Novel Insights into the Mechanism of the Ortho/Para Spin Conversion of Hydrogen Pairs: Implications for Catalysis and Interstellar Water

Abstract

The phenomenon of exchange coupling is taken into account in the description of the magnetic nuclear spin conversion between bound ortho- and para-dihydrogen. This conversion occurs without bond breaking, in contrast to the chemical spin conversion. It is shown that the exchange coupling needs to be reduced so that the corresponding exchange barrier can increase and the given magnetic interaction can effectively induce a spin conversion. The implications for related molecules such as water are discussed. For ice, a dipolar magnetic conversion and for liquid water a chemical conversion are predicted to occur within the millisecond timescale. It follows that a separation of water into its spin isomers, as proposed by Tikhonov and Volkov (Science 2002, 296, 2363), is not feasible. Nuclear spin temperatures of water vapor in comets, which are smaller than the gas-phase equilibrium temperatures, are proposed to be diagnostic for the temperature of the ice or the dust surface from which the water was released.

Ortho-to-para ratio of interstellar heavy water

“…This can occur in either the gas phase by reactions with ions in which actual nuclei change places, or on the grain surfaces by interaction with electron spins or, perhaps, even other nuclear spins (e.g. Le Bourlot 2000;Limbach et al 2006). Although little is known about the spin exchange in the gas phase, it is usually assumed that this is a slow process and that the OPR is likely to retain information about the moment of its formation.…”

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] .…”

A Search for Variation in the H 2 O Ortho-Para Ratio and Rotational Temperature in the Inner Coma of Comet C/2004 Q2 (Machholz)

“…Another possibility is that of dipolar magnetic spin conversion on ice surfaces (Limbach et al 2006). Notably, Limbach et al predict the ortho-para ratio (OPR) to change in an isolated hydrophilic surface, implying that the OPR measured in H 2 O gas released from a cold ice surface would reflect the surface temperature.…”