Isotopologue

V. B. Polyakov, V. B., Horita, J., Cole, D. R. and Chialvo, A. A. Novel corresponding-states principle approach for the equation of state of isotopologues H2180 as an example. J. Phys. Chem. B-lll, 393 (2007). 

Eiler JM (2007) The study of naturaUy-occuring multiply-substituted isotopologues. Earth Planet Sci Lett 262 309-327... 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

In addition to the VMP of water isotopologues described above, VMP of many other triatomic, as well as tetratomic molecules, has been extensively studied and reviewed in detail in Refs. [30, 31] and, on ethyne isotopologues, in Ref. [32]. As explained in these papers, VMP theories and experiments that deal with tetratomic... 

Similarly, Lippmaa and coworkers evaluated the relative acidities of linear and branched carboxylic acids from the variation with degree of protonation of the measured 13C NMR shifts.23 The method was then extended to secondary deuterium IEs, evaluated from the variation with degree of protonation of the measured 13C NMR shifts of a mixture of isotopologues.24 The data were fit by nonlinear least squares to Equation (17), where 6H and 6D are the observed chemical shifts of undeuterated and deuterated isotopologues, 6H and <5d are those chemical shifts in the deprotonated form, < >]) and are those chemical shifts in the protonated form, R = K /K, and n is the degree of protonation of the undeuterated material. This is the same equation as Equation (15), but adapted to deuteration, and again n is evaluated from chemical shifts as (hH — <5h)/(<5h - h)-... 

A variant that combines this NMR titration method with NMR pH titration was developed much earlier by Forsyth and Yang.34 The 19F isotope shift between A-methyl or A-, A -dinictliyl-4-fluoroaniline and its CD3 or (CD3)2 isotopologue shows a maximum at a pH near the pAa of the anilinium ion. Rearranging Equation (19) and ignoring intrinsic isotope shifts leads to Equation (20), where SA and <5ah+ are chemical shifts of the aniline and the anilinium ion, respectively, assumed the same for both isotopologues. From this equation the IE on acidity could be evaluated at any pH, or preferably by averaging over all the pH values near the pKa. 

In early studies of secondary deuterium IEs on acidity, benzoic- acid and phenol-fi 5 were found to be 2.4 0.6 and 12 2% weaker acids, respectively, than their hs isotopologues.37 Phenol shows the larger IE because its oxyanion is better conjugated with the ring. The IE was attributed to the electron-donating ability of the deuterium, arising through anharmonicity. 

Many chromatographic separations of isotopologues of various amines that are biologically active or have drug properties have been achieved. Among these are perdeuteronucleosides 137 a tetracyclic 3,3,4,4-c/4-piperazine with antimigraine activity 138 the tetracyclic antidepressant mianserin with 2H or 3H in the A-methyl 139 A-CH3-tritiated chlorpromazine... 

A key feature of the competitive isotope fractionation measurements is the use of natural abundance O2. Isotope effects are, therefore, determined for the reactions of the most abundant isotopologues 160-160 and 180-160. It is the intermolecular competition of these species that is reflected in the isotope effect. Aside from the obvious advantage of not requiring costly enriched materials, the competitive measurements also avoid the error that could arise from small leaks in the vacuum manifold and dilution due to ambient air. 

The reduced isotopic partition functions are computed from the vibrational frequencies of the heavy and light isotopologues within the reactant and product states. The ZPE and EXC terms describe the isotope effects emanating from the quantized vibrational energy levels of these states, whereas the MMI represents the isotope effect that derives from translational and rotational modes.27 The formulas for... 

Calculations of 180 EIEs upon reactions of natural abundance O2 require the normal mode stretching frequencies for the 160—160 and 180—160 isotopologues (16 16j/ and 18 16, ). These values can often be obtained directly from the literature or estimated from known force constants. DFT calculations can be used to obtain full sets of vibrational frequencies for complex molecules. Such calculations are actually needed to satisfy the requirements of the Redlich-Teller product rule. In the event that the full set of frequencies is not employed, the oxygen isotope effects upon the partition functions change and are redistributed in a manner that does not produce a physically reasonable result. 

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