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

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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.