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

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.