Primary isotope effects defined

Primary isotope effects are defined as concerning reactions in which, at some stage -normally the rate determining step - a bond to the isotopically labelled compound... 

One method has been proposed for testing the n = 0 model for monobasic acids (Kreevoy, 1965b). That involves the relation between kkaIkda (defined in Section IIA3) and ha/ da- If the n = 0 mechanism is correct, and if there are no isotope effects other than the primary isotope effect, kha/ da is given by equation (54). To test this equation... 

Primary isotope effects are defined as the difference in magnetic shielding of the isotopes of nuclei of the same element. Secondary isotope effect on nuclear shielding are considered as intrinsic isotope effects and equilibrium isotope effects. The former effects are caused by isotopic substitution, the later by conformational changes or shifts in equilibria as a consequence of isotopic substitution. 

Isotope effects on coupling constants are more difficult to recognize than isotope effects on chemical shielding. The difference of chemical shifts between isotopomers is easier to determine than slightly differences in signals splitting. The primary isotope effect on coupling constant defined as ... 

In this review, primary isotope effects are defined as concerning reactions in which, at some stage, a bond to the isotopically substituted atom is either broken or formed. A secondary isotope effect will refer to reactions in which no bond to the isotopically substituted atom is broken and none is formed. The strengths of the bonds to the substituted atoms may, however, be altered and, in general, will be to some degree. 

In cases where one site is thought to give rise to a primary isotope effect - one in which bonds are being made or broken at the isotopic element - the subscripts will be used. In cases where secondary isotope effects (by definition, those that are not primary) need to be distinguished, the superscripts will be used. The middle part of Eq. (11.1) illustrates one use of the notation, as a primary H/T isotope effect measured when D occupies a secondary site. As a final example, the right-hand part of Eq. (11.1) shows a Swain-Schaad exponent r defined for a single site of isotopic substitution. The HD subscript serves to connect the exponent to the H/D isotope effect it acts on to produce the H/T isotope effect. In this example, the superscripts were omitted because there is either no second site of isotopic substitution or, in all cases, the isotope is protium. 

Vibrational i. relevant to kinetic primary isotope effects.. ibration that defines the reaction coordinate. B. In-plane and out-of-plane bends at the transition state. C. Symmetric stretch that forms at the transition state. 

The bond between a-carbon and deuterium is not broken in the solvolysis transition state, but the presence of deuterium, bonded to the reaction center, causes the reaction rate decrease. Isotope effects such as in (1.14.20) and (1.14.21) are called secondary kinetic isotope effects, to be clearly distinguished from primary isotope effects. Secondary isotope effects were defined as rate effects caused by isotope substitution on the bond not broken in the rate-determining step. 

These reactions proceed through symmetrical transition states [H H H] and with rate constants kn,HH and kH,DH, respectively. The ratio of rate constants, kH,HH/kH,DH> defines a primary hydrogen kinetic isotope effect. More precisely it should be regarded as a primary deuterium kinetic isotope effect because for hydrogen there is also the possibility of a tritium isotope effect. The term primary indicates that bonds at the site of isotopic substitution the isotopic atom are being made or broken in the course of reaction. Within the limits of TST such isotope effects are typically in the range of 4 to 8 (i.e. 4 < kH,HH/kH,DH < 8). 

In 1983, Huskey and Schowen tested the coupled-motion hypothesis and showed it to be inadequate in its purest form to account for the results. If, however, tunneling along the reaction coordinate were included along with coupled motion, then not only was the exaltation of the secondary isotope effects explained but also several other unusual feamres of the data as well. Fig. 4 shows the model used and the results. The calculated equilibrium isotope effect for the NCMH model (the models employed are defined in Fig. 4) was 1.069 (this value fails to agree with the measured value of 1.13 because of the general simplicity of the model and particularly defects in the force field). If the coupled-motion hypothesis were correct, then sufficient coupling, as measured by the secondary/primary reaction-coordinate amplimde ratio should generate secondary isotope effects that... 

Solvent isotope effects. Much mechanistic information can be obtained about reactions involving proton transfer from solvent kinetic isotope effects, particularly in solvents of mixed isotopic composition. For practical reasons work is essentially confined to H/D effects, especially those in water. Unlike ordinary primary hydrogen isotope effects, solvent isotope effects have to take into account a host of exchangeable sites, subject to equilibrium as well as kinetic isotope effects. A key concept is that of the fractionation factor, (p, which is the deuterium occupancy of a site in a 1 1 H2O/D2O mixture more formally it is defined by equation l.l ... 

C,H) and V(H,H) in CH4 and its isotopomers, /(C,H) was found to increase by 0.088 Hz on raising the temperature from 180 K to 380 K, which agrees with the observed increase of 0.083 Hz on raising the temperature from 200 K to 370 The D primary and secondary isotope effects on /(C,H) were predicted to decrease with increasing temperature. Table 18 shows the total values, separated into stretching, bending and the second-order stretch-bend contributions to /(C,H) and 7 (C,H) in each of the five isotopomers at 300 K, as well as the zero-point corrections. 7 (X,H) is defined as in Eq. (85). 

In general, isotope effects on coupling constants are very small and they are seldom reported. Their observation requires that measurements be carried out with a very good accuracy. In what follows, primary, Ap/, and secondary, AsT, isotope effects are defined as in Eqs (86a) and (86b), respectively. 

Isotope effect on chemical shifts can be of two kinds primary or secondary. The primary ones are defined as... 

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