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.

Understanding the Thermal Denaturation of Myoglobin with IMS-MS: Evidence for Multiple Stable Structures and Trapped Pre-equilibrium States.

Thermal denaturation of holomyoglobin (hMb) in solution (10 mM ammonium acetate at pH = 4.5, 6.8, and 9.0) was monitored by ion mobility spectrometry (IMS) and mass spectrometry (MS) techniques to characterize the stability and investigate structural changes involved in unfolding. We utilize two experimental approaches to induce thermal denaturation: a variable-temperature electrospray ionization (vT-ESI) source that heats the bulk solution in the ESI emitter, and a variable-power 10.6 µm CO2 laser that rapidly heats nanodroplets produced by ESI. These two approaches sample different time scales of the denaturation process; long time scales (seconds to minutes) where the system is at equilibrium using the vT-ESI approach and shorter time scales (µs) by rapid droplet heating in which the system is in a pre-equilibrium state. Increasing the solution temperature (from 28 to 95 °C in the vT-ESI experiments) shifts the charge state distribution from low charge states ([M + 7H]7+ to [M + 9H]9+) to more highly charged species. This is accompanied by loss of the heme group to yield the apomyoglobin (aMb) species, indicating that the protein has unfolded. Monitoring the formation of aMb and the shift in average charge states of aMb and hMb with solution temperature allows for relative quantitation of their individual stabilities, highlighting the stabilizing effects of heme binding. We compare the degree of unfolding induced by heating the bulk solution (using vT-ESI) to the laser droplet heating approach and find that the rapid nature of the laser heating approach allows for transient pre-equilibrium states to be sampled.

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.

Laser-induced- and dispersed-fluorescence studies of rhodamine 590 and 640 ions formed by electrospray ionization: observation of fluorescence from highly-excited vibrational levels of S1 states.

The gas-phase fluorescence excitation and emission properties of rhodamine 590 (R590) and rhodamine 640 (R640) cations were studied using a novel instrument that allows extension of optical excitation to the UV region. Ions were generated by an electrospray ion source, and desolvated ions were accumulated in a room-temperature quadrupole ion-trap. In addition to the well-known S1-S0 transition observed for both dyes, two ultraviolet (UV) bands (that have not been reported previously) were observed for R590. When compared with transitions observed for the dyes in methanol solutions, the S1-S0 transitions in the gas phase are shifted to significantly higher energies. These shifts suggest that the S1 states for both dyes are more stabilized by favorable solvent interactions than the S0 states. The UV bands observed for R590 were assigned to the transitions involving the higher-energy S3 and Sn states. The DF spectra obtained upon UV excitation are similar to those recorded for S1 excitation; however, the emission after UV excitation is shifted slightly to the red region. These observations are consistent with the emission from high-vibrational levels of the S1 state formed upon fast internal conversion from S3 or Sn. In vacuo, the absence of collisions extends the lifetimes of vibrationally-excited S1 states, allowing fluorescence from these states to be observed. A similar DF spectrum was also observed for the UV excitation of R640, suggesting that fluorescence from vibrationally hot S1 states is a general phenomenon for ions in the gas phase.

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 + CO2 → AlO + CO studied by a crossed-beam velocity map imaging technique.

The oxidation reaction of a gas-phase aluminum atom by a carbon dioxide molecule was studied by employing a crossed-beam technique at two collision energies: 27.9 and 52.8 kJ/mol. A (1 + 1) resonance-enhanced multiphoton ionization via the D2Σ+-X2Σ+ transition of AlO was applied to ionize the product. For several rotational levels of AlO in the vibrational ground state, time-sliced ion images were measured for the first time, and the angular-kinetic energy distributions were determined. All angular distributions showed forward and backward peaks; the forward peaks were more pronounced than the backward ones at the two collision energies. The product kinetic energy showed rather narrow distributions whose peaks appeared at near to the highest limit estimated from the available energies. The rotational distributions of the counter product CO, derived from the kinetic energy distributions, suggested that only a limited number of rotational states were formed and that a small amount of energies go into this mode. These results suggested that the reaction proceeds via a short-lived intermediate in which the O-C-O keeps a nearly linear structure.

Excited state reaction dynamics of Ti(a5F(J)) + O2 → TiO(A³Φ, B³Π, C³Δ) + O studied by a crossed-beam velocity map imaging technique.

Oxidation reactions of the gas-phase titanium atom in its excited state with oxygen molecule, Ti(a(5)F(J)) + O2 → TiO(A(3)Φ, B(3)Π, C(3)Δ) + O, were studied by a crossed-beam velocity map imaging technique at 14.3 kJ/mol of collision energy. Metastable excited Ti, Ti(a(5)FJ), was generated by an optical pumping method and the reaction products were detected by single photon-ionization followed by a time-of-flight mass analysis and a two dimensional detection. Three wavelengths were selected to ionize electronically excited TiO*, TiO(A(3)Φ, B(3)Π, C(3)Δ). Time sliced images were measured, and angular and speed distributions of TiO* were determined. In all three ionization wavelengths, the angular distributions showed a forward-backward symmetry with low intensity at the sideway direction. The speed distributions were represented by the distributions based on the statistical energy partition into products. These results suggested that the reaction of Ti(a(5)F(J)) to form TiO(B) and TiO(C) proceeds via a long-lived intermediate and confirmed that the mechanism proposed by the previous chemiluminescence study.

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.

Laser initiated reactions in N2O clusters studied by time-sliced ion velocity imaging technique.

Laser initiated reactions in N2O clusters were studied by a time-sliced velocity imaging technique. The N2O clusters, (N2O)n, generated by supersonic expansion were irradiated by an ultraviolet laser around 204 nm to convert reactant pairs, O((1)D2)-(N2O)n-1. The NO molecules formed from these reactant pairs were ionized by the same laser pulse and their velocity distribution was determined by the time-sliced velocity imaging technique. At low nozzle pressure, lower than 1.5 atm, the speed distribution in the frame moving with the clusters consists of two components. These components were ascribed to the products appeared in the backward and forward directions in the center-of-mass frame, respectively. The former consists of the vibrational ground state and the latter consists of highly vibrational excited states. At higher nozzle pressure, a single broad speed distribution became dominant for the product NO. The pressure and laser power dependences suggested that this component is attributed to the product formed in the clusters larger than dimer, (N2O)n (n ≥ 3).

[A case report of successful treatment for rectosigmoid cancer with peritoneal dissemination after neoadjuvant chemoradiotherapy with XELOX and bevacizumab].

A 60-year-old woman underwent emergency operation for ileus due to rectosigmoid cancer. Intraoperative cytology and peritoneal dissemination were positive. After performing sigmoid colostomy, she underwent neoadjuvant radiation therapy (40 Gy)and eight courses of a XELOX and bevacizumab regimen. FDG-PET did not indicate FDG accumulation after chemoradiotherapy, thus, we performed low anterior resection. Peritoneal dissemination and washing cytology were negative in the second operation. Neoadjuvant chemoradiotherapy with XELOX and bevacizumab were useful for down staging in patients with advanced colorectal cancer.

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.

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.

[A case report of pathologically complete response of locally advanced rectal cancer after neoadjuvant chemoradiotherapy with XELOX and bevacizumab].

A 70-year-old man was admitted to our hospital for constipation. A clinical examination showed locally advanced rectal cancer with possible invasion to the prostate gland and pelvic wall. After performing colostomy, he underwent neoadjuvant radiation therapy (40 Gy) and six courses of a XELOX and bevacizumab regimen. A subsequent examination demonstrated significant reduction of the tumor, so we performed super low anterior resection and colo-anal anastomosis. Pathological examination revealed no residual cancer cells and showed pathological CR. Neoadjuvant chemoradiotherapy with XELOX and bevacizumab were useful for down staging and function-preserving surgery in patients with locally advanced rectal cancer.

Angle-resolved metastable fragment yields spectra of N2 and CO in K-edge excitation energy region.

Angle-resolved metastable fragments yields spectra have been measured in the N 1s ionization region of the N(2) and C 1s ionization region of CO. These spectra are compared with zero kinetic energy electron and photoelectron spectra. It has been shown that an isotropic metastable fragments yields spectra are almost identical with the ZEKE spectrum, whereas metastable fragments yields spectra with the Σ-Σ transition show similarity with photoelectron spectra. This means that these spectra clearly contain information about two shake-up mechanisms: conjugate and direct shake-up processes. All the peaks in the metastable photofragment spectra can be assigned as either satellite states or double/triple excitation states. Thus, it was shown that angle-resolved metastable photofragment spectroscopy could be used to help characterize multi-electron excitation states in general.

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.

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.

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.

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.

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.

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.