Isotope effect secondary thermodynamic

Tetrahedral intermediates, derived from carboxylic acids, spectroscopic detection and the investigation of their properties, 21, 37 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-state chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition state structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1... 

A nonunity ratio (sometimes called a thermodynamic isotope effect) of the equilibrium constants ( ught/ heavy) for two reactions differing only in the isotopic composition at one or more positions of their otherwise chemically identical substances . If the equilibrium isotope effect is attributable to a covalent bond making/breaking, then the effect is often referred to as a primary equilibrium isotope effect. If isotopic substitution at a position other than the scissile bond results in an equilibrium isotope effect, the term secondary equilibrium istope effect is used. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

A physical implication of this assumption is that the occurrence of isotopic substitution in one of the positions of XLm has no effect on the exchange equilibrium of any of the other positions, i.e. the exchange behaviour of the molecule XLm is equivalent to m molecules of a hypothetical solute X L containing one hydrogen nucleus per molecule (Block and Gold, 1959). In other words, thermodynamic secondary hydrogen isotope effects are assumed to be absent. 

The numerical factors in these equations are statistical they correspond to differences in the number of the relevant hydrogen nuclei. The factor consisting of a power of l represents a thermodynamic secondary isotope effect. 

To supplement the data on prolyl isomerization, I will draw on the literature describing rotation about the C-N bond in secondary amides. Early studies in this field were described by Stewart and Siddall in an excellent 1970 review. As we will see, these reactions are related to prolyl isomerization and support the mechanism to be proposed for prolyl isomerization. The mechanism is based on results from a variety of experimental approaches. In all cases, experiments employing kinetic-based probes will be used to obtain an accurate picture of the activated complex in the rate-limiting transition state. The experiments that will be described include thermodynamics, in which activation parameters (i.e., AG, AHt, and ASt) will be described solvent effects, in which the influence of organic solvents and deuterium oxide will be reviewed acid-base catalysis substituent effects and secondary deuterium isotope effects. 

Thus, A"7in may be considered as the average of secondary isotope effects or the averaged value for two equally populated states with equatorial and axial C-D couplings. Therefore, if there is no substantial secondary effect on a coupling, A"7 A "7th holds, and it is possible to determine the isotope effect on conformational equilibrium in I. It was observed that only for the vicinal 7( C,D) coupling does the thermodynamic part predominates (A 7th =-0.0032 Hz, estimated with AT = 1.014, from data of ref. 710 A 7in = -0.0011 Hz), and therefore this coupling can be used to estimate the conformational equilibrium effect. 

Secondary deuterium isotope effects for Cope rearrangements of CH ,CH= C(C2H5)C(CN),CH2CH=CD2 (VA d = 0.94 at 85°C) and CH3CH=C C.,H,y C(CN)2CD2CH=CH.2 (/ h/ d= 1 19 at 85°C) suggest that the transition state more closely resembles reactants than products, as expected from their relative thermodynamic stabilities . ... 

This was substantiated by later work on secondary deuterium isotope effects in deoxymercuration. Coplanarity of the two carbons, and the RO and HgX functions is implied. However, thermodynamic parameters for... 

Let us consider a thermodynamic secondary isotope effect like that on the ionization constant of phenylacetic acid (I-l, 1-3). A general formulation of such an effect would be ... 

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