Theory of mass-independent fractionation of isotopes, phase space accessibility, and a role of isotopic symmetry

Abstract

Key experimental and theoretical features of mass-independent fractionation (MIF) of isotopes, also known as the η-effect, are summarized, including its difference from the exit channel zero-point energy difference effect. The latter exactly cancels in the MIF. One key experimental result is that the MIF for O 3 formation is a low-pressure phenomenon and, moreover, that it decreases with increasing pressure of third bodies at pressures far below the “Lindemann fall-off” pressur e s for three-body recombination of O and O 2 . A possible origin of the MIF is discussed in terms of a role for isotopologue symmetry in intramolecular energy sharing. An explanation is suggested for the large difference in the fall-off pressure for recombination and the pressure for a large decrease in MIF, in terms of a difference between deactivating collisions and what we term here “symmetry-changing collisions”. It is noted that the theory of the MIF involves four recombination rate constants and an equilibrium constant, for each trace isotope, seven rate constants in all and two equilibrium constants. A conceptual shortcut is noted. Experimental and computational information that may provide added insight into the MIF mechanism and tests is described.

Communication: Rigorous quantum dynamics of O + O2 exchange reactions on an ab initio potential energy surface substantiate the negative temperature dependence of rate coefficients

“…[3][4][5][6] It is well established that ozone in the atmosphere is produced through the Chapman cycle, in which the metastable O 3 * formed by collision between O and O 2 is stabilized by energy loss to a third collision partner. While a complete explanation of the strong and surprising isotope effect has not been achieved yet, 7 it is reasonably certain that it has a quantum mechanical origin associated with the zeropoint energy differences among various O 2 isotopomers. 8 A closely related isotope effect has also been observed for the O + O 2 isotope exchange reactions, 9 which competes with the formation of O 3 .…”

Quantum Dynamics of the 18O + 36O2 Collision Process

“…5 Other factors have been evoked, like the η-effect. 6 However, this mass-independent fractionation effect is far from being well understood, for one simple reason: presently, despite a few attempts using classical-quantum hybrid methods (see for example Ref. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Concerning the exchange processes 18 O + 32 O 2 and 16 O + 36 O 2 alone (not even talking of the recombination reaction leading to ozone), significant differences already appear between experimentally measured [8][9][10][11] and theoretically computed rate constants.…”

Detection and analysis of three highly excited vibrational bands of 16O3 by CW-CRDS near the dissociation threshold

“…As the upper studied band is located only 8% below the dissociation threshold of the ground electronic state (D 0 $8560 cm À 1 [14][15][16]), this new information permits considerably augmenting the knowledge of unusual properties of excited ozone such as the selective enrichment of heavy ozone isotopomers in the atmosphere and in laboratory experiments [18,19]. Note that many features related to the process of ozone formation are still far from being understood [20]. In particular the existence of a reef-like structure [18,21,22] on the minimum energy path towards the dissociation, which should have an impact on the ozone dynamics, is an intricate subject of theoretical discussions [23,24].…”