Internal Energy Dependence of the Pyrrole Dimer Cation Structures Formed in a Supersonic Plasma Expansion: Charge-Resonance and Hydrogen-Bonded Isomers.

The structures of the pyrrole dimer cation (Py2+) formed in an electron-ionization-driven supersonic plasma expansion of Py seeded in Ar or N2 are probed as a function of its internal energy by infrared photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. The IRPD spectra recorded in the CH and NH stretch ranges are analyzed by dispersion-corrected density functional theory (DFT) calculations at the B3LYP-D3/aug-cc-pVTZ level. The spectra of the cold Ar/N2-tagged Py2+ clusters, Py2+Ln (n = 1-5 for Ar, n = 1 for N2), indicate the exclusive formation of the most stable antiparallel π-stacked Py2+ structure under cold conditions, which is stabilized by charge-resonance interaction. The bare Py2+ dimers produced in the ion source have higher internal energy, and the observation of additional transitions in their IRPD spectra suggests a minor population of less stable hydrogen-bonded isomers composed of heterocyclic Py/Py+ structures formed after intramolecular H atom transfer and ring opening. These intermolecular isomers differ from the chemically bonded structures proposed earlier in the analysis of IRPD spectra of Py2+ generated by VUV ionization of neutral Pyn clusters.

IR cavity ringdown spectroscopy and density functional theory calculation of pyrrole-diethyl ketone clusters: impacts of carbon-chain flexibility on the diversity of N-H⋯OC hydrogen bonds.

The N-H⋯OC hydrogen bond (H-bond) plays a key role in stabilizing the geometry and energy of biomolecules such as protein folding and DNA double strand. To investigate N-H⋯OC H-bonds in a microscopic view, we apply IR cavity ringdown spectroscopy (IR-CRDS) and density functional theory (DFT) calculation to pyrrole-diethyl ketone (Py-Dek) clusters in the gas phase. Dek exhibits a pentane carbon chain, which provides various conformations such as anti, gauche, and their mixtures. An introduction of the carbon-chain flexibility to Py-Dek clusters is expected to cause a diversity of the N-H⋯OC H-bond formation. In the observed IR spectra, there are seven prominent bands of the NH stretches due to Py-Dek clusters. These bands are classified into three groups: one for Py1-Dek1, two for Py1-Dek2, and four for Py2-Dek1. Stable structures and their harmonic frequencies obtained by DFT calculations provide the proper NH band assignments and appropriate cluster structures. Py1-Dek1 exhibits a single isomer, which is formed by an ordinary N-H⋯OC H-bond between Py and anti-conformation of Dek (Dek(a)) with a linear carbon-chain. Py1-Dek2 shows two isomeric structures, in which both isomers are commonly constructed by the N-H⋯OC H-bond for the first Dek and by the stacking interaction between π electrons of Py and the second Dek. Both isomers exhibit the Dek(a) for the stacking interaction, but are distinguished between Dek(a) and gauche-conformation Dek (Dek(g)) for the N-H⋯OC H-bond. Py2-Dek1 shows a triangular cyclic structure, which is formed by the N-H⋯OC H-bond, the N-H⋯π H-bond, and the stacking interaction between Py and Dek. The observed four bands are assigned to two N-H⋯OC and two N-H⋯π H-bonds for two isomeric structures due to Dek(a) and Dek(g). Not only smaller clusters but also higher hetero-tetramers are characterized based on the architecture of smaller clusters. In particular, Py2-Dek(a)2(I) was the first to be found with a highly symmetric (Ci) cyclic structure. Calculated potential energy surfaces of Py-Dek clusters shed light on the impact of Dek flexibility on N-H⋯OC H-bond diversity. Selective formation of isomeric structures for Py-Dek clusters is discussed in terms of a mechanism of a two- and three-body collision process in a supersonic expansion.

Microhydration of the Pyrrole Cation (Py+) Revealed by IR Spectroscopy: Ionization-Induced Rearrangement of the Hydrogen-Bonded Network of Py+(H2O)2.

Microhydration of heterocyclic aromatic molecules can be an appropriate fundamental model to shed light on intermolecular interactions and functions of macromolecules and biomolecules. We characterize herein the microhydration process of the pyrrole cation (Py+) by infrared photodissociation (IRPD) spectroscopy and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Analysis of IRPD spectra of mass-selected Py+(H2O)2 and its cold Ar-tagged cluster in the NH and OH stretch range combined with geometric parameters of intermolecular structures, binding energies, and natural atomic charge distribution provides a clear picture of the growth of the hydration shell and cooperativity effects. Py+(H2O)2 is formed by stepwise hydration of the acidic NH group of Py+ by a hydrogen-bonded (H2O)2 chain with NH···OH···OH configuration. In this linear H-bonded hydration chain, strong cooperativity, mainly arising from the positive charge, strengthens both the NH···O and OH···O H-bonds with respect to those of Py+H2O and (H2O)2, respectively. The linear chain structure of the Py+(H2O)2 cation is discussed in terms of the ionization-induced rearrangement of the hydration shell of the neutral Py(H2O)2 global minimum characterized by the so-called "σ-π bridge structure" featuring a cyclic NH···OH···OH···π H-bonded network. Emission of the π electron from Py by ionization generates a repulsive interaction between the positive π site of Py+ and the π-bonded OH hydrogen of (H2O)2, thereby breaking this OH···π hydrogen bond and driving the hydration structure toward the linear chain motif of the global minimum on the cation potential.

IR Cavity Ringdown Spectroscopy and Density Functional Theory for Jet-Cooled Pyrrole-Cyclopentanone Binary Clusters: Effect of Pseudorotation on N-H···O═C Hydrogen Bonds.

The geometry and energetics of the N-H···O═C hydrogen bond (H-bond) are important to understand the stability and flexibility of biomolecules, such as protein and DNA. Jet-cooled pyrrole-cyclopentanone (Py-Cp) binary clusters are appropriate models to investigate the N-H···O═C H-bond from a microscopic point of view. In this study, NH stretching vibrations of the Py-Cp binary clusters were observed by IR cavity ringdown spectroscopy. Furthermore, density functional theory calculations revealed geometric structures, harmonic vibrations, intermolecular energies, and donor-acceptor interactions for various sizes of binary clusters. The IR spectra of the Py-Cp binary clusters were measured under various conditions of the vapor pressures of Py and Cp in He buffer gas for a supersonic expansion. The dependence of the IR band intensities on the vapor pressure provides vibrational assignments of the NH stretching vibrations, which were reproduced by calculated frequencies of Py1-Cp1, Py1-Cp2, and Py2-Cp1. An admixture of Ar in He buffer gas for a supersonic expansion was also applied to produce Py1-Cp2 in order to differentiate several NH stretches of isomeric structures due to the pseudorotation of Cp molecules. Py1-Cp1 is formed by the N-H···O═C H-bond. Py1-Cp2 has a cyclic structure that is formed by the N-H···O═C H-bond and stacking interactions among Py and two Cp molecules. Py2-Cp1 also has a cyclic structure that is formed by not only the N-H···O═C H-bond but also a N-H···π H-bond between two Py molecules and a stacking interaction between Py and Cp. A comparison of the H-bond geometries between Py2-Cp1 and the corresponding pyrrole-acetone binary cluster reveals that the stacking interaction between Py and Cp strengthens the N-H···O═C H-bond through a cooperative effect.

An effective Hamiltonian analysis of a Franck-Condon-like pattern in the IR spectra of phenol-alkylsilane dihydrogen-bonded clusters in the S1 state.

Infrared (IR) spectra in a region of the OH stretch band of phenol (PhOH)-ethyldimethylsilane (EDMS), phenol (PhOH)-triethylsilane (TES), and phenol (PhOH)-t-butyldimethylsilane (BDMS) dihydrogen-bonded clusters in the S1 state were observed. All of the species exhibited unconventional band patterns in which many combination bands appeared with comparable intensities to those of allowed bands. Such a behavior is sometimes called a Franck-Condon-like pattern. In the case of the PhOH-BDMS, one intermolecular vibrational mode is involved in this behavior. The observed IR spectra were well reproduced based on the concept of the Franck-Condon-like behavior. As an alternative treatment, we analyzed the band patterns on the concept of intensity borrowing due to the vibrational anharmonic interaction. The analysis was based on an effective Hamiltonian involving an anharmonic interaction between the OH stretch and intermolecular vibrational modes. Two treatments provided the same results. Thus, it was confirmed that the Franck-Condon-like behavior originates from vibrational anharmonic interactions. In the cases of the PhOH-EDMS and PhOH-TES, we carried out a two-dimensional Franck-Condon and an effective Hamiltonian analysis to interpret the Franck-Condon-like patterns. We examined vibrational wave functions obtained by the latter analysis. Shapes of the wave functions suggest that a recombination of the intermolecular vibrational modes occurs during the excitation of OH stretch mode in these clusters, which is a similar behavior to the Duschinsky effect in the electronic transition.

A comprehensive infrared spectroscopic and theoretical study on phenol-ethyldimethylsilane dihydrogen-bonded clusters in the S0 and S1 states.

To investigate microscopic characters of Si-H⋯H-O type dihydrogen bonds, we observed OH and SiH stretch bands in both the S0 and S1 states of phenol-ethyldimethylsilane (PhOH-EDMS) clusters by infrared (IR)-ultraviolet (UV) and UV-IR double resonance spectroscopies. Density functional theory (DFT) calculations and energy decomposition analysis were also performed. Structures of two isomers identified were unambiguously determined through the analysis of IR spectra and DFT calculations. To discuss the strength of dihydrogen bond in various systems, we performed theoretical calculations on clusters of EDMS with several acidic molecules in addition to PhOH. It was revealed that charge-transfer interaction energies from a bonding σ orbital of SiH bond to an anti-bonding σ* orbital of OH bond well reflect strengths of dihydrogen bonds. Additionally, it was found that the red shift of SiH stretch frequencies can be used as a crude measure of the strength of dihydrogen bonds. Relationship between the red shifts of OH/SiH stretch frequencies and various electrostatic components of the interaction energy was examined. In the S1 state, large increases in red shifts were observed for both the OH and SiH stretch frequencies. Since the EDMS moiety is not associated with the electronic excitation in a cluster, the strength of dihydrogen bonds in the S1 and S0 states was able to be directly compared based on the red shifts of the SiH stretch bands. A significant increase in the red shift of SiH stretch frequency indicates a strengthening of the dihydrogen bonds during the electronic excitation of the PhOH moiety.

Aromatic Charge Resonance Interaction Probed by Infrared Spectroscopy.

Charge resonance is a strong attractive intermolecular force in aromatic dimer radical ions. Despite its importance, this fundamental interaction has not been characterized at high resolution by spectroscopy of isolated dimers. We employ vibrational infrared spectroscopy of cold aromatic pyrrole dimer cations to precisely probe the charge distribution by measuring the frequency of the isolated N-H stretch mode (νNH ). We observe a linear correlation between νNH and the partial charge q on the pyrrole molecule in different environments. Subtle effects of symmetry reduction, such as substitution of functional groups (here pyrrole replaced by N-methylpyrrole) or asymmetric solvation (here by an inert N2 ligand), shift the charge distribution toward the moiety with lower ionization energy. This general approach provides a precise experimental probe of the asymmetry of the charge distribution in such aromatic homo- and heterodimer cations.

Microsolvation of the pyrrole cation (Py+) with nonpolar and polar ligands: infrared spectra of Py+-Ln with L = Ar, N2, and H2O (n ≤ 3).

The solvation of aromatic (bio-)molecular building blocks has a strong impact on the intermolecular interactions and function of supramolecular assemblies, proteins, and DNA. Herein we characterize the initial microsolvation process of the heterocyclic aromatic pyrrole cation (Py+) in its 2A2 ground electronic state with nonpolar, quadrupolar, and dipolar ligands (L = Ar, N2, and H2O) by infrared photodissociation (IRPD) spectroscopy of cold mass-selected Py+-Ln (n ≤ 3) clusters in a molecular beam and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Size- and isomer-specific shifts in the NH stretch frequency (ΔνNH) unravel the competition between various ligand binding sites, the strength of the respective intermolecular bonds, and the cluster growth. In Py+-Ar, linear H-bonding of Ar to the acidic NH group (NHAr) is competitive with π-stacking to the aromatic ring, and both Py+-Ar(H) and Py+-Ar(π) are observed. For L = N2 and H2O, the linear NHL H-bond is much more stable than any other binding site and the only observed binding motif. For the Py+-Ar2 and Py+-(N2)2 trimers, the H/π isomer with one H-bonded and one π-bonded ligand strongly competes with a 2H isomer with two bifurcated nonlinear NHL bonds. The latter are equivalent for Ar but nonequivalent for N2. Py+-H2O exhibits a strong and linear NHO H-bond with charge-dipole configuration and C2v symmetry. IRPD spectra of cold Py+-H2O-L clusters with L = Ar and N2 reveal that Ar prefers π-stacking to the Py+ ring, while N2 forms an OHN2 H-bond to the H2O ligand. The ΔνNH frequency shifts in Py+-Ln are correlated with the strength of the NHL H-bond and the proton affinity (PA) of L, and a monotonic correlation between ΔνNH of the Py+-L(H) dimers and PA is established. Comparison with neutral Py-L dimers reveals the strong impact of the positive charge on the acidity of the NH group, the strength of the NHL H-bond, and the preferred ligand binding motif.

Identification of crystalline structures in jet-cooled acetylene large clusters studied by two-dimensional correlation infrared spectroscopy.

We investigated the crystalline structures of jet-cooled acetylene (C2H2) large clusters by laser spectroscopy and chemometrics. The CH stretching vibrations of the C2H2 large clusters were observed by infrared (IR) cavity ringdown spectroscopy. The IR spectra of C2H2 clusters were measured under the conditions of various concentrations of C2H2/He mixture gas for supersonic jets. Upon increasing the gas concentration from 1% to 10%, we observed a rapid intensity enhancement for a band in the IR spectra. The strong dependence of the intensity on the gas concentration indicates that the band was assigned to CH stretching vibrations of the large clusters. An analysis of the IR spectra by two-dimensional correlation spectroscopy revealed that the IR absorption due to the C2H2 large cluster is decomposed into two CH stretching vibrations. The vibrational frequencies of the two bands are almost equivalent to the IR absorption of the pure- and poly-crystalline orthorhombic structures in the aerosol particles. The characteristic temperature behavior of the IR spectra implies the existence of the other large cluster, which is discussed in terms of the phase transition of a bulk crystal.

Reaction dynamics of Al + O2 → AlO + O studied by a crossed-beam velocity map imaging technique: vib-rotational state selected angular-kinetic energy distribution.

Oxidation reaction of a gas-phase aluminum atom by a molecular oxygen was studied by a crossed-beam condition at 12.4 kJ/mol of collision energy. A (1+1) resonance-enhanced multiphoton ionization (REMPI) via the D(2)Σ(+)-X(2)Σ(+) transition of AlO was applied to ionize the product. The REMPI spectrum was analyzed to determine rotational state distributions for v = 0-2 of AlO. For several vib-rotational states of AlO, state selected angular and kinetic energy distributions were determined by a time-sliced ion imaging technique for the first time. Kinetic energy distributions were well represented by that taken into account initial energy spreads of collision energy and the population of the spin-orbit levels of the counter product O((3)P(J)) determined previously. All angular distributions showed forward and backward peaks, and the forward peaks were more pronounced than the backward one for the states of low internal energy. The backward peak intensity became comparable to the forward one for the states of high internal energy. These results and the rotational state distributions suggested that the reaction proceeds via an intermediate which has a lifetime comparable to or shorter than its rotational period.

Contribution of the π electron to the N-H···O=C hydrogen bond: IR spectroscopic studies of the jet-cooled pyrrole-acetone binary clusters.

To investigate the π bonding electron contribution to N-H···O=C hydrogen-bond (H-bond) formation, we applied IR cavity ringdown spectroscopy to jet-cooled pyrrole-acetone (Py-Ac) binary clusters. The observed NH stretching vibrations were analyzed by density functional theory (DFT), in which the energetically optimized structures, harmonic frequencies, and interaction energies were calculated for various sizes of binary clusters. We observed three NH stretching vibrations, ascribed to binary clusters at 3406, 3388, and 3335 cm(-1). These were assigned to H-bonded NH stretches of the Py(2)-Ac(1), Py(1)-Ac(1), and Py(1)-Ac(2) clusters, respectively. The Py(1)-Ac(1) cluster has a single N-H···O=C H-bonded structure with C(s) symmetry, while the Py(1)-Ac(2) cluster has a cyclic structure formed by a single N-H···O=C H-bond, dipole-dipole interactions, and weak CH H-bonds. A natural bond orbital (NBO) analysis was performed to reveal the H-bond strength in Py-Ac binary clusters. For the Py(1)-Ac(2) cluster, we found that the donor-acceptor interactions are not only the n →σ* type (O atom lone pair to the NH anti-bonding orbitals), but also the π→σ* type (the CO π bonding to the NH anti-bonding orbitals). By analyzing the relationship between the frequency shift and the stabilization energy in donor-acceptor interactions, we concluded that larger red-shift of the NH stretching vibration in the Py(1)-Ac(2) can be explained by not only the lone pair and the π electron contributions to the N-H···O=C H-bond, but also the dipole-interaction between Py and non-H-bonded Ac. We also discussed the structures of Py(2)-Ac(1) clusters.

Reaction dynamics of Mo + O(2) → MoO + O studied by a crossed-beam velocity map imaging technique.

The oxidation reaction dynamics of gas-phase molybdenum atoms by oxygen molecules was studied under a crossed-beam condition. The product MoO was detected by a time-of-flight mass spectrometer combined with laser multi-photon ionization. An acceleration lens system designed for the ion-velocity mapping condition, a two-dimensional (2D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products at three collision energies: 10.0, 17.8, and 50.0 kJ/mol. The angular distributions showed forward and backward peaks, whose relative intensities changed by the collision energy. While two peaks had similar intensities at low collision energies, the forward peak became dominant at the highest collision energy, 50 kJ/mol. The product kinetic energy distributions showed a good correlation with the initial collision energies, i.e., almost the same energy as the collision energy appeared as the product kinetic energy. These results suggested that the reaction proceeds via an intermediate complex, and the lifetime of the complex becomes shorter than its rotational period at high collision energy.

Fish-Bite structure by three-dimensional hydrogen-bond acceptor: IR spectroscopy of pyrrole and N-methylpyrrole binary clusters.

The N-H...π hydrogen-bonded (H-bonded) structures of pyrrole (Py) and N-methylpyrrole (NMPy) binary clusters have been studied by IR cavity ringdown spectroscopy and density functional theory calculations. The Py(1)-NMPy(1) cluster has an "L-shape" structure, which is formed by an ordinary H-bond between a N-H donor of Py and a π-electron cloud acceptor of NMPy. The Py(2)-NMPy(1) cluster has a "Cyclic" structure, which is also formed by ordinary N-H...π H-bonds as well as the weak C-H...π H-bond between the methyl CH group and the π cloud acceptor of Py. On the other hand, the Py(1)-NMPy(2) cluster shows an extraordinary structure, in which the single donor NH group is surrounded by a three-dimensional H-bond acceptor formed by two aromatic π electron clouds. We call the Py(1)-NMPy(2) cluster as the "Fish-Bite" structure. The Py(1)-NMPy(2) cluster exhibits a redshifted NH stretch by 157 cm(-1) from the Py monomer, which is larger than 94 cm(-1) of the Py(1)-NMPy(1) cluster. However, both Py(1)-NMPy(1) and Py(1)-NMPy(2) clusters have calculated IR intensities of 169 and 163 km/mol, respectively. This result indicates that not only the N-H...π H-bonds but also the dipole-dipole interaction between Py and NMPy contributes to the Fish-Bite Py(1)-NMPy(2) cluster formation.

Time-sliced ion-velocity imaging study of the reaction Y + O2 → YO + O.

The oxidation reaction dynamics of the gas-phase yttrium atoms by oxygen molecules was studied under crossed-beam conditions. The product YO was detected using a time-of-flight mass spectrometer combined with laser single-photon ionization. An acceleration lens system designed for the ion-velocity mapping conditions, a two-dimensional (2-D) detector, and a time-slicing technique were used to obtain the velocity and angular distributions of the products. Two ionization wavelengths were used for the internal (vibrational and/or electronic) energy selective detection of YO. The single photon of the shorter wavelength (202.0 nm) can ionize all states of YO(X (2)Σ, A' (2)Δ, and A (2)Π), while electronically excited YO(A' and A) are dominantly ionized at a longer wavelength (285.0 nm). Time-sliced images were converted to the velocity and angular distributions in the center-of-mass frame. The general features of the velocity distributions of YO, determined at two wavelengths, were well represented by those expected from the statistical energy disposal model. The forward-backward symmetry was also observed for two images. These results suggest that the reaction proceeds via long-lived intermediates, and that this mechanism is consistent with previous chemiluminescence/LIF studies.

Hydrogen-bonded structures for self-aggregates of 2,5-dimethylpyrrole and its binary clusters with pyrrole studied by IR cavity ringdown spectroscopy.

N-H···π hydrogen-bonded (H-bonded) structures were studied by applying vibrational spectroscopy to self-aggregate clusters of 2,5-dimethylpyrrole (DMPy) and its binary clusters with pyrrole (Py). The NH stretching vibrations of jet-cooled clusters were observed by IR cavity ringdown spectroscopy. A combination of experiments and density functional theory calculations revealed the stable structures, intermolecular binding energies, and harmonic vibrational frequencies. The IR spectrum of the DMPy self-aggregate clusters was very similar in spectral features to that of the Py clusters in a previous work. The observed NH stretching vibrations at 3505, 3420, 3371, and 3353 cm(-1) are simultaneously red-shifted by ∼25 cm(-1) from the Py monomer, dimer, trimer, and tetramer, respectively. Based on a spectral analogy of DMPy with Py, and a consistency of the calculated harmonic frequencies with experiments, the H-bonded structures of the DMPy clusters were determined to be of a T-shape for a dimer and a cyclic for a trimer and a tetramer. For the DMPy-Py binary clusters, we discussed the stability and geometry of the N-H···π interactions in the T-shaped dimer and the cyclic trimer. The binary dimer showed the only single NH stretch at 3419 cm(-1) in the IR spectrum. A vibrational analysis of the H-bonded NH stretches as well as the calculated stabilization energies deduced that only the binary dimer by DMPy as an acceptor and Py as a donor can exist in a supersonic jet. For binary trimers, NH stretches were observed due to both (DMPy)(2)-(Py)(1) and (DMPy)(1)-(Py)(2). They were found to have different vibrational patterns from each other; the former showed three dispersed NH stretches, and the other had two quasi-degenerate NH stretches. Throughout this study, we also considered the intermolecular geometries, such as the H-bond distance and the angle in terms of the methyl group substitution effect.

Two-Dimensional Correlation Spectroscopy (2D-COS) of Gas-Phase Pyrrole Clusters in a Supersonic Jet: Treatment of Sharp Bands on a Broad Background.

Two-dimensional correlation spectroscopy (2D-COS) is a useful technique to analyze any intensity behavior of optical spectra that exhibit a complicated feature with overlapped bands. In this study, we apply 2D-COS to the infrared (IR) spectra of gas-phase pyrrole (Py) clusters. The NH stretching vibrations of the Py clusters are measured by cavity ringdown spectroscopy. The observed IR spectra of the Py clusters consist of sharp bands, full width half-maximum (FWHM) ∼1 cm-1, and a broad background (FWHM >50 cm-1). The 2D asynchronous correlation spectra reveal that the sharp bands and a broad background are assigned to small clusters of dimer to pentamer and large clusters with bulk-like structures, respectively, which support the results of our previous study. The sharp bands are also analyzed using another 2D asynchronous correlation spectrum, which is obtained by decomposing the observed IR spectra into sharp and broad components. Because the asynchronous signals are consistent with those obtained from the IR spectra without decomposition, the result would suggest that we need not to decompose the IR spectra into sharp and broad components before applying 2D-COS. However, our model simulations of 2D-COS showed a counterexample that gives an incorrect result without removing a broad background component from the IR spectra. This study strongly suggests that we need to undertake a careful treatment of the complicated IR spectrum with various widths of bands.

Hydrogen-bonded structures of pyrrole-solvent clusters: infrared cavity ringdown spectroscopy and quantum chemical calculations.

The hydrogen-bonded structures of pyrrole-solvent (H(2)O,CH(3)OH,C(2)H(5)OH) binary clusters were studied by the combination of experimental and theoretical techniques. Infrared cavity ringdown spectroscopy was applied to observe the NH and OH stretching vibrations of the clusters. The structures, binding energies, and normal modes of the binary clusters were obtained by quantum chemical calculations of the MP2/6-31+G(d,p) and B3LYP/6-311+G(d,p) levels. For the 1:1 clusters of pyrrole-H(2)O, pyrrole-CH(3)OH, and pyrrole-C(2)H(5)OH, the hydrogen-bonded NH stretching vibrations were observed at 3448, 3414, and 3408 cm(-1), respectively. They were redshifted from the NH stretching vibration of the pyrrole monomer, and the amounts of the redshift were proportional to the proton affinities of the solvent molecules. MP2 level calculations revealed that the sigma-type (NH...O) hydrogen-bonded structures had 7.6-9.0 kJ/mol larger binding energies than the pi-type structures (OH...pi electron cloud of pyrrole), and that the vibrational frequencies of the sigma-type structures are consistent with the observed spectra. In addition to the 1:1 clusters, the NH or OH stretching vibrations of pyrrole-CH(3)OH binary clusters were observed at 3432 and 3549 cm(-1). Among three optimized structures of the pyrrole-(CH(3)OH)(2), the sigma-pi bridge pyrrole-(CH(3)OH)(2) provided a reasonable agreement between the observed and calculated vibrational frequencies. For the pyrrole-H(2)O binary clusters, three new bands were observed at 3414, 3435, and 3541 cm(-1). These bands are consistent with the calculated NH and OH stretching vibrations of the (pyrrole)(2)-H(2)O cluster, which has a closed cyclic hydrogen-bonded structure.

Excited state reaction dynamics of Ti(a(5)F(J)) + O2 --> TiO(A,B) + O studied by a crossed-beam technique.

Oxidation reactions of the gas-phase titanium atom in its excited state with oxygen molecule, Ti(a(5)F(J)) + O(2) --> TiO(A(3)Phi,B(3)Pi) + O, were studied by a crossed-beam technique. Metastable excited Ti, Ti(a(5)F(J)), was generated by an optical pumping method and the reaction products were detected by the chemiluminescence spectroscopy. The chemiluminescence from TiO(A(3)Phi,B(3)Pi) was analyzed to determine vib-rotational state distributions of both excited states and their branching ratio. The vib-rotational state distribution of TiO(B) was represented by the statistical energy disposal and the branching ratio of TiO(A)/TiO(B) was also consistent with the statistical expectation. These results suggested the presence of long-lived intermediates in the course of the reactions of the excited Ti(a(5)F(J)) atom with O(2). Also observed was the significant deviation of the vibrational state distribution of TiO(A) from the statistical one and another reaction pathway which may not proceed via the long-lived intermediates was implied.

NH stretching vibrations of pyrrole clusters studied by infrared cavity ringdown spectroscopy.

The IR spectra for various sizes of pyrrole clusters were measured in the NH stretching vibration region by infrared cavity ringdown spectroscopy. The hydrogen-bonded structures and normal modes of the pyrrole clusters were analyzed by a density functional theory calculation of the B3LYP/6-311+G(d,p) level. Two types of pulsed nozzles, a slit and a large pinhole, were used to generate different cluster size distributions in a supersonic jet. A rotational contour analysis of the NH stretching vibration for the monomer revealed that the slit nozzle provides a warmer jet condition than the pinhole one. The IR spectra, measured under the warmer condition, showed the intense bands at 3444, 3392, and 3382 cm(-1), which were assigned to hydrogen-bonded NH stretching vibrations due to the dimer, the trimer, and the tetramer, respectively. On the other hand, the IR spectra measured under a lower temperature condition by a pinhole nozzle showed a broad absorption feature in addition to sharp bands. This broad absorption was reproduced by the sum of two Gaussians peaks at 3400 and 3372 cm(-1) with widths of 30 and 50 cm(-1) (FWHM), respectively. Compared with the spectra of the condensed phase, two bands at 3400 and 3372 cm(-1) were assigned to hydrogen-bonded NH stretching vibrations of larger clusters having liquid-like and solid-like structures, respectively.

Detection of O(3P(J)) atoms formed by reaction, Al+O2--> AlO+O under crossed-beam condition.

The vacuum ultraviolet laser-induced fluorescence technique was employed to detect the oxygen atoms formed by the reaction, Al+O(2)--> AlO+O. The measurements were carried out under the crossed-beam condition at 12.2 kJmol of collision energy. The relative populations of three spin-orbit states of O((3)P(J)) were determined to be 3.8, 1.0, and 0.2 for J=2, 1, and 0, respectively. They show nonstatistical populations, i.e., more population in O((3)P(2)) and less population in O((3)P(0)) than the statistical expectation. These populations were almost identical for two Al beam conditions where the relative concentrations of two spin-orbit states of Al, (2)P(1/2), and (2)P(3/2), are different. These results suggest that the reaction of Al with O(2) proceeds via an intermediate complex where the memory of the initial spin-orbit state is lost. Deviation from the statistical population of O((3)P(J)) implies the occurrence of the interaction among potential surfaces in the exit channel.