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Infrared spectra of the CS2 dimer are observed in the region of the CS2 ν3 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 (CO2)2, (N2O)2, and (OCS)2, the CS2 dimer exhibits a cross-shaped structure with D2d 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) Å. 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 CS2 dimer. [ABSTRACT FROM AUTHOR]

Infrared spectra of the nitrous oxide tetramer, (N2O)4, are studied in the region of the N2O ν1 fundamental band (∼2200 cm-1). The spectra are observed using a tunable diode laser to probe a pulsed supersonic jet expansion. Parallel (ΔK = 0) bands are observed for the previously observed isomer of (N2O)4, which is confirmed by isotopic substitution to have an oblate symmetric rotor structure with D2d symmetry. A distinct new isomer of (N2O)4 is observed by means of a perpendicular (ΔK = ±1) band. It has a prolate symmetric rotor structure with S4 symmetry. These isomers represent two distinct, but almost equally favorable, ways of bringing together a pair of nonpolar N2O dimers to form a tetramer. It is not clear at present which one represents the true ground state. [ABSTRACT FROM AUTHOR]

The infrared spectrum of weakly bound OCS–C6H6 is studied in the region of the ν1 fundamental band of OCS (∼2050 cm-1) using a tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. This is one of the first direct infrared observations of a benzene-containing van der Waals complex. A very simple band is observed, corresponding to the parallel transition of a symmetric top. It is shifted by -11.1 cm-1 with respect to the free OCS monomer. The isotopologues OCS–13C 12C5H6 and OC 34S–C6H6 are observed, and the derived structure has OCS located along the benzene C6 symmetry axis in an S-bonded configuration with a center of mass separation of 4.42 Å, in good agreement with previous microwave spectra. Similar bands are observed for the trimers OCS–C6H6–He and OCS–C6H6–Ne, whose structure is obtained by adding an on-axis rare gas atom to the other side of the benzene. However, the analogous band for OCS–C6H6–Ar is not detected, raising the possibility that the stable form of this trimer may not have the same symmetrical structure. A “mystery” feature is observed close to the OCS–C6H6 band origin and its possible assignment to a cluster such as OCS–(C6H6)3 is discussed. [ABSTRACT FROM AUTHOR]

Many weakly bound OCS-hydrocarbon complexes exhibit a relatively simple rotation-vibration band, characteristic of a T-shaped structure, which is redshifted (by 5–12 cm-1) from the OCS monomer ν1 frequency. Spectra of OCS with seven chain and ring hydrocarbons are described here. They allow a straightforward comparison of intermolecular force effects (vibrational shift and intermolecular separation) over a range of molecules, which could be extended to other hydrocarbons and other probes such as CO2 and N2O. [ABSTRACT FROM AUTHOR]

Infrared spectra of OCS-CO2 complexes are studied in a pulsed supersonic slit-jet expansion using a tunable diode laser probe in the 2060 cm-1 region of the C–O stretching fundamental of OCS. Two bands are observed and analyzed, corresponding to two distinct isomers of the complex. Isomer a is the known form which has been previously studied in the microwave region. Isomer b is a new form, expected theoretically but first observed here. Structures are determined with the help of isotopic substitution. Both isomers are planar, with slipped near-parallel geometries. In isomer a, the intermolecular (center of mass) separation is 3.55 Å and the C atom of the CO2 is closer to the S atom of the OCS. In isomer b, the C atom of CO2 slides closer to the O atom of OCS and the center of mass separation increases to 3.99 Å. Isomer a is the lowest energy form, but paradoxically isomer b appears to be stronger in our infrared spectra. Predicted pure rotational transition frequencies are given to help in a search for the microwave spectrum of isomer b. [ABSTRACT FROM AUTHOR]

Improved quantum chemistry (coupled-cluster) results are presented for spectroscopic parameters and the potential energy surface for the N2O dimer. The calculations produce three isomer structures, of which the two lowest energy forms are those observed experimentally: a nonpolar C2h-symmetry planar slipped-antiparallel geometry (with inward-located O atoms) and a higher-energy polar Cs-symmetry planar slipped-parallel geometry. Harmonic vibrational frequencies and infrared intensities for these isomers are calculated. The low-frequency intermolecular vibrational mode predictions should be useful for future spectroscopic searches, and there is good agreement in the one case where an experimental value is available. The frequency shifts for the high-frequency intramolecular stretching vibrations, relative to the monomer, were calculated and used to help locate a new infrared band of the polar isomer, which corresponds to the weaker out-of-phase combination of the ν1 antisymmetric stretch of the individual monomers. The new band was observed in the region of the monomer ν1 fundamental for both (14N2O)2 and (15N2O)2 using a tunable infrared diode laser to probe a pulsed supersonic jet expansion, and results are presented. [ABSTRACT FROM AUTHOR]

Infrared spectra of N2O trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by Miller and Pedersen [J. Chem. Phys. 108, 436 (1998)] in the N2O ν1+ν3 combination band region (∼3480 cm-1) showed the trimer structure to be noncyclic, with three inequivalent N2O monomer units which could be thought of as an N2O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N2O fundamental band regions ν3 (∼1280 cm-1) and ν1 (∼2230 cm-1). In the ν3 region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the ν1+ν3 region, but in the ν1 region all three possible trimer bands are observed. Relationships among the various bands are considered with reference to their rotational intensity patterns, their vibrational shifts, and the properties of the related N2O dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully 15N-substituted species in the ν1 region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer. [ABSTRACT FROM AUTHOR]

The spectra of the weakly bound complex CO2–N2O are studied in the regions of the ν1 and ν3 fundamentals of N2O and the ν3 fundamental of CO2 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. Five bands are measured and analyzed, of which four have not been previously studied. Two bands at 2225.75 and 1279.58 cm-1, with a/b-type rotational structure, are assigned to the ν1 and ν3 fundamentals of the N2O component of the complex. Small perturbations are noted in the ν3 band, similar to those observed previously for the nonpolar dimer (N2O)2 in the ν3 and ν1+ν3 regions. The previously known band at 2348.86 cm-1 in the region of the ν3 CO2 stretch is analyzed in improved detail. Weaker c-type bands at 2251.46 and 2374.67 cm-1 are assigned as combinations of the intermolecular torsional (out-of-plane) vibration plus the N2O ν1 and CO2 ν3 stretches, respectively. The resulting torsional frequency of CO2–N2O is experimentally determined for the first time to be 25.8 cm-1. [ABSTRACT FROM AUTHOR]

A new parallel (ΔK=0) band of the cyclic CO2 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 CO2 monomer ν3 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 C3h symmetry and zero nuclear spins. [ABSTRACT FROM AUTHOR]

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 ν1 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. [ABSTRACT FROM AUTHOR]

Spectra of the nitrous oxide dimer (N2O)2 are studied in the region of the N2O ν1 fundamental band around 2230 cm-1 using a rapid-scan tunable diode laser spectrometer to probe a pulsed supersonic jet expansion. The previously known band of the centrosymmetric nonpolar dimer is analyzed in improved detail, and a new band is observed and assigned to a polar isomer of (N2O)2. This polar form of the dimer has a slipped parallel structure, rather similar to the slipped antiparallel structure of the nonpolar form but with a slightly larger intermolecular distance. The accurate rotational parameters determined here should enable a microwave observation of the polar N2O dimer. The need for a modern ab initio investigation of the N2O–N2O intermolecular potential energy surface is emphasized. [ABSTRACT FROM AUTHOR]

Infrared spectra of the OCS–He van der Waals complex and of OCS–HeN clusters have been studied in the region of the OCS ν1 fundamental band using a tunable diode laser to probe a pulsed supersonic slit jet. For the complex, the spectrum of the normal isotope, 16O12C32S–4He, has been considerably extended and the 34S- and 13C-substituted forms have been recorded for the first time. The data could be analyzed satisfactorily using a conventional asymmetric rotor Hamiltonian with sextic centrifugal distortion terms. For the clusters, the 34S- and 13C-substituted forms have been observed and assigned for N=2–7, including some transitions with higher J values than previously reported for the normal isotope, e.g., R(5). The observed vibrational shifts, relative to the free OCS molecule, were very similar to those of the normal isotope, and most of the difference could be explained by simple scaling. These results constitute a subtle and precise probe of intermolecular forces and dynamical effects in a system which is of current interest for cluster studies. [ABSTRACT FROM AUTHOR]

The torsional spectrum of disilane was recorded for the first time under high-pressure-pathlength conditions and at a spectral resolution of 0.007 cm-1 using a Bruker IFS-120 HR Fourier transform spectrometer. The spectrum shows six distinct Q branches. The most prominent Q branch is near 130 cm-1 which is a blend of four components of the torsional fundamental. Of the remaining five, four were assigned to the first torsional hot band (v4=2←1) and one to the second torsional hot band (v4=3←2). Over 350 transitions were identified. An analysis of the torsional fundamental, the first torsional hot band, and the lower state combination differences from frequencies of the vibrational bands ν9 and ν9+ν4-ν4 was made to characterize the torsion-rotation Hamiltonian in the ground vibrational state. The barrier height, barrier shape, and the rotational constant about the Si–Si bond were determined to be 404.344(83) cm-1, 2.255(65) cm-1, and 43208(28) MHz, respectively. Comparison of simulated and the experimental spectra yielded (μ∥-μ⊥)/μ⊥=-4(1) for the torsional dipole moments. This ratio compares well with -3.39(6) for ethane. A comparison of molecular parameters obtained here is made with those for methyl silane and ethane. [ABSTRACT FROM AUTHOR]

The C–C stretching fundamental ν[sub 3] and the difference band 2ν[sub 3]-ν[sub 3] of the CCO radical have been measured in the gas phase using a tunable infrared diode laser spectrometer. The CCO radical was produced using a flowing mixture of carbon suboxide and helium subjected to a hollow-cathode discharge. The spectral region between 1035–1085 cm[sup -1] was probed. Fifty-five and fifty-three rovibrational transitions were measured in the ν[sub 3] and 2ν[sub 3]-ν[sub 3] bands, respectively. The analyses of these bands yielded spectroscopic constants for (001) and (002) vibrational states. The band origins for ν[sub 3] and 2ν[sub 3]-ν[sub 3] were determined to be 1066.82407(54) and 1061.91084(63) cm[sup -1], respectively. To check the consistency of the data in this work and those in the C–O stretching region, several lines from the ν[sub 1]+ν[sub 3]-ν[sub 1] band were also recorded. These lines were observed at their predicted frequency.© 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Analyzes the Fourier transform measurements of the lowest frequency degenerate fundamental band of ethane together with infrared torsional spectra. Investigation of vibration-torsion-rotation effects in a symmetric top molecule; Derivation of the vibration-torsion-rotation Hamiltonian for ethane from that for a methyl silane-like molecule.

The weakly-bound molecular clusters (D2O)2–CO and (D2O)3–CO are observed in the C–O stretch fundamental region (≈2150 cm−1), and their rotationally-resolved infrared spectra yield precise rotational parameters. The corresponding H2O clusters are also observed, but their bands are broadened by predissociation, preventing detailed analysis. The rotational parameters are insufficient in themselves to determine cluster structures, so ab initio calculations are employed, and good agreement between the experiment and theory is found for the most stable cluster isomers, yielding the basic cluster geometries as well as confirming the assignments to (D2O)2–CO and (D2O)3–CO. The trimer, (D2O)2–CO, has a near-planar geometry with one D atom from each D2O slightly out of the plane. The tetramer, (D2O)3–CO, has the water molecules arranged in a cyclic quasi-planar ring similar to the water trimer, with the carbon monoxide located 'above' the ring and roughly parallel to its plane. The tunneling effects observed in the free water dimer and trimer are quenched by the presence of CO. The previously observed water–CO dimer together with the trimer and tetramer reported here represent the first three steps in the solvation of carbon monoxide. [ABSTRACT FROM AUTHOR]

The torsional spectrum of gaseous CH3CD3 has been measured between 230 and 280 cm-1 under relatively large pressure-path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm-1 and an absorption path of 20 m. The gas temperature and pressure were 190 K and 82 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v6=1←0). The P and R branches consist of broad peaks from 0.3 to 0.5 cm-1 wide showing partially resolved rotational structure. A frequency analysis combining the present far-infrared frequencies with molecular beam, microwave, and mm-wave measurements from previously reported experiments yielded an improved set of torsion–rotation parameters. An intensity analysis was carried out to obtain the torsional dipole moments. The torsional dipole components μT⊥ and (μT||-μT⊥) were determined to be 14.0(1.5) and -39.6(4.0) μD, respectively. The conditions under which there is agreement between the experimental values and those calculated from theoretical expressions are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The infrared vibrational frequencies and intensities of the CnO linear chains, n=3–9, in their electronic ground state is predicted at the Becke–Lee–Yang–Parr (BLYP) level of theory. The computational model is assessed in three steps: (i) comparison of calculated and experimental rotational parameters for the whole series; (ii) comparison of experimental and calculated infrared frequencies, intensities and isotopic shifts for C3O (this molecule can be considered the prototype of the chains whose ground electronic state is 1Σ+); (iii) comparison of calculated and experimental infrared frequencies and intensities for C4O (this molecule can be considered the prototype of the chains whose ground electronic state is 3Σ-). The excellent agreement between experimental and computational results allows the prediction of the infrared pattern to 20 cm-1 for the frequencies and a few percent for the relative intensities. Analysis of the infrared intensities in terms of local atomic oscillators within the chains shows that while for short chains the intensity arises from the motion of the two carbon atoms nearest to the oxygen, for C7O and C9O the intensity arises in conjunction with the motion of carbon atoms close to, but not at, the other end of the molecule. For these two molecules, the infrared intensity is therefore similar in nature to that of pure carbon chains. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The infrared absorption spectrum of the linear C4 radical has been studied in an extension of the original observation of gas-phase C4 by Heath and Saykally [J. Chem. Phys. 94, 3271 (1991)]. The experiment was performed using a flowing mixture of acetylene and helium subjected to a hollow-cathode discharge, which was probed in the 1525–1570 cm-1 spectral region using a tunable diode laser spectrometer. Transitions with N-values up to 60 were measured. Their analysis yielded band origins, rotational, and centrifugal distortion parameters for the lower and upper vibrational states, and l-type doubling parameters for the degenerate bending states ν5 and ν3+ν5. In particular, the ν3 origin was determined to be 1548.6128(4) cm-1, the ground state rotational and centrifugal distortion parameters were B=4979.89(21) MHz and D=0.848(44) kHz, and the l-doubling parameters for ν5 was q5=10.98(13) MHz. This value for q5 was used to estimate the ν5 frequency of gas-phase C4 to be 160±4 cm-1. Both the l=0 and 2 components of the ν3+2ν5-2ν5 sequence band were also tentatively observed, but a detailed analysis was not yet possible. The results were completely consistent with a linear structure for the triplet ground state of C4, and showed no effects of quasilinearity such as that exhibited by C3. [ABSTRACT FROM AUTHOR]

The pure rotational spectra of CH3CD3 in the three lowest torsional states has been observed using a mm-wave spectrometer. A total of 87 rotational frequencies were measured between 230–363 GHz for J=7←6 to 11←10 in v6=0, 1, and 2, where v6 is the torsional quantum number. For the lowest two torsional states, the spectra have the classic form expected for a symmetric top (with no internal rotation) in the ground vibrational state. For v6=2 and for a given (J+1)←J, a markedly different splitting pattern is observed as a result of the (K,σ) dependence of the effective rotational constant B, where σ labels the torsional sublevels. In order to identify the individual features in the (v6=2) spectrum, an assignment procedure was developed which is based on the fact that the ratio of moment of inertia of the top about the molecular symmetry axis to that of the whole molecule about the same axis is to a very good approximation 1/3. The torsion–rotation Hamiltonian discussed earlier in connection with CH3SiH3 [N. Moazzen-Ahmadi et al. J. Mol. Spectrosc. 119, 299 (1986)] was used to analyze the rotational frequencies along with the molecular beam anticrossing data and the origin of the torsional fundamental. Several constants which characterize the J-dependence of the energy levels were determined. Effective values for the barrier height V3 and the shape parameter V6 associated with the first-order correction in the Fourier expansion of the potential function were obtained. The effect of redundancies on the interpretation of the measurements is discussed. [ABSTRACT FROM AUTHOR]

Gas-phase infrared spectra of 13C-substituted C3 molecules have been studied in the region of the ν3 vibration around 5 μm, using a tunable diode laser spectrometer and a hollow-cathode discharge in a flowing mixture of acetylene and helium. Three bands were observed and assigned: the ν3 fundamentals of 13C3 and of 12C 12C 13C, and the ν3+ν2-ν2 sequence band of 13C3 . The observed band origins are consistent with previous matrix-isolation spectroscopy of substituted C3 . The rotational and centrifugal distortion parameters are consistent with previous gas-phase work on 12C3 , and they also reflect the highly anharmonic nature of the bending vibration in this molecule. The ν3 band of 12C 12C 13C, measured here for the first time, provides a sensitive means to measure 13C abundances in astrophysical sources containing C3 . [ABSTRACT FROM AUTHOR]

The far-infrared spectrum of gaseous CH3CH3 has been measured in the region of the torsional fundamental under relatively large pressure-path length conditions. Observations were made at resolutions of 0.08, 0.02, and 0.016 cm-1 and under various experimental conditions using two different Fourier transform spectrometers. In addition, a very high resolution (0.0014 cm-1) spectrum of the lowest lying degenerate fundamental ν9 was obtained. From a frequency analysis of the far-infrared spectrum and 737 lower state combination differences in ν9 and its associated hot band ν9+ν4-ν4, an improved set of torsion–rotation parameters was obtained. An intensity analysis of the torsional bands was carried out to obtain torsional dipole moments. The torsional dipole components μ⊥ and (μ||-μ⊥ ) were determined to be 9.37(70) and -31.7(2.0) μD, respectively. These experimental values are in good agreement with calculated values of μ⊥=10.13 μD and (μ||-μ⊥ )=-35.10 μD. The shape of the torsional bands can be reproduced in detail with the model adopted here. [ABSTRACT FROM AUTHOR]

The far-infrared spectrum of gaseous CD3CD3 has been measured in the region of the torsional fundamental under relatively large pressure-path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm-1, an absorption path of 20 m, and a gas temperature and pressure of 168 K and 80 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v4=1←0) and the Q branch of the first torsional overtone (v4=2←1). The P and R branches of the fundamental consist of peaks about 0.2 cm-1 wide due to unresolved structure. In addition, a very high resolution (0.0014 cm-1) spectrum of the lowest lying degenerate fundamental ν9 of CD3CD3 was obtained. From the combined intensity and frequency analyses of the far-infrared spectrum and 442 lower state combination differences in ν9 and its associated hot band ν9+ν4-ν4, the effective leading coefficients V3=989.10(68) cm-1 and V6=9.47(29) cm-1 in the Fourier expansion of the hindering potential were determined. The rotational constant B was found to be 13 802.658(39) MHz and four distortion parameters were obtained. The torsional dipole constants μ⊥ and μ||-μ⊥ were determined to be 7.41(74) and -19.9(2.0) μD, respectively. The experimental values of μ⊥ and μ||-μ⊥ are shown to be in good agreement with values calculated from theoretical expressions. The shape of the torsional bands can be reproduced in detail with the model adopted here. [ABSTRACT FROM AUTHOR]

The spectrum of the linear carbon chain molecule C5 in the gas phase has been studied around 2170 cm-1, the region of the highest asymmetric stretching vibration ν3. The results were obtained using a tunable diode laser spectrometer and a cooled hollow cathode discharge in a flowing mixture of acetylene and helium. Four vibration–rotation bands were assigned and analyzed: the fundamental, a hot band arising from the v7=1, l=1 vibrational level, a second hot band arising from v7=2, l=0, and a third hot band tentatively ascribed to v5=1, l5=1. Small local perturbations were found to affect the upper vibrational states of two of the bands. Analysis of the data yielded accurate values for a number of molecular parameters for C5, e.g., the band origin ν3= 2169.4410(2) cm-1, the rotational constant, B0 =2557.63(9) MHz, and the l-type doubling parameters, q7=3.99(6) MHz, and q5=2.36(9) MHz. The value of q7 may be used to estimate a value of 118 cm-1 for the lowest bending frequency of the molecule. There is no evidence in C5 for quasilinear behavior such as that shown by C3 and C3O2. [ABSTRACT FROM AUTHOR]

The perturbation-allowed torsional spectrum of gaseous CH3CH3 has been measured between 225 and 340 cm-1 at a resolution of 0.015 cm-1 using a Fourier transform spectrometer. The absorption path length was 124 m; the gas temperature and pressure were 295 K and 107 Torr, respectively. The P, Q, and R branches have been observed for all four torsional sublevels in the bands v4=1←0 and 2←1. A torsion–rotation Hamiltonian containing seven parameters was used to analyze 204 frequencies assigned in the P and R branches of the two bands. Effective values were obtained for the reduced rotational constant AR, the two leading coefficients V3 and V6 in the Fourier expansion of the hindering potential, the B rotational constant, and three distortion parameters. Estimates for the three zeroth order torsional parameters free of higher order contributions are: AR =2.6711(50) cm-1, V3=1012.0(1.0) cm-1, and V6=10.71(0.60) cm-1. The torsional energies calculated from the current model are compared to those obtained by earlier experiments. [ABSTRACT FROM AUTHOR]

Examines the 2v[sub 1]-v[sub 1] and v[sub 1]+v[sub 3]-v[sub 3] difference bands of the CCO radical in the gas phase. Use of a tunable infrared diode laser spectroscopy; Production of the CCO radical; Measurement of the rovibrational transitions in the bands; Formation of spectroscopic constants for the vibrational states.

Presents the vibration-torsion-rotation analysis of the v[sub 12] band of CH[sub 3]CD[sub 3] by measuring the Fourier transform spectrum. Determination of coupling parameters; Use of the electric-resonance molecular-beam technique in splitting measurement; Impact of internal rotation on intrinsic splitting.

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. [ABSTRACT FROM AUTHOR]

The fundamental band for the OC-C2H2 dimer and two combination bands involving the intermolecular bending modes ν9 and ν8 in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tuneable infrared quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for OC-DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C2H2. A similar band for the fully deuterated isotopologue CO-(C2D2)2 confirms our assignment. [ABSTRACT FROM AUTHOR]

Only a few weakly-bound complexes containing the O2 molecule have been characterized by high resolution spectroscopy, no doubt due to the complications added by the oxygen molecule's unpaired electron spin. Here we report an extensive infrared spectrum of CO–O2, observed in the CO fundamental band region using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion. The rotational energy level pattern derived from the spectrum consists of stacks of levels characterized by the total angular momentum, J, and its projection on the intermolecular axis, K. Five such stacks are observed in the ground vibrational state, and ten in the excited state (ν(CO) = 1). They are divided into two groups, with no observed transitions between groups. The groups correspond to different projections of the O2 electron spin, and correlate with the two lowest fine structure states of O2, (N, J) = (1, 0) and (1, 2). The rotational constant of the lowest K = 0 stack implies an effective intermolecular separation of 3.82 Å, but this should be interpreted with caution since it ignores possible effects of electron spin. A new high-level 4-dimensional potential energy surface is developed for CO–O2, and rotational energy levels are calculated for this surface, ignoring electron spin. By comparing calculated and observed levels, it is possible to assign detailed quantum labels to the observed level stacks. [ABSTRACT FROM AUTHOR]