Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.     

Infrared spectrum of the CS2 trimer: observation of a structure with D3 symmetry

Infrared spectra of a carbon disulfide trimer formed in a pulsed supersonic slit-jet expansion are obtained via direct absorption of a tuneable diode laser in the region of the CS(2)ν(3) fundamental (～1535 cm(-1)). This is the first high-resolution spectroscopic observation of (CS(2))(3). Two bands sharing the same lower state are assigned to ((12)C(32)S(2))(3). These correspond to the two infrared active trimer vibrations (a parallel and a perpendicular band) of the constituent CS(2) monomer asymmetric stretches. The weaker perpendicular band is centered at 1524.613 cm(-1), shifted by -10.74 cm(-1) with respect to the free CS(2) monomer. The parallel band is centered at 1545.669 cm(-1), a vibrational shift of +10.31 cm(-1). Transitions with K≠ 3n and those with K = 0, J = odd in the ground state are absent, establishing that this trimer has D(3) symmetry. The two parameters required to define this structure are determined to be 3.811 ? for the C-C bond distance and 61.8° for the angle between a monomer axis and the plane containing the C atoms. In addition, a parallel band arising from trimers with a single (34)S substitution is observed around 1544.46 cm(-1). Together with the recently observed cross-shaped CS(2) dimer, these results indicate a tendency for CS(2) to form highly symmetric clusters.     

Spectroscopic observation and structure of CS2 dimer

Infrared spectra of the CS(2) dimer are observed in the region of the CS(2) ν(3) fundamental band (～1535 cm(-1)) using a tunable diode laser spectrometer. The weakly bound complex is formed in a pulsed supersonic slit-jet expansion of a dilute gas mixture of carbon disulfide in helium. Contrary to the planar slipped-parallel geometry previously observed for (CO(2))(2), (N(2)O)(2), and (OCS)(2), the CS(2) dimer exhibits a cross-shaped structure with D(2d) symmetry. Two bands were observed and analyzed: the fundamental (C-S asymmetric stretch) and a combination involving this mode plus an intermolecular vibration. In both cases, the rotational structure corresponds to a perpendicular (ΔK = ±1) band of a symmetric rotor molecule. The intermolecular center of mass separation (C-C distance) is determined to be 3.539(7) A?. Thanks to symmetry, this is the only parameter required to characterize the structure, if the monomer geometry is assumed to remain unchanged in the dimer. From the band centers of the fundamental and combination band an intermolecular frequency of 10.96 cm(-1) is obtained, which we assign as the torsional bending mode. This constitutes the first high resolution spectroscopic investigation of CS(2) dimer.     

New infrared bands of nonpolar OCS dimer and experimental frequencies for two intermolecular modes

Spectra of the nonpolar carbonyl sulfide dimer in the region of the OCS ν(1) fundamental band were observed in a slit-jet supersonic expansion. The jet was probed using radiation from a tunable diode laser employed in a rapid-scan signal averaging mode. Six new bands were observed and analyzed, all of which originate from the dimer ground vibrational state. Three were vibrational fundamentals involving the ((18)OCS)(2) and (16)OCS-(18)OCS isotopologues. They enabled an estimate to be made of the frequency of the infrared-forbidden mode corresponding to in-phase vibration of the OCS monomers in the dimer, a value needed to obtain an intermolecular vibrational frequency from one of the observed combination bands. A relatively weak b-type dimer band centered at 2103.105 cm(-1) was assigned to the OCS 4ν(2) (l = 0) bending overtone. Combination bands were observed involving the geared bend and van der Waals stretch intermolecular modes. The resulting experimental frequencies of 37.5(20) cm(-1) for the bend and 42.9727(1) cm(-1) for the stretch are in good agreement with a recent high level theoretical calculation.     

Infrared spectra of the OCS trimer

Infrared spectra of the barrel-shaped trimer (OCS)(3), previously known from its microwave spectrum, are reported for the first time. The observations are carried out in a supersonic slit-jet expansion of a He+OCS gas mixture which is probed with a tunable diode laser. Three rotationally resolved bands associated with the nu(1) fundamental vibration of OCS (2062.20 cm(-1)) are observed, at about 2047, 2053, and 2077 cm(-1). Small perturbations are noted in the 2077 cm(-1) band and may also be present in the 2053 cm(-1) band, which is weak and hence more difficult to analyze precisely. Employing a variety of evidence, we suggest a plausible assignment for the nature of the OCS vibrations in each of the three bands.     

The cyclic CO2 trimer: observation of a parallel band and determination of an intermolecular out-of-plane torsional frequency

A new parallel (DeltaK=0) band of the cyclic CO(2) trimer is observed at 2364 cm(-1). The trimers are generated in a pulsed supersonic expansion from a slit-jet nozzle and probed with a tunable infrared diode laser. The band is assigned as a combination of an intramolecular CO(2) monomer nu(3) stretch and an intermolecular out-of-plane torsion, giving a torsional frequency of 12-13 cm(-1). The band is surprisingly strong and completely unperturbed, providing a rare and near perfect example for a parallel band of a symmetric top molecule with C(3h) symmetry and zero nuclear spins.     

Infrared spectroscopic investigation of two isomers of the weakly bound complex OCS-(CO2)2

Vibration-rotation spectra of the OCS-(CO(2))(2) van der Waals complex were studied by means of direct infrared absorption spectroscopy. Complexes were generated in a supersonic slit-jet apparatus, and the expansion gas was probed using a rapid-scan tunable diode laser. Infrared bands were observed for two different isomeric forms of the complex. A relatively strong band centered at 2058.799 cm(-1) was assigned to the most stable isomer, which has a barrel-shaped geometry and is already known from microwave spectroscopy. A weaker infrared band centered at 2050.702 cm(-1) was assigned to a new isomeric form, observed here for the first time, which was expected on the basis of ab initio calculations. Infrared bands for seven isotopomers were recorded in an attempt to quantify the structure of the new isomer. Because it has no symmetry elements, nine parameters are needed to fully define the geometry. It was possible to determine six of these which define the relative position of the OCS monomer with respect to the CO(2) dimer fragment in the complex while the remaining three were fixed at their ab initio values. Similarities and differences between the faces of the two isomers of OCS-(CO(2))(2) and the associated dimers are discussed.     

Measurement and revised analysis of the torsional combination band of the nonpolar N2O dimer at 2249 cm(-1)

The high-resolution infrared spectrum of the weakly-bound dimer (N(2)O)(2) is studied using a rapid-scan tunable-diode laser spectrometer to probe a pulsed supersonic jet expansion. An observed band with c-type rotational structure is assigned as a combination of the intramolecular N(2)O nu(1) stretching vibration and the intermolecular out-of-plane dimer torsional vibration, with a vibrational origin at 2249.360 cm(-1). The resulting torsional frequency for the nonpolar N(2)O dimer is about 21.5 cm(-1). The present rotational analysis is completely different from that reported previously for the same band [Hecker et al., Phys. Chem. Chem. Phys., 2003, 5, 2333], which gave a band origin some 1.53 cm(-1) lower.     

Molecular structure and infrared spectra of (HXeCN)n (n=2, 3 or 4)

The structure, energetics, and vibrational spectra of the (HXeCN)2 dimer were investigated at the CCSD(T), MP2 and B3LYP levels. Such properties of the (HXeCN)3 trimer and (HXeCN)4 tetramer were investigated at the B3LYP level. The dimer, trimer, and tetramer were predicted to have a C2h, C2v, and D2d structure, respectively. In all of these oligomers, the N?Xe intermonomeric interaction is the most important one for holding the monomers together. Included with the ZPVE and BSSE, the stabilization energy of the dimer is 12.36 kcal/mol at the CCSD(T) level, while those of the dimer, trimer, and tetramer are 10.42, 18.23, and 31.34 kcal/mol, respectively, at the B3LYP level. At the B3LYP level, with respect to those of the isolated monomer, the C-Xe and Xe-H asymmetric stretching frequencies are shifted by -11.2 and +128.0 cm(-1) for the dimer, -51.6, +220.7 and -11.5, +96.6 cm(-1) for the trimer, and -14.1 and +201.8 cm(-1) for the tetramer.     

Competition between hydrogen bonding and dispersion interactions in the indole···pyridine dimer and (indole)2···pyridine trimer studied in a supersonic jet

Structures of the indole···pyridine dimer and (indole)2···pyridine trimer have been investigated in a supersonic jet using resonant two-photon ionization (R2PI) and IR-UV double resonance spectroscopic techniques combined with quantum chemistry calculations. R2PI spectra of the dimer and the trimer recorded by electronic excitation of the indole moiety show that the red-shift in the band origin of the dimer with respect to the 0(0)(0) band of the monomer is larger compared to that of the trimer. The presence of only one conformer in the case of both the dimer and the trimer has been confirmed from IR-UV hole-burning spectroscopy. The structures of the dimer and the trimer have been determined from resonant ion dip infrared (RIDIR) spectra combined with ab initio as well as DFT/M05-2X and DFT/M06-2X calculations. It has been found that the dimer, observed in the experiment, has a V-shaped geometry stabilized by N–H···N and C–H···N hydrogen bonding interactions, as well as C–H···π and π···π dispersion interactions. The geometry of the trimer has been found to be a cyclic one stabilized by N–H···N, N–H···π, C–H···π, and C–H···N interactions. The most important finding of this current study is the observation of the mixed dimer and trimer, which are stabilized by hydrogen bonding as well as dispersion interactions.     

Communication: rigorous calculation of dissociation energies (D0) of the water trimer, (H2O)3 and (D2O)3

Using a recent, full-dimensional, ab initio potential energy surface [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)] together with rigorous diffusion Monte Carlo calculations of the zero-point energy of the water trimer, we report dissociation energies, D(0), to form one monomer plus the water dimer and three monomers. The calculations make use of essentially exact zero-point energies for the water trimer, dimer, and monomer, and benchmark values of the electronic dissociation energies, D(e), of the water trimer [J. A. Anderson, K. Crager, L. Fedoroff, and G. S. Tschumper, J. Chem. Phys. 121, 11023 (2004)]. The D(0) results are 3855 and 2726 cm(-1) for the 3H(2)O and H(2)O + (H(2)O)(2) dissociation channels, respectively, and 4206 and 2947 cm(-1) for 3D(2)O and D(2)O + (D(2)O)(2) dissociation channels, respectively. The results have estimated uncertainties of 20 and 30 cm(-1) for the monomer plus dimer and three monomer of dissociation channels, respectively.     

Infrared and infrared emission spectroscopy of the zinc carbonate mineral smithsonite

Infrared emission and infrared spectroscopy has been used to study a series of selected natural smithsonites from different origins. An intense broad infrared band at 1440cm(-1) is assigned to the nu(3) CO(3)(2-) antisymmetric stretching vibration. An additional band is resolved at 1335cm(-1). An intense sharp Raman band at 1092cm(-1) is assigned to the CO(3)(2-) symmetric stretching vibration. Infrared emission spectra show a broad antisymmetric band at 1442cm(-1) shifting to lower wavenumbers with thermal treatment. A band observed at 870cm(-1) with a band of lesser intensity at 842cm(-1) shifts to higher wavenumbers upon thermal treatment and is observed at 865cm(-1) at 400 degrees C and is assigned to the CO(3)(2-)nu(2) mode. No nu(2) bending modes are observed in the Raman spectra for smithsonite. The band at 746cm(-1) shifts to 743cm(-1) at 400 degrees C and is attributed to the CO(3)(2-)nu(4) in phase bending modes. Two infrared bands at 744 and around 729cm(-1) are assigned to the nu(4) in phase bending mode. Multiple bands may be attributed to the structural distortion ZnO(6) octahedron. This structural distortion is brought about by the substitution of Zn by some other cation. A number of bands at 2499, 2597, 2858, 2954 and 2991cm(-1) in both the IE and infrared spectra are attributed to combination bands.     

Observation of the "missing" polar OCS dimer

A new infrared band at 2069.3 cm-1 is observed and assigned to the long-anticipated polar isomer of the OCS dimer, helping to explain apparent discrepancies among earlier studies. The data reported here should enable direct observation of the microwave spectrum of polar (OCS)2 and motivate new theoretical works on the energetics of OCS dimer isomers and interconversion energy barriers.     

In situ ATR-IR study of prochiral 2-methyl-2-pentenoic acid adsorption on Al2O3 and Pd/Al2O3

Adsorption and desorption of trans-2-methyl-2-pentenoic acid (MPeA) in dichloromethane (CH(2)Cl(2)) were investigated by using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. A liquid flow-through spectroscopic cell allowed for high quality spectra to be obtained from deposited thin films of Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) on a ZnSe internal reflection element. The MPeA molecules adsorb on both Al(2)O(3) and Pd surfaces molecularly and dissociatively under the concentration range examined (2-16 mM). In the case of molecular adsorption, both monomer (ν(C=O) ~ 1720 cm(-1)) and dimer (ν(C=O) ~ 1685 cm(-1)) species are observed to adsorb, with the relative amount of monomer to dimer dependent on the surface and the liquid phase acid concentration. In the case of dissociative adsorption, the acid adsorbs predominantly in a bridged bidentate configuration, as adjudged by the ca. 150-220 cm(-1) separation between asymmetric and symmetric vibrational bands. All of these species are found to be strongly adsorbed on both Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) surfaces, even under pure solvent flow after adsorption.     

Infrared spectroscopy of hydrogen-bonded CHCl3-SO2 in the gas phase

A molecular association between chloroform and sulfur dioxide in the gas phase at room temperature was studied by Fourier transform infrared spectroscopy. Since the intensity of the CH-stretching fundamental vibration of monomer chloroform is very weak but much stronger upon complexation, a simple subtraction procedure isolated the CH-stretching vibration spectrum of the complex. The presence of a 1:1 complex was confirmed by two dilution series, where the monomer concentrations were varied. The molecular association manifested itself as a shift of the peak absorbance of the CH-stretching vibration of CHCl3-SO2 by +7 cm(-1) and of the CD-stretching vibration of CDCl3-SO2 by +5 cm(-1) to higher wave numbers compared to monomer chloroform, accompanied by a considerable broadening of the band contour. In agreement with previous ab initio calculations, this indicates a "blueshifting" or more appropriately, a "C-H contracting" hydrogen bond between chloroform and sulfur dioxide. An estimate of the complex concentration was made based on ab initio calculations for the integrated band strength and the measured spectrum. With this estimate, the equilibrium constant Kp (295 K)=0.014 (po=10(5) Pa) for the dimerization was calculated, providing one of the very few cases where the formation of a hydrogen-bonded gas phase complex at room temperature could be quantitatively studied by infrared spectroscopy.     

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the M?ller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.     

Rh—V／SiO2上H2的化学吸附方式

ＣＯ加氢反应机理是许多研究者感兴趣的课题．负载的Ｒｈ是ＣＯ加氢反应的优良催化剂．ＣＯ在Ｒｈ上吸附态的研究已有许多报道［１～７］,而对Ｈ２有关的吸附态的研究却少见报道．Ｗｏｒｌｅｙ等［８］利用高压超纯Ｈ２在２．２％Ｒｈ／Ａｌ２Ｏ３膜上首次观测到Ｒｈ—Ｈ...     

An experimental value for the B(1u) C-H stretch mode in benzene

We here present experimental infrared spectra on two (C(6)H(6))(C(6)D(6)) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C-H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm(-1) to the red, compared to the modes that are known for the monomer. For the nu(13) B(1u) C-H stretch fundamental mode of benzene, a first experimental value of 3015(+2) (-5) cm(-1) is determined, in excellent agreement with anharmonic frequency calculations presented here.     

Observation of rovibrational transitions of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets

We report the infrared spectra of HCl, (HCl)2, and H2O-HCl in liquid helium nanodroplets in the frequency region between 2680 and 2915 cm(-1). For the HCl monomer a line width of 1.0 cm(-1) (H35Cl) corresponding to a lifetime of 5.3 ps was observed. The line broadening indicates fast rotational relaxation similar to that previously observed for HF. For (HCl)2 the free HCl as well as the bound HCl stretching band has been observed. The nu2+ bands of (HCl)2 could be rotationally resolved, and rotational constants were deduced from the spectra. We observed both the allowed and the symmetry forbidden transition. However, the forbidden "broken symmetry" tunneling transition of the mixed dimer shows an intensity that is considerably enhanced compared to the gas phase. Upon the basis of the present measurements we were able to calculate the tunneling splitting in the excited state. The tunneling splitting is found to be reduced by 28% compared to the gas phase. Transitions from the ground state to the Ka=1 level of the free HCl stretch (nu1) are recorded and show considerable line broadening with a line width of 2 cm(-1). The excited state Ka=1 has an additional rotational energy of about 10 cm(-1), thereby allowing fast rotational relaxation by coupling to helium excitations. In addition we observed the HCl stretch of the HCl-H2O dimer, which exhibits an unusually large width (1.7 cm(-1) for H35Cl)) and large red shift (8.5 cm(-1)), compared to the gas-phase values. The large-amplitude motion originating from the libration mode of the HCl-H2O complex is supposed to act as a fast relaxation manifold.     

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.