M. Rezaei

Nonpolar nitrous oxide dimer: Observation of combination bands of (14N2O)2 and (15N2O)2 involving the torsion and antigeared bending modes

Spectra of the nonpolar nitrous oxide dimer in the region of the N(2)O ν(1) 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 (A(g) antigeared bend) for ((14)N(2)O)(2) and ((15)N(2)O)(2), the previously reported torsional combination band for ((14)N(2)O)(2) with improved signal-to-noise ratio, and the same torsional combination band for ((15)N(2)O)(2). The resulting frequencies for the intermolecular antigeared mode are 96.0926(1) and 95.4912(1) cm(-1) for ((14)N(2)O)(2) and ((15)N(2)O)(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.

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) 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 CS(2) dimer.

Towards an understanding of the helium–acetylene van der Waals complex

The weakly bound complex He–C2D2 is studied in the ν 3 fundamental band of C2D2 (≈2440 cm−1) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. This is the first published spectrum for helium–acetylene. Transitions observed in the region of the C2D2 R(0) line are assigned with the help of theoretical results based on an ab initio intermolecular potential, and fitted using a simple Coriolis model. The results indicate that the complex is rather close to the free rotor limit, helping to explain the absence of previous data. Scaled parameters from the model are used to predict a spectrum for He–C2H2.

New combination bands of N2O-CO2, N2O-OCS, and N2O-N2 complexes in the N2O ν1 region

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 a-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 a-inertial axis, in agreement with a previous structural determination of this complex by rotational spectroscopy.

Spectroscopic observation of nitrous oxide pentamers

Two new infrared bands in the ν(1) fundamental region of N(2)O are observed in a supersonic jet expansion and assigned to nitrous oxide pentamers. Each band is measured using both (14)N(2)(16)O and (15)N(2)(16)O. 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 N(2)O 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 N(2)O pentamers.

Fundamental and combination bands of CO2–C2H2 and CO2–C2D2 in the mid-infrared region

Spectra of the weakly bound CO2–C2H2 and CO2–C2D2 complexes are observed in the regions of CO2 ν3 (≈ 2349 cm−1) and C2D2 ν3 (≈ 2440 cm−1) fundamental vibrations, using an infrared optical parametric oscillator to probe a pulsed supersonic slit-jet expansion. Five bands are measured and analysed: the fundamental asymmetric stretch of the C2D2 component, two combination bands involving the out-of-plane torsional vibrations (C2D2 ν3 + torsion and CO2 ν3 + torsion) for CO2–C2D2, and two combination bands involving an intermolecular in-plane bending vibration for CO2–C2H2 and CO2–C2D2. The resulting intermolecular frequencies are 61.408(1), 54.5(5), 39.9(5), and 39.961(1) cm−1 for CO2–C2H2 and CO2–C2D2 in-plane vibrations, and CO2–C2D2 out-of-plane torsional vibrations in CO2 and C2D2 regions, respectively. This is the first experimental determination of these intermolecular vibrational frequencies.

Communication: Spectroscopic evidence for a planar cyclic CO trimer.

A high-resolution spectrum in the region of 2144 cm(-1) is assigned to the previously elusive CO trimer. In spite of interference from the CO dimer and some remaining unexplained details, there is strong evidence for a planar, cyclic, C-bonded trimer structure, with C(3h) symmetry and 4.42 Å intermolecular separation, in agreement with theoretical calculations. A modest vibrational blueshift of +0.85 cm(-1) is observed for the CO trimer, as compared to +0.71 cm(-1) for the C-bonded form of the dimer.

Five intermolecular vibrations of the CO2 dimer observed via infrared combination bands

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-of-plane 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.