (NH3)4+ are determined through the observation of infrared spectra and vibrational calculations based on ab initio calculations at the MP2/6-31G** and 6-31++G** levels. (NH3)2+ is found to be of the "hydrogen-transferred" form having the (H3N+-...NH2) composition. In contrast, (NH3)4+ exhibits the "head-to-head" dimer cation (H3...NH3+ core structure, where the positive charge is shared between two ammonia molecules in the core, and two other molecules are hydrogen bonded onto the core. An unequivocal assignment of the infrared spectrum of (NH3)3+ has not been achieved, because the presence of two isomeric structures could be suggested by the observed spectrum and theoretical calculations.