A new feature in the internal heavy isotope distribution in ozone

Ozone produced by discharge or photolysis of oxygen has unusually heavy isotopic composition (¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratio) which does not follow normal mass fractionation rule: δ¹⁷O ∼ 0.52^*δ¹⁸O, expressed as an anomaly Δ¹⁷O = δ¹⁷O − 0.52^*δ¹⁸O. Ozone molecule being an open isosceles triangle can have the heavy isotope located either in its apex or symmetric (s) position or the base or asymmetric (as) position. Correspondingly, one can define positional isotopic enrichment, written as δ¹⁸O (s) or δ¹⁸O (as) (and similarly for δ¹⁷O) as well as position dependent isotope anomaly Δ¹⁷O (s) and Δ¹⁷O (as). Marcus and co-workers have proposed a semi-empirical model based in principle on the RRKM model of uni-molecular dissociation but with slight modification (departure from statistical randomness assumption for symmetrical molecules) which explains many features of ozone isotopic enrichment. This model predicts that the bulk isotope anomaly is contained wholly in the asymmetric position and the Δ¹⁷O (s) is zero. Consequently, Δ¹⁷O (as) = 1.5 ^* Δ¹⁷O (bulk) (named here simply as the "1.5 rule") which has been experimentally confirmed over a range of isotopic enrichment. We now show that a critical re-analysis of the earlier experimental data demonstrates a small but significant departure from this 1.5 rule at the highest and lowest levels of enrichments. This departure provides the first experimental proof that the dynamics of ozone formation differs from a statistical model constrained only by restriction of symmetry. We speculate over some possible causes for the departure