The structure of the CO2–CO2–H2O van der Waals complex determined by microwave spectroscopy

Abstract

Rotational spectra of CO2 –CO2 –H2 O, CO2 –CO2 –D2 O, 13 CO2 –13 CO2 –H2 O and CO2 –CO2 –H2 18 O have been measured using a pulsed-molecular-beam Fabry–Perot Fourier-transform microwave spectrometer. An asymmetric top spectrum is observed with rotational constants, A=3313.411(5) MHz, B=1470.548(3) MHz, and C=1308.850(3) MHz for the normal species. The dipole moment obtained is μT =μb =1.989(2) D. Only b-type transitions are observed with the transitions showing a 3 to 1 intensity alternation depending on whether Ka +Kc is odd or even, respectively. This indicates a structure with twofold symmetry with the C2v axis of the water subunit aligned with the C2 axis of the complex. The CO2 subunits lie in a plane which is perpendicular to the C2 axis and located 2.47 Å below the oxygen atom of the water subunit; the C–C bond length is 3.413(2) Å. The orientation of the CO2 subunits in CO2 –CO2 –H2 O is very similar to that observed in CO2 –CO2 although the C–C bond length is 0.19 Å shorter in the trimer. The C–O bond distances between the H2 O and two CO2 subunits are both 3.00(2) Å which is 0.16 Å longer than that found in the CO2 –H2 O dimer. The hydrogens of the H2 O subunit are directed away from the CO2 –CO2 plane although their angular orientation around the b axis is not well determined.

The OCS Trimer: Isotopic Studies, Structure, and Dipole Moment

“…This trend is broken by the C−O−O−C dihedral angle in (CO 2 ) 2 OCS, which has the considerably smaller value of 7.7°. Other trimers with similar structures, such as (CO 2 ) 2 HCN, (CO 2 ) 2 H 2 O, , (N 2 O) 3 , and (CO 2 ) 3 , also have dihedral angles that are considerably different from the ∼34° angle seen in the complexes discussed here. Thus, perhaps the similarity between the dihedral angles of (OCS) 3 , (CO 2 ) 2 N 2 O, and (OCS) 2 CO 2 is no more than a coincidence.…”

“…This structure is similar to that proposed by Connelly et al in a previous paper and it conforms to a pattern of structures seen in other trimers of linear monomers. Structures with this triangular arrangement of monomers are common and include CO 2 trimer, N 2 O trimer, (CO 2 ) 2 H 2 O, , (CO 2 ) 2 HCN, (CO 2 ) 2 N 2 O, (CO 2 ) 2 OCS, , and (OCS) 2 CO 2 , for example. The structures in which the three monomer units are roughly parallel maximize the dispersion interactions, while the slipping and twisting of the sticks enhances the attractive electrostatic forces.…”

Rotational Spectrum and Structure of the (OCS)2−CO2Trimer

“…The CO 2 −(OCS) 2 trimer has been found to possess the tilted barrel shape that has also been observed in a number of trimer systems to date. For example, the three homomolecular trimers (OCS) 3 , (CO 2 ) 3 , and (N 2 O) 3 and the mixed trimers (CO 2 ) 2 −OCS 8,9 and (CO 2 ) 2 −H 2 O , all possess structures that resemble CO 2 −(OCS) 2 . Only for the (CO 2 ) 3 system has a second, planar-pinwheel isomer also been observed .…”

Spectroscopy of Hydrothermal Reactions. 1. The CO2−H2O System and Kinetics of Urea Decomposition in an FTIR Spectroscopy Flow Reactor Cell Operable to 725 K and 335 bar

“…The resulting vibrational frequency is red-shifted from the center of the P and R branches of gas-phase CO 2 which is located at 2349 cm -1 . The similarity of these frequencies indicates that the carbon atom of CO 2 weakly associates with the oxygen atom of H 2 O, rather than the hydrogen atoms. ,− Above 575 K, ν 3 (CO 2 ) becomes increasingly asymmetric and begins to resolve into the P and R branches above 625 K. This gradual transition indicates that diffusional rotation of CO 2 is occurring in this temperature range. By 700 K, which was the temperature limit of measurement and is well above the critical point of the solution, the P and R branches of CO 2 are broad and resolved, which is consistent with the dense gas−fluid supercritical state.…”