Yoshiyuki Matsuda

Vibrational spectroscopy of 2-pyridone and its clusters in supersonic jets: Structures of the clusters as revealed by characteristic shifts of the NH and C=O bands

Vibrational spectroscopy of the key functional vibrations of 2-pyridone and its hydrogen-bonded clusters with water, methanol, dioxane, dimethylether, as well as its dimer, has been carried out by using infrared-ultraviolet (IR–UV) and stimulated Raman–UV double resonance methods combined with fluorescence detection. The characteristic spectral changes upon the cluster formation have been observed for the NH and C=O stretching vibrations of the bare molecule and also for the OH stretching vibrations of the solvent molecules. The cluster structures were investigated by comparing the observed spectra with the simulated ones of the energy-optimized structures obtained by ab-initio molecular orbital calculations. It was found that the “ring-type” hydrogen-bonded structure is appropriate for the cluster with water or methanol, while the “linear-type” hydrogen-bonded structure is appropriate for the cluster with dioxane or dimethylether. The symmetric form of 2-pyridone dimer was confirmed by the observed spect...

Population labeling spectroscopy for the electronic and the vibrational transitions of 2-pyridone and its hydrogen-bonded clusters

The S1–S0 electronic spectra, and the vibrational spectra of jet-cooled 2-pyridone (2PY) and its hydrogen bonded clusters, 2PY–H2O and 2PY dimer, have been investigated by population labeling and various double-resonant vibrational spectroscopies. For bare 2PY, the S1–S0 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 S0 and S1. The results led to the conclusion that 2PY has two close lying electronic states in the S1 region, whose structures are slightly different with respect to the NH group. It was also found that the NH stretching frequency becomes smaller in S1 than in S0, indicating that the NH bond strength of 2PY becomes weaker in S1. 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-bo...

On the zirconium oxide neutral cluster distribution in the gas phase: Detection through 118 nm single photon, and 193 and 355 nm multiphoton, ionization

Zirconium oxide clusters are generated in the gas phase by laser ablation of the metal into a flow of ca. 5% O2/95% He at 100 psig and supersonic expansion into a vacuum chamber. Mass spectra of neutral gas phase zirconium oxide clusters are obtained through photoionization at three different laser wavelengths: 118, 193, and 355 nm. Ionization of the clusters with 118 nm laser radiation is through a single photon ionization mechanism, while ionization by 193 and 355 nm laser radiation is through a multiphoton (three or more photon) mechanism. Fragment ion features are observed in the mass spectra of ZrmOn+ for only the 193 nm and 355 nm ionization schemes. The true neutral ZrmOn cluster distribution is obtained only through 118 nm single photon ionization, as verified by mass spectral peak linewidths and calculations of the cluster binding energies, ionization energies, and fragmentation rates. The neutral cluster distribution consists mainly of the series ZrmO2m and ZrmO(2m+1) for m = 1,..., approximately 30.

On the iron oxide neutral cluster distribution in the gas phase. II. Detection through 118 nm single photon ionization

Neutral clusters of iron oxide are created by laser ablation of iron metal and subsequent reaction of the gas phase metal atoms, ions, clusters, etc., with an O2/He mixture. The FemOn clusters are cooled in a supersonic expansion and detected and identified in a time-of-flight mass spectrometer following laser ionization at 118 nm (10.5 eV), 193 nm (6.4 eV), or 355 nm (3.53 eV) photons. With 118 nm radiation, the neutral clusters do not fragment because single photon absorption is sufficient to ionize all the clusters and the energy/pulse is approximately 1 microJ. Comparison of the mass spectra obtained at 118 nm ionization (single photon) with those obtained at 193 nm and 355 nm ionization (through multiphoton processes), with regard to intensities and linewidths, leads to an understanding of the multiphoton neutral cluster fragmentation pathways. The multiphoton fragmentation mechanism for neutral iron oxide clusters during the ionization process that seems most consistent with all the data is the loss of one or two oxygen atoms. In all instances of ionization by laser photons, the most intense features are of the forms FemOm+, FemO(m+1)+, and FemO(m+2)+, and this strongly suggests that, for a given m, the most prevalent neutral clusters are of the forms FemOm, FemO(m+1), and FemO(m+2). As the value of m increases, the more oxygen rich neutral clusters appear to increase in stability.

The Intermolecular SH⋅⋅⋅Y (Y=S,O) Hydrogen Bond in the H2S Dimer and the H2S–MeOH Complex

The nature of the S−H⋅⋅⋅S hydrogen-bonding interaction in the H2 S dimer and its structure has been the focus of several theoretical studies. This is partly due to its structural similarity and close relationship with the well-studied water dimer and partly because it represents the simplest prototypical example of hydrogen bonding involving a sulfur atom. Although there is some IR data on the H2 S dimer and higher homomers from cold matrix experiments, there are no IR spectroscopic reports on S−H⋅⋅⋅S hydrogen bonding in the gas phase to-date. We present experimental evidence using VUV ionization-detected IR-predissociation spectroscopy (VUV-ID-IRPDS) for this weak hydrogen-bonding interaction in the H2 S dimer. The proton-donating S−H bond is found to be red-shifted by 31 cm(-1) . We were also able to observe and assign the symmetric (ν1 ) stretch of the acceptor and an unresolved feature owing to the free S−H of the donor and the antisymmetric (ν3 ) SH stretch of the acceptor. In addition we show that the heteromolecular H2 S-MeOH complex, for which both S−H⋅⋅⋅O and O−H⋅⋅⋅S interactions are possible, is S-H⋅⋅⋅O bound.

Infrared predissociation spectroscopy of cluster cations of protic molecules, (NH3)n+, n=2-4 and (CH3OH)n+, n=2,3.

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.

On the copper oxide neutral cluster distribution in the gas phase: Detection through 355 nm and 193 nm multiphoton and 118 nm single photon ionization

The distribution of neutral copper oxide clusters in the gas phase created by laser ablation is detected and characterized through time-of-flight mass spectroscopy (TOFMS). The neutral copper oxide clusters are ionized by two different approaches: Multiphoton absorption of 355 and 193 nm radiation; and single photon absorption of 118 nm radiation. Based on the observed cluster patterns as a function of experimental conditions (e.g., copper oxide or metal sample, ablation laser power, expansion gas, etc.) and on the width of the TOFMS features, one can uncover the true neutral cluster distribution of CumOn species following laser ablation of the sample. Ablation of a metal sample generates only small neutral CumOn clusters for m less, similar 4 and n approximately 1, 2. Ablation of copper oxide samples generates neutral clusters of the form CumOm (m < or = 4) and CumO(m-1) (m > 4). These clusters are directly detected without fragmentation using single photon, photoionization with 118 nm laser radiation. Using 355 and 193 nm multiphoton ionization, the observed cluster ions are mostly of the form Cu2mOm+ for 4 < or = m < or = 10 (193 nm ionization) and CumO1,2 (355 nm ionization) for copper oxide samples. Neutral cluster fragmentation due to multiphoton processes seems mainly to be of the form CumO(m,m-1) --> CumO(m/2,m/2+1). Neutral cluster growth mechanisms are discussed based on the cluster yield from different samples (e.g., Cu metal, CuO powder, and Cu2O powder).

Size-Selected Infrared Predissociation Spectroscopy of Neutral and Cationic Formamide-Water Clusters: Stepwise Growth of Hydrated Structures and Intracluster Hydrogen Transfer Induced by Vacuum-Ultraviolet Photoionization

Vibrational spectroscopy of size-selected formamide-water clusters, FA-(H2O)n , n = 1-4, prepared in a supersonic jet is performed with vacuum-ultraviolet-ionization detected-infrared predissociation spectroscopy (VUV-ID-IRPDS). The cluster structures are determined through comparisons of the observed IR spectra with theoretical calculations at the MP2/6-31++G** level. The FA-(H2O)n , n = 1-3, clusters have ring-type structures, where water molecules act as both single donor and single acceptor in the hydrogen-bond network between the amino and carbonyl groups of FA. For FA-(H2O)4, on the other hand, the absence of the free NH stretching vibration indicates formation of a double ring type structure, where two NH bonds of the amino group and the carbonyl oxygen of FA form hydrogen bonds with water molecules. An infrared spectrum of the formamide-water cluster cation, [FA-H2O](+), is also observed with infrared predissociation spectroscopy of vacuum-ultraviolet-pumped ion (IRPDS-VUV-PI). No band is observed for the free OH stretches of neutral water. This shows [FA-H2O](+) has such a structure that one of the hydrogen atoms of the water moiety is transferred to the carbonyl oxygen of FA(+).

Observation of an Isolated Intermediate of the Nucleophilic Aromatic Substition Reaction by Infrared Spectroscopy

Extensive studies have shown that nucleophilic aromatic substitution reactions occur upon ionization of the gas-phase clusters formed from halogen-substituted benzene and polar solvent molecules. In such systems, ionization causes the aromatic compounds to become highly electron deficient, thus ensuring the high reaction efficiencies. Recently, several s-complex-type structures, such as HC6H6, H C6H5F, (C6H6NH3) , and [CH3OC6H3(NO2)3] have been observed in the gas phase by spectroscopy. However, the elimination step is unfavorable in these cases because of their poor leaving group (X=H). As a result, the high yields of the s complexes sacrificed their ability to act as reaction intermediates. Such intermediate structures have not so far been reported for systems in which the substitution reactions can actually occur. Intracluster substitution reactions have only been observed in the cases of halobenzenes. The cluster cations formed from hexafluorobenzene (C6F6) and polar molecules are very attractive targets to probe the intermediate structures in the reaction because there are no ortho/meta/para isomers and only one type of structure for the s complex is expected. In this study, we report the first experimental observation of a stable s complex for a system in which aromatic substitution actually occurs. We employed the efficient direct ionization of C6F6 by coherent vacuum ultraviolet (VUV) light, and confirmed the high substitution reactivity of the C6F6 /NH3 system to produce C6F5NH2 . We also measured the infrared spectrum of the (C6F6-NH3) + cluster cation and showed that the cluster cation forms an intermediate s complex. Figure 1 shows the mass spectrum of the ions produced by the one-photon ionization of the C6F6/NH3/He mixture by VUV light. The most intense signal at m/z 186 corresponds to C6F6 . The signal at m/z 183 is uniquely assigned to the substitution reaction product C6F5NH2 . The intensity of this signal is comparable to that of C6F6 , and it clearly demonstrates that, upon ionization, the reaction efficiently proceeds in the C6F6/NH3 system with the elimination of HF.

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization

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: (CH(3)COOH)(2), CH(3)COOH-CH(3)OH, and CH(3)COOH-H(2)O. 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, (CH(3)COOH)(2) (+) and (CH(3)COOH-CH(3)OH)(+) form proton-transferred structures, where the acetic acid moiety donates the proton to the counter molecule. (CH(3)COOH-H(2)O)(+) has a non-proton-transferred structure, where CH(3)COOH(+) and H(2)O 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.