Isotope effects on ion abundances

The publications covered here are those whose concern is with isotope effects on the rates of ion decompositions or rearrangements. These are a small proportion of the total number of papers reporting mass spectra of isotopically labelled compounds, the majority of which are concerned with identifying the atoms involved in molecular rearrangements. 

The possibility of partial or even complete randomisation of isotopic labels always exists with gaseous ions and its occurrence makes the 

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With thermal systems either in the gas phase or in solution, it is in ter -molecular isotope effects which are more commonly studied. Intramolecular isotope effects involve distinguishing and measuring two, or more, chemically identical but isotopically different products produced in the same reaction vessel from the same reactant. The situation is different in mass spectrometry. Intramolecular isotope effects are conveniently studied, because the chemically identical products are naturally separated according to their masses. Intermolecular isotope effects on ion abundances are also easily measured, but, as regards kinetics and mechanism of reaction, their value is limited. Whereas an intramolecular isotope effect (on ion abundances) reflects kinetic isotope effects, an intermolecular isotope effect (on ion abundances) reflects kinetic isotope effects, isotope effects on the internal energy distribution, P(E), and other factors as well and the effects cannot be easily separated (vide infra). 

In this review, isotope effects on ion abundances will be denoted /h//d, IchJIcd3, IhJId2 and so on, where appropriate corrections will already have been made for numbers of equivalent pathways. The subscripts indicate the neutral species formed in the decomposition. 

RELATIONSHIP BETWEEN INTRAMOLECULAR ISOTOPE EFFECTS ON ION ABUNDANCES AND INTRAMOLECULAR KINETIC ISOTOPE EFFECTS... 

Intermolecular isotope effects observed [534] in metastable ion decompositions of (C3H8)t and (C3D8)t again evidence the fact that intermolecular isotope effects on ion abundances are not a reliable guide to kinetic isotope effects. The isotope effect on methane loss (lcH4/fcD ) is 7 cf. 87 for (ICH4 //ch,d) with CH3CD2CHD2 (above). 

To digress, the method of internal reference is a device for making interpretation of intermolecular isotope effects on ion abundances more sound, even if not watertight. What is done is to use as an internal reference a reaction whose rate coefficient, kREF(E), is not expected to be affected by the isotope substitution. The product abundance for the isotopic reaction of interest in each molecule is divided by the product abundance for the reference reaction in that molecule (intramolecular comparison). These normalised abundances for the different molecules are then compared to give the isotope effects (intermolecular comparison). 

Taken as a whole, the literature on isotope effects in mass spectrometry exhibits two salient features. The values of isotope effects for different molecules and different experimental conditions vary greatly and the isotope effects for some decompositions are very large (>100). These isotope effects are based on ion abundances, as has already been emphasised, but the kinetic isotope effects if measured would show not dissimilar variety and magnitudes. Both features arise because the range of internal energies encountered in reactive ions is very wide and the isotope effects are dependent upon internal energy, usually increasing... 

Equation (27) indicates how the intermolecular isotope effect, /j//n, on ion abundances depends on a number of factors in addition to the intermolecular kinetic isotope effect ki(E)/kn(E). The two most important of these additional factors are the internal energy distributions and the competing reactions. 

Isotopic substitution affects the ionization process and thereby the internal energy distribution, P(E), of a molecular ion [471, 681]. In general, Pi(E) i=Pn(E). One way of expressing the influence of the differing internal energy distributions on the isotope effect Ij/In on ion abundances (or rates) is to say that isotopic substitution alters the number of ions initially formed with energies appropriate for decomposition... 

The observation window, At, is altered by isotopic substitution. The observation windows are determined by times of flight, which depend upon the masses of ions, and masses of the reactant ions M and Mjj differ. The shift in the observation window will be small, but could be significant in studies of isotope effects on metastable ion abundances. Shifts in observation window are not a problem in FIK. 

Because of these various factors, determination of intermolecular kinetic isotope effects ftj(E)/ku(E) from intermolecular isotope effects Ij/In on ion abundances (or rates) is, in general, not possible. 

A metastable peak for the loss of H from [(CH3)3CH]f has been reported, whereas no loss of D (or H ) could be seen with [(CH3)3CD]t [59]. By taking the instrument s detection limit as an upper level for the missing metastable peak s abundance, the intermolecular isotope effect on the ion abundances has been given as > 1000. Again, the figure provides no reliable information about the kinetic isotope effect. A metastable peak could not be observed for loss of D" from (CD3CD2CD2CH3)t and the intramolecular isotope effect, 7H //D, has been given as at least 240 [778]. Disappearance of metastable peaks on deuteration has also been reported with cyclopentane [639] and cyclopentanone [638]. 

From a study of metastable ion decompositions of [C2(H, D)sS] + ions, the average isotope effect, i, for acetylene loss was reported as 1.6 [137]. Isotope effects on metastable ion decompositions of (C3H7S)+ ions have proved difficult to study, because of hydrogen randomisation and facile isomerisation of ion structures. Nevertheless, the metastable ion abundances for H2S and HDS loss from [CH3(CD3)C = SH]+ have been shown to be in the ratio 2.2 1 [136]. 

The natural abundance of 13C has been exploited in an investigation of carbon isotope effects on loss of methane from metastable butane ions [64]. An inverse isotope effect /12cH //j3ch of about 0.9 was reported. The significance of this result is unclear as there appear to be several reaction pathways contributing to the loss of methane [928]. 

More recently Buckler et al. (210) and Lutz et al. (211) have studied the isotope effect on Mo shielding in molybdate ( 0/ 0) and tetrathiomolybdate ( " S/ S). In the latter case the isotope peak can be observed at the natural abundance of (4-2%) on the species (14%). An isotope effect of 0-25 ppm towards lower frequencies is observed upon exchange of one by The isotope shift induced by (relative to S) in M0S4" was found to be 0 09 ppm to low frequency. This represents a rare case where the shielding isotope effects are known for two isotope pairs and identical molecular geometries since both ions are tetrahedrally coordinated. As a first approximation the isotope effect on the shielding should be proportional to (mi" — ) with mi and m2 representing the mass... 

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