First direct observation of orientationally correlated molecules in CO2(l) by neutron diffraction

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Abstract
The structure factors S(Q) of the molecular fluid CO2 have been obtained at four different thermodynamic states, viz., CO2(l) at 220 K, 58 bar; 290 K, 60 bar; 303 K, 77 bar and CO2(g) at 303 K, 60 bar. The S(Q) of CO2(l) at 220 K shows a shoulder on the high-Q side of the main diffraction peak which is taken by some authors as evidence of the existence of orientational correlations between neighbouring molecules. The effect of strong quadrupole-quadrupole interactions in CO2 on its structure is examined in terms of orientational correlations. The real space G(r) results show for the first time direct evidence for the existence of orientationally correlated molecules in CO2(l) at a very short range (∼ 3·3 Å). These correlations disappear with increase of temperature. None of the models or the computer simulation data reported so far has reproduced these or similar effects.