The discovery of the first chemically produced mass-independent isotope effect in 1983 by Thiemens & Heidenreich opened a broad variety of applications, including physical chemistry studies, atmospheric chemistry, paleoclimatology, biologic primary productivity assessment, Solar System origin and evolution, planetary atmospheres (Mars), and the origin and evolution of life in Earth's earliest environment. This chapter reviews the history of the field as well as all of the various applications since the first report of the mass-independent isotope effect.

History and applications of mass-independent isotope effects

Many gas-phase chemical reactions proceed via reaction intermediates, supported by potential wells. The characteristics of such complex-forming reactions differ drastically from those for direct reactions that involve barriers. For example, the reaction path for a complex-forming reaction is often barrierless, which results in weak and sometimes negative temperature dependence for its rate constant. The product angular and internal distributions of such reactions also bear clear signatures. Specifically, the angular distribution (i.e. differential cross-section) of a complex-forming reaction is often dominated by scattering in the forward and backward directions, and the product rotational state distribution usually peaks near the highest accessible rotational state, while vibrational state distribution often decays monotonically. While the quantum dynamics of direct reactions is well established, our understanding of complex-forming reactions is still far from complete. Given the importance of such react...

Quantum dynamics of complex-forming bimolecular reactions

Isotope effects in the chemistry of atmospheric trace compounds.

We present an ab initio potential energy surface for the ground electronic state of ozone. It is global, i.e., it covers the three identical C2v (open) minima, the D3h (ring) minimum, as well as the O(3P)+O2(3Σg−) dissociation threshold. The electronic structure calculations are performed at the multireference configuration interaction level with complete active space self-consistent-field reference functions and correlation consistent polarized quadruple zeta atomic basis functions. Two of the O–O bond distances, R1 and R2, and the O–O–O bending angle are varied on a regular grid (ca. 5000 points with R1⩾R2). An analytical representation is obtained by a three-dimensional cubic spline. The calculated potential energy surface has a tiny dissociation barrier and a shallow van der Waals minimum in the exit channel. The ring minimum is separated from the three open minima by a high potential barrier and therefore presumably does not influence the low-temperature kinetics. The dissociation energy is reproduce...

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The vibrational energies of ozone up to the dissociation threshold: Dynamics calculations on an accurate potential energy surface

Dynamical studies of the recombination of O and O2 to form ozone are reviewed. The focus is the intriguing isotope dependence of the recombination rate coefficient as observed by Mauersberger and coworkers in the last decade. The key quantity for understanding of this dependence appears to be the difference of zero-point energies of the two fragmentation channels to which excited ozone can dissociate, i.e., X + YZ XY + Z, where X, Y, and Z stand for the three isotopes of oxygen. Besides the isotope dependence, the variation of the recombination rate coefficient with pressure and temperature is also addressed. Despite the numerous approaches of recent years, the recombination of ozone is far from being satisfactorily explained; there are still several essential questions to be solved by detailed theoretical analysis. We mainly discuss--and critically assess--the results of our own investigations of the ozone kinetics. The work of other research groups is also evaluated.

DYNAMICAL STUDIES OF THE OZONE ISOTOPE EFFECT: A Status Report

The oxygen isotope anomaly in ozone was first identified in isotopomers 49O3 and 50O3 with enrichments of over 10%. Recent rate coefficient measurements imply a new interpretation of the anomaly. The large variability of the rate coefficients of ozone formation O + O2
→ O3 of about 50% characterizes the ozone isotope effect and demonstrates its kinetic origin. Surprisingly, the rate coefficients correlate with the zero-point energy change of the oxygen molecules participating in the isotope exchange reactions but not with the mass of the ozone molecules. Rate coefficients of symmetric molecules depart from this linear relationship by about 20%.

Jã¼rgen Guenther

Papers

Kinetic origin of the ozone isotope effect: a critical analysis of enrichments and rate coefficients