Mechanisms of Dipolar Ortho/Para-H₂O Conversion in Ice

Abstract

In this paper a possible explanation for an unexpected ortho/para-water ratio in the gas clouds of comets is given. The description is based on the quantum-mechanical density matrix formalism and the spin temperature concept. Only the nuclear spin system is treated quantum-mechanically. Employing the model of a four spin system, created by two nearest neighbour water molecules, spin eigenstates and their dynamics under the influence of their mutual dipolar interactions are studied. It is shown that a fast conversion between ortho- and para-states occurs on a msec time scale, caused by the intermolecular homonuclear magnetic dipolar interaction. Moreover the spin eigenstates of water in an ice crystal are determined by magnetic dipolar interactions and are not given by normal ortho- and para-H2O states of gaseous water. As a result of this the spin temperature of gaseous water evaporated from ice depends strongly on its evaporation history and the ortho/para-ratio of water molecules are only an indirect measure of the temperature of ice crystals from where they descend. This result could explain the unexpected experimentally observed ortho/para-ratios in the clouds of comets.

Uncertainties in water chemistry in disks: An application to TW Hydrae

“…Theoretical calculations of Buntkowsky et al (2008) show that this happens in an ice matrix even on timescales of milliseconds. Recent experiments (Hama et al 2011) suggest that the o/p ratio of thermally desorbed water -originally deposited at 8 K -is always close to three, which is the high temperature limit.…”

Water Ice Formation and the o/p Ratio

“…Buntkowsky et al (2008) have studied the case of two water molecules from a theoretical point of view. They have shown that the frustration of the rotation reduces considerably the difference in energy of the rotational levels.…”

“…I will present some results obtained in noble gas matrix that demonstrate the dependence of the NSC on the concentration of H 2 O, suggesting that the NSC is accelerated with water-water interaction. I will recall the theoretical results obtained by Buntkowsky et al (2008), that show that the interaction of 2 water molecules should produce NSC in the millisecond timescale. I will finally discuss the possibility that selective desorption of water in function of the spin isomer can produce a disequilibrium in the opr of the nascent molecules detected in the gas phase.…”

Separating Para and Ortho Water

“…However, unlike other small polyatomic molecules exhibiting spin isomerism, such as fluoromethane or ethylene [15,16] , which were spin-isomerically enriched using the light induced drift technique [7] , this has not been achieved for water. Separation through selective adsorption on surfaces was reported [17] , but these results remain controversial and could not be reproduced [18][19][20][21] . Thus, studies of spin-conversion dynamics in water have been limited to water embedded in rare gas matrices, with relative spin populations modified by the sample temperature [22] .…”