Isotopologs

The most straightforward experimental approach is isotopic labeling of certain atoms in the reactants. The detection method must distinguish between the possible isotopologs of the products. For example, in the reaction... 

The potential energy surface [47] for this reaction (Fig. 5) shows many potentially competitive pathways, labeled A-F, leading to the two most exothermic product channels. Many of these pathways can be isotopically separated by reaction of 02 with HCCO in normal abundance, as diagramed in Fig. 5. Zou and Osbom used time-resolved Fourier transform emission spectroscopy to detect the CO and CO2 products of this reaction [47]. Rotationally resolved infrared (IR) spectroscopy can easily identify all the possible isotopologs. For example. Fig. 6 shows a single... 

Figure 2. Representations of the three distinct topologies derived by simulated annealing based on data for a lithium gallosilicate (orthorhombic, Pna2i a = 18.5A, c = 7.5 A, 2 unique T-sites, 8 T-atoms in the unit cell) [66], The correct model (which proves isotopological with the parent zeolite Li-A(BW)) is (c).

By selective labeling experiments, it has been shown that, in contrast with CH5+, the proton scrambling does not occur in the isoC H,1 and C3H9+species. Rather, the exchange takes place between the alkane and the acid and not between the isotopolog... 

The absorption cross-sections for the isotopically substituted species are the same and thus for a given photon flux and path length, the photon absorption scales with the abundance of the isotopolog. Under conditions of the interstellar medium and solar nebula, isotope-selective chemistry may occur for abundant CO molecules (e.g. Dalgarno Stephens 1970 Thiemens Heidenreich 1983 Draine Bertoldi 1996 Lee et al. 1996 van Dishoeck Black 1988). 

A chemically based, mass-independent fractionation process was first observed during ozone formation through the gas-phase recombination reaction (Thiemens Heidenreich 1983) O + O2 + M - O3 + M. The product ozone possesses equally enriched heavy-oxygen isotopes I7 IS0. by approximately lOO /oo with respect to the initial oxygen, with a slope value of unity in a three-isotope oxygen plot. This discovery led to the conclusion that a symmetry-dependent reaction can produce meteoritic isotopic anomalies (Thiemens 1999, 2006). Recently, theoretical calculations of Gao Marcus (2001) established the major role of symmetry in isotopolog-specific stabilization of vibrationally excited ozone molecules that give rise to the mass-independent compositions. 

However, the lack of analytical methods to distinguish clearly diastereoisomers such as 7a and 7b casts some doubt on the above assumptions. Specifically, it cannot be ensured with ultimate certainty that the reactions have really occurred in the desired manner or whether epimerization has taken place to a certain extent. In the case of 7a/Fe+ and 7b/Fe+, for example, the experimental data clearly reveal the occurrence of diastereoselective dehydrogenation, but the intrinsic SE might even be larger if the samples were not the pure diastereoisomers 7a and 7b but only enriched samples. Epimerization at C(3) is in fact facile via keto/enol tautomerism of the carbonyl compound occurring upon work-up. Therefore, the isotopolog 7c was prepared via an independent synthetic route (N2D2 reduction of 2-methyl-but- -2-enoic acid). As the results obtained with... 

Isotopolog/isotopomeric samples allow identification of specific interactions. 

The kinetics of the HHHH-transfer in the cyclic tetramer of 3,5-diphenyl-4-pyr-azole (DPP) has been evaluated recently [27]. The overall kinetic HHHH/DDDD isotope effects were found to be only around 12. This value indicated absence of a single barrier HHHH process where one would expect a larger overall effect. Instead, the Arrhenius pattern depicted in Fig. 6.36 could be explained in terms of a stepwise HH+HH process according to the profile of Fig. 6.17, where two hydrons are transferred in each step, leading to the expected isotope effects depicted in Fig. 6.19(b) and (c). This means that the rate constants of the HHHD and the HDHD reaction are very similar, and also those of the DDHH, DDHD, DDDD reactions. This leads to a very special dependence of the rate constants observed on the deuterium fraction in the mobile proton sites. The mole fractions of all isotopologs according to a statistical distribution are depicted in Fig. 6.37(a), and the sums of mole fractions of the relevant species exhibiting similar rate constants in Fig. 6.37(b). It is clear, that practically only three different species and rate constants are observed in this case. 

Figure 6.37 Statistical mole fractions of isotopologs (a) and ensembles of isotopologs (b) ofthe tetrameric DPP. Adapted from Ref [27],...

The degree and the order of this automorphism group are respecUvdy p = 1 + 2(1+ 3)+1(1 + 3)= 13 andh = l-2(l-6) -l(l 6) = 2- = 432. The automorphism groups of isotopological molecules have equal degrees though their orders may be different. 

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