Mahin Afshari

Nitrous oxide dimer: observation of a new polar isomer.

Spectra of the nitrous oxide dimer (N2O)2 are studied in the region of the N2O nu1 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.

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.

High-resolution infrared spectroscopy of carbon dioxide dimers, trimers, and larger clusters

From 1984 to 1996 there was rapid progress in the spectroscopy of CO2 dimers and trimers, but since then little work has appeared. Here we report a number of new high-resolution infrared results. For the dimer, a combination band provides the first experimental intermolecular vibrational frequency, and 13C isotopic spectra clarify the role of resonant and non-resonant vibrational shifts. For the cyclic trimer, a new parallel combination band involving an out-of-plane intermolecular mode is observed, and various 13C isotope results are analysed. In addition to these dimer and trimer results, the spectra also contain features which must be due to larger (CO2) N clusters in the range N ≈ 5–15.

Nitrous oxide dimer: An ab initio coupled-cluster study of isomers, interconversions, and infrared fundamental bands, and experimental observation of a new fundamental for the polar isomer

Improved quantum chemistry (coupled-cluster) results are presented for spectroscopic parameters and the potential energy surface for the N(2)O dimer. The calculations produce three isomer structures, of which the two lowest energy forms are those observed experimentally: a nonpolar C(2h)-symmetry planar slipped-antiparallel geometry (with inward-located O atoms) and a higher-energy polar C(s)-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 nu(1) antisymmetric stretch of the individual monomers. The new band was observed in the region of the monomer nu(1) fundamental for both ((14)N(2)O)(2) and ((15)N(2)O)(2) using a tunable infrared diode laser to probe a pulsed supersonic jet expansion, and results are presented.

New infrared spectra of the nitrous oxide trimer.

Infrared spectra of N(2)O 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 N(2)O nu(1)+nu(3) combination band region ( approximately 3480 cm(-1)) showed the trimer structure to be noncyclic, with three inequivalent N(2)O monomer units which could be thought of as an N(2)O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N(2)O fundamental band regions nu(3) ( approximately 1280 cm(-1)) and nu(1) ( approximately 2230 cm(-1)). In the nu(3) region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the nu(1)+nu(3) region, but in the nu(1) 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 N(2)O dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully (15)N-substituted species in the nu(1) region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer.

The torsional vibration of the CO2–N2O complex determined from its infrared spectrum

The spectra of the weakly bound complex CO(2)-N(2)O are studied in the regions of the nu(1) and nu(3) fundamentals of N(2)O and the nu(3) fundamental of CO(2) 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 nu(1) and nu(3) fundamentals of the N(2)O component of the complex. Small perturbations are noted in the nu(3) band, similar to those observed previously for the nonpolar dimer (N(2)O)(2) in the nu(3) and nu(1)+nu(3) regions. The previously known band at 2348.86 cm(-1) in the region of the nu(3) CO(2) 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 N(2)O nu(1) and CO(2) nu(3) stretches, respectively. The resulting torsional frequency of CO(2)-N(2)O is experimentally determined for the first time to be 25.8 cm(-1).

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 spectra of the OCS-CO2 complex: Observation of two distinct slipped near-parallel isomers

Infrared spectra of OCS-CO(2) 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 A and the C atom of the CO(2) is closer to the S atom of the OCS. In isomer b, the C atom of CO(2) slides closer to the O atom of OCS and the center of mass separation increases to 3.99 A. 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.

Ubiquitous T-shaped isomers of OCS-hydrocarbon van der Waals complexes

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 nu(1) 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 CO(2) and N(2)O.