Atoms isotopic

Element Atomic Isotope Atomic mass of isotope % abundance... 

Atom Isotope Natural abundance Monoisotopic Average mass mass ... 

SIMS) to map specific compounds in cross-sections, and atomic isotopic analysis for dating using accelerator mass spectrometry (AMS). 

Atomic isotopes help detection so that the shift for 13 CO is perfectly predictable and isotope ratios are known from various sources, as we shall see later on. The change in the rotational constant with reduced mass is easily calculated and hence the change in the frequency of the J = J = 0 can be calculated, as we saw in Section 3.3. 

ATOMS, ISOTOPES, ELECTRON ORBITALS, AND THE PERIODIC TABLE... 

Atoms, isotopes, electron orbitals, and the periodic table 219... 

The observation of a heavy-atom isotope effect, therefore, allows one to determine whether C—X bond weakening (a decrease in force constant) has occurred when the reactant is converted into the transition state of the rate-determining step. Calculations by... 

Saunders10 and by Sims and coworkers11 have shown that the magnitude of the leaving-group heavy-atom isotope effect varies linearly with the extent of C—X bond rupture in the transition state for concerted elimination reactions and for nucleophilic substitution reactions, respectively. Since the magnitude of the isotope effect is directly related to the amount of C—X bond rupture in the transition state, these isotope effects provide detailed information about the structure of the transition state. 

Carbon atom Isotope fractionation k(12C)/k(13C) from Equation 7.23... 

Further Discussion, Heavy Atom Isotope Effects, Secondary Isotope Effects 319... 

For historic and practical reasons hydrogen isotope effects are usually considered separately from heavy-atom isotope effects (i.e. 160/180, 160/170, etc.). The historic reason stems from the fact that prior to the mid-sixties analysis using the complete equation to describe isotope effects via computer calculations was impossible in most laboratories and it was necessary to employ various approximations. For H/D isotope effects the basic equation KIE = MMI x EXC x ZPE (see Equations 4.146 and 4.147) was often drastically simplified (with varying success) to KIE ZPE because of the dominant role of the zero point energy term. However that simplification is not possible when the relative contributions from MMI (mass moment of inertia) and EXC (excitation) become important, as they are for heavy atom isotope effects. This is because the isotope sensitive vibrational frequency differences are smaller for heavy atom than for H/D substitution. Presently... 

For heavy atom isotope effects tunneling is relatively unimportant and the TST model suffices. As an example the dehalogenation of 1,2-dichloroethane (DCE) to 2-chloroethanol catalyzed by haloalkane dehalogenase DhlA is discussed below. This example has been chosen because the chlorine kinetic isotope effect for this reaction has been computed using three different schemes, and this system is among the most thoroughly studied examples of heavy atom isotope effects in enzymatic reactions. 

HEAVY ATOM ISOTOPE EFFECT KINETIC ISOTOPE EFFECTS... 

---