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Structural calculations and high-resolution infrared spectra are reported for trimers and tetramers containing CO2 together with CO and/or N2. Among the 9 clusters studied here, only (CO2)2–CO was previously observed by high-resolution spectroscopy. The spectra, which occur in the region of the ν3 fundamental of CO2 (≈2350 cm−1), were recorded using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The trimers (CO2)2–CO and (CO2)2–N2 have structures in which the CO or N2 is aligned along the symmetry axis of a staggered side-by-side CO2 dimer unit. The observation of two fundamental bands for (CO2)2–CO and (CO2)2–N2 shows that this CO2 dimer unit is non-planar, unlike (CO2)2 itself. For the trimers CO2–(CO)2 and CO2–(N2)2, the CO or N2 monomers occupy equivalent positions in the 'equatorial plane' of the CO2, pointing toward its C atom. To form the tetramers CO2–(CO)3 and CO2–(N2)3, a third CO or N2 monomer is then added off to the 'side' of the first two. In the mixed tetramers CO2–(CO)2–N2 and CO2–CO–(N2)2, this 'side' position is taken by N2 and not CO. In addition to the fundamental bands, combination bands are also observed for (CO2)2–CO, CO2–(CO)2, and CO2–(N2)2, yielding some information about their low-frequency intermolecular vibrations. [ABSTRACT FROM AUTHOR]

The weakly bound van der Waals dimer (CO2)2 has long been of considerable theoretical and experimental interest. Here, we study its low frequency intermolecular vibrations by means of combination bands in the region of the CO2 monomer ν3 fundamental (?2350 cm-1), which are observed using a tunable infrared laser to probe a pulsed supersonic slit jet expansion. With the help of a recent high level ab initio calculation by Wang, Carrington, and Dawes, four intermolecular frequencies are assigned: the in-plane disrotatory bend (22.26 cm-1); the out-ofplane torsion (23.24 cm-1); twice the disrotatory bend (31.51 cm-1); and the in-plane conrotatory bend (92.25 cm-1). The disrotatory bend and torsion, separated by only 0.98 cm-1, are strongly mixed by Coriolis interactions. The disrotatory bend overtone is well behaved, but the conrotatory bend is highly perturbed and could not be well fitted. The latter perturbations could be due to tunneling effects, which have not previously been observed experimentally for CO2 dimer. A fifth combination band, located 1.3 cm-1 below the conrotatory bend, remains unassigned. [ABSTRACT FROM AUTHOR]

Infrared spectra of Ne-C2D2 are observed in the region of the 3 fundamental band (asymmetric C-D stretch, 2440 cm-1) using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion from a cooled nozzle. Like helium-acetylene, this system lies close to the free rotor limit, making analysis tricky because stronger transitions tend to pile up close to monomer (C2D2) rotation-vibration transitions. Assignments are aided by predicted rotational energies calculated from a published ab initio intermolecular potential energy surface. The analysis extends up to the j = 32 band, where j labels C2D2 rotation within the dimer, and is much more complete than the limited infrared assignments previously reported for Ne-C2H2 and Ne-C2HD. Two previous microwave transitions within the j = 1 state of Ne-C2D2 are reassigned. Coriolis model fits to the theoretical levels and to the spectrum are compared. Since the variations observed as a function of C2D2 vibrational excitation are comparable to those noted between theory and experiment, it is evident that more detailed testing of theory will require vibrational averaging over the acetylene intramolecular modes. [ABSTRACT FROM AUTHOR]

Infrared spectra in the carbon monoxide CO stretch region (≈2150 cm-1) are assigned to the previously unobserved O-bonded form of the CO2-CO dimer ("isomer 2"), which has a planar T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective center of mass intermolecular distances are 3.58 Å for isomer 2 as compared to 3.91 Å for isomer 1. In addition to the fundamental band, two combination bands are observed for isomer 2, yielding values for two intermolecular vibrational modes: 14.19 cm-1 for the in-plane CO bend and 22.68 cm-1 for the out-of-plane bend. [ABSTRACT FROM AUTHOR]

All singly substituted 13C, 18O, and 34S isotopomers of the previously known OCS trimer are observed in natural abundance in a broad-band spectrum measured with a chirped-pulse Fourier transform microwave spectrometer. The complete substitution structure thus obtained critically tests (and confirms) the common assumption that monomers tend to retain their free structure in a weakly bound cluster. A new OCS trimer isomer is also observed, and its structure is determined to be barrel-shaped but with the monomers all approximately aligned, in contrast to the original trimer which is barrel-shaped with two monomers aligned and one anti-aligned. An OCS tetramer spectrum is assigned for the first time, and the tetramer structure resembles an original trimer with an OCS monomer added at the end with two sulfur atoms. Infrared spectra observed in the region of the OCS v1 fundamental (≈2060 cm-1) are assigned to the same OCS tetramer, and another infrared band is tentatively assigned to a different tetramer isomer. The experimental results are compared and contrasted with theoretical predictions from the literature and from new cluster calculations which use an accurate OCS pair potential and assume pairwise additivity. [ABSTRACT FROM AUTHOR]

Extensive infrared spectra of the weakly-bound CO2-CO dimer are observed in the carbon dioxide ν3 asymmetric stretch region (≈2350 cm−1) using a tunable infrared OPO laser source to probe a pulsed slit jet supersonic expansion. Both C-bonded and O-bonded isomers are analysed for the normal isotopologue as well as for 13CO2-CO and 16O13C18O-CO, the latter being the first observation of an asymmetrically substituted form for which all values of Ka are allowed. Combination bands involving the lowest in-plane intermolecular bending modes are also studied for both isomers. Weak bands near 2337 cm−1 are assigned to CO2 hot band transitions (v1, v 2 l 2 , v3) = (0111) ← (0110), yielding the splitting of the degenerate CO2 ν2 bend into in-plane and out-of-plane components due to the presence of the CO. This splitting has rather different values for the C- and O- bonded isomers, 4.57 and 1.59 cm−1, respectively, with the out-of-plane mode higher in energy than the in-plane for both cases. [ABSTRACT FROM AUTHOR]

The infrared spectrum of the weakly bound CO2–Xe complex is studied in the region of the carbon dioxide ν3 fundamental vibration (≈2350 cm−1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. The Xe isotope dependence of the spectrum is modelled by scaling the vibrational and rotational parameters, with the help of previous microwave data. The scaling model provides a good simulation of the observed broadening and (partial) splitting of transitions in the fundamental band, and it is essential for understanding the intermolecular bending combination band where some transitions are completely split by isotope effects. The combination band is influenced by a significant bend–stretch Coriolis interaction and by the relatively large Xe isotope dependence of the intermolecular stretch frequency. The weak CO2–Xe spectrum corresponding to the (0111) ← (0110) hot band of CO2 is also detected and analysed, providing a measurement of the symmetry breaking of the CO2 bending mode induced by the nearby Xe atom. This in-plane/out-of-plane splitting is determined to be 2.14 cm−1. [ABSTRACT FROM AUTHOR]

Spectra of the weakly bound complexes N2O-CO2, N2O-OCS, and N2O-N2 were studied in the region of the ν1 fundamental of N2O (∼2224 cm-1) using a tunable quantum cascade laser to probe a pulsed supersonic jet expansion with an effective rotational temperature of about 2.5 K. One new combination band was observed for each complex: a band involving an intermolecular in-plane bending mode for N2O-N2, a band involving the disrotation (in-plane geared bend) for of N2O-CO2, and a band involving the out-of-plane torsional vibration for isomer b of N2O-OCS. Small perturbations were noted for the N2O-OCS band. Because of the absence of theoretical prediction, the nature of the intermolecular bending mode for N2O-N2 has not been identified. The resulting intermolecular frequencies are 34.175(1), 17.107(1), and 22.334(1) cm-1 for N2O-CO2, N2O-OCS, and N2O-N2, respectively. In addition, the previously known fundamental band of N2O-N2 at 2225.99 cm-1 was analyzed in improved detail. This band exhibits very weak α-type transitions which were not detected in the first infrared observation of this complex, indicating that N2O-N2 is not exactly T-shaped. That is, the N2O molecular axis is not exactly perpendicular to the α-inertial axis, in agreement with a previous structural determination of this complex by rotational spectroscopy. [ABSTRACT FROM AUTHOR]

The weak infrared spectrum of CO2–Ar corresponding to the (0111) ← (0110) hot band of CO2 is detected in the region of the carbon dioxide ν3 fundamental vibration (≈2340 cm−1), using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. While this method was previously thought to cool clusters to the lowest rotational states of the ground vibrational state, here we show that under suitable jet expansion conditions, sufficient population remains in the first excited bending mode of CO2 (1–2%) to enable observation of vibrationally hot CO2–Ar, and thus to investigate the symmetry breaking of the intramolecular bending mode of CO2 in the presence of Ar. The bending mode of the CO2 monomer splits into an in-plane and an out-of-plane mode, strongly linked by a Coriolis interaction. Analysis of the spectrum yields a direct measurement of the in-plane/out-of-plane splitting measured to be 0.8770 cm−1. Calculations were carried out to determine if key features of our results, i.e., the sign and magnitude of the shift in the energy for the two intramolecular bending modes, are consistent with a quantum chemical potential energy surface. This aspect of intramolecular interactions has received little previous experimental and theoretical consideration. Therefore, we provide an additional avenue by which to study the intramolecular dynamics of this simplest dimer in its bending modes. Similar results should be possible for other weakly-bound complexes. [ABSTRACT FROM AUTHOR]

The jet-cooled spectrum ( T r o t = 12 K) of the H 2 O–CO 2 van der Waals complex has been recorded in the 1.4 μm region by cavity ring-down spectroscopy. Two b-type vibrational bands have been observed and analysed. The rotational assignment has been achieved using a different asymmetric rotor Hamiltonian for each nuclear spin species, accounting for the internal rotation of the H 2 O and CO 2 units. The band at 7247 cm − 1 is assigned to (101) in terms of the ( v 1 v 2 v 3 ) vibrational quantum number of the H 2 O monomer. The band at 7238 cm − 1 is assigned to (200) + an intermolecular mode ( v 12 ) excited in the complex. Vibration-rotation constants are provided for the excited states. The symmetry of the wavefunction, the effect of vibrational excitation on the tunnelling dynamics and the vibrational assignment are discussed. [ABSTRACT FROM AUTHOR]

Spectra of the nonpolar carbonyl sulfide dimer in the region of the OCS ν1 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 (18OCS)2 and 16OCS-18OCS 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 4ν2 (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. [ABSTRACT FROM AUTHOR]

Two new infrared bands in the ν1 fundamental region of N2O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both 14N216O and 15N216O. Although they are similar in appearance, the bands have slightly different lower state rotational parameters, and are thus assigned to distinct structural isomers of the pentamer. Cluster calculations using two N2O intermolecular potentials give results in good agreement with the observed spectra, and indicate that the two isomers probably have the same basic structure (which is unsymmetrical), but differ in the alignment (N-N-O or O-N-N) of one or two of the constituent monomers. Calculations using a resonant dipole interaction model also support the proposed assignment and structure. These are the first reported high-resolution spectra for N2O pentamers. [ABSTRACT FROM AUTHOR]

Spectra of the nonpolar nitrous oxide dimer in the region of the N2O ν1 fundamental band were observed in a supersonic slit-jet apparatus. The expansion gas was probed using radiation from a quantum cascade or a tunable diode laser, with both lasers employed in a rapid-scan signal averaging mode. Four bands were observed and analyzed: new combination bands involving the intermolecular conrotation of the monomers (Ag antigeared bend) for (14N2O)2 and (15N2O)2, the previously reported torsional combination band for (14N2O)2 with improved signal-to-noise ratio, and the same torsional combination band for (15N2O)2. The resulting frequencies for the intermolecular antigeared mode are 96.0926(1) and 95.4912(1) cm-1 for (14N2O)2 and (15N2O)2, respectively. This is the third of the four intermolecular frequencies which has now been measured experimentally, the others being the out-of-plane torsion and the geared bend modes. Our experimental results are in good agreement with two recent high level ab initio theoretical calculations. [ABSTRACT FROM AUTHOR]

Thirteen specific infrared bands in the 2350 cm-1 region are assigned to carbon dioxide clusters, (CO2)N, with N = 6, 7, 9, 10, 11, 12 and 13. The spectra are observed in direct absorption using a tuneable infrared laser to probe a pulsed supersonic jet expansion of a dilute mixture of CO2 in He carrier gas. Assignments are aided by cluster structure calculations made using two reliable CO2 intermolecular potential functions. For (CO2)6, two highly symmetric isomers are observed, one with S6 symmetry (probably the more stable form), and the other with S4 symmetry. (CO2)13 is also symmetric (S6), but the remaining clusters are asymmetric tops with no symmetry elements. The observed rotational constants tend to be slightly (≈2%) smaller than those from the predicted structures. The bands have increasing vibrational blueshifts with increasing cluster size, similar to those predicted by the resonant dipole-dipole interaction model but significantly larger in magnitude. [ABSTRACT FROM AUTHOR]