CO2 Dimer Research Articles

Local orientational correlations and short time anisotropic motion in molecular liquids: Computer simulations of liquid CO2

Molecular dynamic (MD) simulations of liquid CO2 at 250 MPa pressure and room temperature have been performed using a flexible model potential. A detailed analysis of the data reveal the full three-dimensional local structure of coordination shells that exhibit significant deviations from spherical symmetry with strong angular correlations among the molecules which form the inner coordination shell of the local liquid structure. Structures resembling T-shaped and offset-parallel CO2 dimers similar to those found in molecular beam and low temperature experiments have been identified, the T-shaped dimer having higher probability to be formed than the offset-parallel configuration. Local motion on short time scale is found to be different along three principal directions of a local coordinate frame.

Kinetics of molecular association and relaxation in a pulsed supersonic expansion

The formation of polar Ar/HCN dimers and trimers in supersonic jets has been studied by observing the intensity of rotational transitions in the expanded pulsed jet as a function of input gas composition at a backing pressure of 1 to 2 atm. A programmable gas flow system was employed for accurate control of the composition of the gas mixture supplied to the pulsed nozzle. The formation of Ar–HCN was found to be first order in [Ar] and in [HCN]; (HCN)2 to be second order in [HCN]; (HCN)3 to be second order in [HCN]; and both Ar2–HCN and (HCN)2–Ar to be first order in [Ar] and in [HCN]. The three trimers are formed in two consecutive bimolecular steps at steady state, with (HCN)2 as the intermediate for (HCN)3, and Ar–HCN as the intermediate for both Ar2–HCN and (HCN)2–Ar. Less extensive measurements were made on the HCN/CO2 system, in which formation of the T-shaped HCN–CO2 dimer was found to be first order in [HCN] and in [CO2]. The rare gases He, first-run Ne (70% Ne, 30% He), and Ar were used as carrier gases. The carrier gas plays a major role as a third body in the formation steps. The relative effectiveness of the carrier gases as a relaxant in forming the dimers and trimers is Ar>Ne (first run)≫He. The method and results are discussed.

Rotational spectrum and structure of the linear CO2–HCN dimer: Dependence of isomer formation on carrier gas

A linear hydrogen-bonded dimer, OCO–HCN, has been identified and characterized via its microwave rotational spectrum. The study was made using the pulsed nozzle Fourier transform method with the Flygare/Balle Mark II spectrometer. A T-shaped HCN–CO2 dimer was reported earlier by the Klemperer group. Rotational constants have been determined for all seven monoisotopically substituted species of the linear form. B0 , DJ , and χaa (14 N) for the normal isotopic dimer are 1057.9397(2) MHz, 1.372(8) kHz, and −4.2466(5) MHz, respectively. The average torsional displacements of the OCO and HCN monomers about their center of mass (c.m.) are found to be 7.66° and 12.40°, based on the substitution O–C and C–N bond distances for the dimer. With these values for α and γ, the B0 for the normal isotopic dimer corresponds to a c.m. to c.m. distance R=5.035 Å. Bending and stretching force constants and the well depth (ε∼590 cm−1 ) are estimated from the centrifugal distortion. The relative concentrations of the linear and T-shaped isomers are unusually sensitive to the carrier gas used in the supersonic jet expansion. The linear form could not be detected at all with argon as the carrier gas but gave a strong signal in neon first run (70% Ne, 30% He). In contrast, the T form gave strong signals in both carrier gases. However, a carrier-gas effect was not found for the N2 O/HF dimer pair, which has a high barrier between the bent NNO–HF and linear FH–NNO isomers. Similar results were obtained for chlorocyclohexane (CCH) and ethyl formate (EF), which have two conformational isomers. In CCH which has a high barrier to a↔e interconversion, the two conformers gave strong signals in both Ar and He. In EF, with a low barrier, the gauche conformer could not be detected in Ar but gave a strong signal in He, while the trans form gave strong signals in both carrier gases.

Spatial distribution of CO2 dimers in axisymmetric gas jets expanding in a vacuum

Spatial distribution of CO2 dimers in axisymmetric gas jets expanding in a vacuum

Vibrational predissociation in the CO2 dimer and trimer and rare gas–CO2 complexes

Vibrational predissociation linewidths for the CO2 dimer and trimer and the Ne–CO2 and Ar–CO2 complexes have been resolved using a bolometer-detected (optothermal) molecular-beam color-center laser spectrometer. Observations were made on the pair of vibrations near 3715 and 3613 cm−1 corresponding to the ν1+ν3/2ν02+ν3 Fermi diad of CO2. Homogeneous linewidths of from ∼0.5 to ∼22 MHz (FWHM) were measured for these related complexes, with Ne–CO2 exhibiting both the broadest and the sharpest lines for the upper and lower bands, respectively. Ar–CO2 and (CO2)2 showed mode-independent intermediate predissociation rates while only the lower band of (CO2)3 could be found. The results indicate that V→V energy transfer processes are the dominant predissociation channels with symmetry selection or propensity rules and specific resonances playing a role.

Structure and vibrational dynamics of the CO2 dimer from the sub-Doppler infrared spectrum of the 2.7 μm Fermi diad

Sub-Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color-center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C--C separation and C--C–O angle are determined to be 3.599(7) Å and 58.2(8)°, respectively. The nearest neighbor O--C distance is 3.14 Å which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.

Dimer concentrations in supersonic molecular beams of argon and carbon dioxide

The terminal dimer concentrations in supersonic molecular beams of Ar and CO2 were measured as functions of the stagnation pressure P0. For both Ar and CO2, the relative dimer densities were found to obey a power law, n/sub d//n/sub m/ proportional to P0/sub /, where n/sub d/ and n/sub m/ are the terminal number densities of the dimer and the monomer, respectively. The values of the exponent were determined to be 1.46 +/- 0.19 for Ar and 2.03 +/- 0.23 for CO2, being significantly different from each other. It was shown that the equilibrium concentration of (CO2)2 was more strongly dependent on the temperature than that of Ar2 because of the existence of the degrees of freedom of internal rotations in (CO2)2, and that this stronger temperature dependence caused a steeper P0 dependence of the CO2 dimer concentration. Information was also obtained on the rate constants for the rate-determining step in the dimer formation processes. 26 references, 6 figures.

The reaction of CO2 dimer ions with sulfur dioxide

The rate coefficient and the product distribution have been determined for the reaction CO2-CO+2 with sulfur dioxide.

Condensation in CO2 free jet expansions. I. Dimer formation

A theoretical and experimental investigation of the kinetics of CO2 dimer formation in the adiabatically expanding core of a free jet expansion has been carried out. The theoretical analysis is based on dimer formation by an efficient termolecular process and dimer destruction by bimolecular collisions. Account is taken of the expected variation of the specific heat ratio γ during the expansion and of the effective increase in the termolecular collision rate at low temperatures due to the presence of loosely bound orbiting pairs. Experimental dimer concentrations were measured mass spectrometrically by forming a molecular beam from the expanded gas mixture after the onset of molecular flow. Observed dimer concentrations, for a range of pre-expansion conditions of 400–900 torr pressure and room temperature, are in good agreement with the theoretical analysis, provided account is taken of the effect of loosely bound orbiting pairs at low temperatures.