Hydration of carbon dioxide: The structure of H2O–H2O–CO2 from microwave spectroscopy

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Abstract
The structure of the gas‐phase trimeric complex H2O–H2O–CO2 is determined through an analysis of the rotational spectra of ten isotopically substituted species. These spectra were measured in the region between 7.5 and 18 GHz using a pulsed‐molecular‐beam Fourier‐transform microwave spectrometer. The nondeuterated species display two sets of transitions separated by ∼1 MHz. The splittings of the perdeuterated form are smaller and three partially deuterated forms have no splittings. The rotational constants for the lower frequency set of transitions of the normal species are A=6163.571(4) MHz, B=2226.157(2) MHz, C=1638.972(1) MHz, δJ=0.000 83(3) MHz, ΔJ=0.002 98(4) MHz, ΔJK=−0.0005(2) MHz. The differences in the rotational constants between the upper and lower states are ΔA=498 kHz, ΔB=520 kHz, and ΔC=−133 kHz. The dipole moments are μa=1.571(5) D and μb=0.761(4) D with μc=0 D. The dipole moments and the intertial defect of −0.620 uÅ2 both indicate an essentially planar complex. The structure is found to be cyclical with the dimer‐type bond lengths within the trimer being approximately the same as those found in the free heterodimers. One water molecule is oxygen bound to the carbon atom of the CO2 and is also hydrogen bonded to the oxygen of the second water molecule. The second water molecule is in turn hydrogen bonded to one of the oxygens of the CO2 molecule. The observed splittings are most likely due to a hydrogen‐exchanging internal rotation of this second water molecule.

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