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The infrared spectrum of CO2‐Kr complex is studied in the region of the carbon dioxide ν3 fundamental vibration (≈2350 cm−1). Tunable IR radiation from an OPO is employed to record absorption spectrum of the complex generated in a pulsed supersonic slit jet. The spectrum exhibits broadening and splitting of transitions due to mass dependence of the five Kr isotopes with natural abundances exceeding 2 %. Good simulation of the spectrum is achieved by scaling the vibrational and rotational parameters. This scaling model is particularly important for the combination band involving the intermolecular bending mode where many isotope splittings are observed. As with other CO2‐rare gas complexes, we also observe weak hot bands of CO2‐Kr transitions corresponding to the hot band originating in the CO2 intramolecular bend. From these we determine a splitting of 1.418 cm−1 between the in‐plane and out‐of‐plane bend of CO2, due to the neighboring Kr atom. [ABSTRACT FROM AUTHOR]

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]

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]

A detailed description of a new ab initio interaction potential energy surfaces for the H2-CO complex computed on a six-dimensional grid (i.e., including the dependence on the H-H and C-O separations) is presented. The interaction energies were first calculated using the coupled-cluster method with single, double, and noniterative triple excitations and large basis sets, followed by an extrapolation procedure. Next, a contribution from iterative triple and noniterative quadruple excitations was added from calculations in smaller basis sets. The resulting interaction energies were then averaged over the ground-state and both ground- and first-excited-states vibrational wave functions of H2 and CO, respectively. The two resulting four-dimensional potential energy surfaces were fitted by analytic expressions. Theoretical infrared spectra calculated from these surfaces have already been shown [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)] to agree extremely well, to within a few hundredth of wavenumber, with the experimental spectra of the para and orthoH2-CO complex. In the latter case, this agreement enabled an assignment of the experimental spectrum, ten years after it had been measured. In the present paper, we provide details concerning the development of the surfaces and the process of spectral line assignment. Furthermore, we assign some transitions for paraH2-CO that have not been assigned earlier. A completely new element of the present work are experimental investigations of the orthoH2-CO complex using microwave spectroscopy. Vast parts of the measured spectrum have been interpreted by comparisons with the infrared experiments, including new low-temperature ones, and theoretical spectrum. Better understanding of the spectra of both para and orthoH2-CO complexes provides a solid foundation for a new search of the bound H2-CO complex in space. [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]

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]

Clusters of para-H2 and/or ortho-H2 containing a single carbon dioxide molecule are studied by high resolution infrared spectroscopy in the 2300 cm-1 region of the CO2 ν3 fundamental band. The (H2)N-CO2 clusters are formed in a pulsed supersonic jet expansion from a cooled nozzle and probed using a rapid scan tunable diode laser. Simple symmetric rotor type spectra are observed with little or no resolved K-structure, and prominent Q-branch features for ortho-H2 but not para-H2. Observed rotational constants and vibrational shifts are reported for ortho-H2 up to N = 7 and para-H2 up to N = 15, with the N > 7 assignments only made possible with the help of theoretical simulations. The para-H2 cluster with N = 12 shows clear evidence for superfluid effects, in good agreement with theory. The presence of larger clusters with N > 15 is evident in the spectra, but specific assignments are not possible. Mixed para- + ortho-H2 cluster transitions are well predicted by linear interpolation between corresponding pure cluster line positions. [ABSTRACT FROM AUTHOR]

Spectra of solid para-H2 doped with CH3F at 1.8 K are studied in the ν3 region (∼1040 cm-1) using a quantum cascade laser source. As shown previously, residual ortho-H2 in the sample (∼1000 ppm) gives rise to distinct spectral features due to clusters of the form CH3F-(ortho-H2)N, with N = 0, 1, 2, 3, etc. Brief annealing at 7 K is found to give narrower spectral lines (≥0.006 cm-1) than conventional (5 K) annealing, and causes the N = 3 and 4 lines to fragment into two or more components. The N = 3 line is observed to be particularly stable and persistent. The N = 0 line (no ortho-H2 neighbors) is resolved into two closely spaced (≈0.007 cm-1) components which are assigned to the K = 0 and 1 states of CH3F rotating around its C3v symmetry axis (ortho- and para-CH3F, respectively). Similar K-structure is also evident for other lines. Weak but persistent features ('N = 1/2 lines') are observed mid way between N = 0 and 1. [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]

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]