Isotopes atomic

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

Isotope Atomic mass Half-hfe, Decay mode... 

Properties. Strontium is a hard white metal having physical properties shown in Table 1. It has four stable isotopes, atomic weights 84, 86, 87, and 88 and one radioactive isotope, strontium-90 [10098-97-2] which is a product of nuclear fission. The most abundant isotope is strontium-88. 

Isotopes Atoms of the same element (having the same atomic number) that differ in mass number. 

A kinetic isotope effect that is a result of the breaking of the bond to the isotopic atom is called a primary kinetic isotope effect. Equation (6-88) is, therefore, a very simple and approximate relationship for the maximum primary kinetic isotope effect in a reaction in which only bond cleavage occurs. Table 6-5 shows the results obtained when typical vibrational frequencies are used in Eq. (6-88). Evidently the maximum isotope effect is predicted to be very substantial. 

A kinetic isotope effect that results when the bond to the isotopic atom is not broken is called a secondary isotope effect. Here are two examples ... 

In these examples B is a base. The first example is called a secondary isotope effect of the first kind, the next one is a secondary isotope effect of the second kind. The distinction between these is that in the first kind bonds to the isotopic atom have undergone spatial (i.e., structural) change. Halevi has reviewed secondary isotope effects on equilibria and rates. 

Isotope Atomic weight Isotope Atomic weight... 

Steric isotope effects may be ascribed to differences in effective size of isotopic atoms. The early part of our discussion will be concerned with the problem of the meaning of this concept. The experimental results which are to be explained are differences in the positions of chemical equilibria and in reaction velocities arising from this difference in size . 

Isotopes Atoms with the same number of protons and electrons but with a different number of neutrons in the nucleus. Isotopes of an element act the same chemically but differ in mass. 

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

Isotope Atomic number Mass number Isotopic mass (amu) Relative abundance (%)... 

Relative isotopic, atomic and molecular masses are measured on a scale in which the mass of an atom of carbon-12 is exactly 12 atomic mass units (a.m.u.). 

Nuclei of natural isotopes (atoms of chemical elements differing in the number of neutrons in their nuclei) may possess angular momentum or spin and therefore magnetic moments. One defines spin by the following equation ... 

Table 1. Natural abundance of stable isotopes (atom %) (Oebelmann et al., 2000)...

Since the bond to the isotopic atom is not formed or broken in the transition state of the rate-determining step of the reaction, the difference between the rate constant for the reaction of the undeuterated and deuterated substrates is usually small. As a result, secondary deuterium KIEs are usually close to unity, i.e. the maximum secondary deuterium KIE is 1.25 per deuterium (Shiner, 1970a) and most of these KIEs are less than 1.10 (Westaway, 1987a). Therefore, careful kinetic measurements with an error of approximately 1 % in each rate constant or specially designed competitive methods are required to determine them with an acceptable degree of accuracy. 

In the preceding sections, the bond to the isotopic atom is broken or formed in the rate-determining step of the reaction. In these cases, the change in rate is referred to as a primary kinetic isotope effect. Isotopic substitution at other sites in the molecule has much smaller effects on the rate. These small isotope effects are collectively referred to as secondary kinetic isotope effects. 

We have seen that Equation 4.95 for (s2/si)f involves the difference between the sums of the squares of the frequencies for two isotopomers. Consider now two isotopic atoms X and with masses ma and mp and two isotopomers AX and AXP with mp > ma. Then, from Equations 4.95 and 4.99, according to the first quantum correction... 

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). 

The title indicates the scope of the text. The term isotope effects is used rather than applications of isotopes to indicate clearly that it deals with differences in the properties of isotopically substituted molecules, for example differences in the chemical and physical properties of water and the heavy waters (H2O, HDO, D2O, HTO, etc.). Thus H20, HDO and D2O have different thermodynamic properties. Also reactions in solvent mixtures of light and heavy water proceed at different rates than they do in pure H2O. On the other hand, the differences are not large and consequently, to the extent the difference in properties can be ignored, HDO or HTO can be used as tracers for H2O. An important point, however, is that this book does not deal with isotopes as tracers in spite of the widespread importance of tracer studies, particularly in the bio and medical sciences. Also the title specifically does not mention physics which would necessarily have been included if the term Physical Sciences had been used. Thus the text does not deal with differences in the nuclear properties of isotopic atoms. Such differences are in the realm of nuclear physics and will not be discussed. 

If secondary isotope effects arise strictly from changes in force constants at the position of substitution, with none of the vibrations of the isotopic atom being coupled into the reaction coordinate, then a secondary isotope effect will vary from 1.00 when the transition state exactly resembles the reactant state (thus no change in force constants when reactant state is converted to transition state) to the value of the equilibrium isotope effect when the transition state exactly resembles the product state (so that conversion of reactant state to transition state produces the same change in force constants as conversion of reactant state to product state). For example in the hydride-transfer reaction shown under point 1 above, the equilibrium secondary isotope effect on conversion of NADH to NAD is 1.13. The kinetic secondary isotope effect is expected to vary from 1.00 (reactant-like transition state), through (1.13)° when the stmctural changes from reactant state to transition state are 50% advanced toward the product state, to 1.13 (product-like transition state). That this naive expectation... 

Isotope atoms of the same element containing different numbers of neutrons. 

At one time, the hydrogen atom with one proton and no neutron was used as the standard to define 1 atomic mass unit (1 amu). Today, chemists use carbon-12, the most abundant isotope of carbon for the standard amu, which is defined as 1/12 of the C-12 atom. Therefore, the actual atomic weight for an element is in average mass units (numbers), taking into account all the isotopes (atoms) of that element. 

A kinetic isotope effect observed by a single reactant, having isotopic atoms at equivalent reactive positions, which reacts to produce isotopomeric products with a nonstatistical distribution. The pathway favored will be the one having lower force constants for the displacement of the isotopic nuclei in the transition state. 

Depending on the position of the isotopic atom(s) in the labeled substrate or product, various exchanges may be examined. The types of exchanges subject to measurement can be illustrated by considering a bisubstrate reaction ... 

Molecules having the same type and number of isotopic atoms, albeit in different positions. CH2DCH (NH3+)C00- and CH3CD(NH3+)COQ- are examples of isotopomers. Certain stereoisomers and cis-trans isomers can also be regarded as isotopomers for example, (R)- and (5)-l-deuteroethanol, CH3CHDOH. 

A parameter (often symbolized as R) used in the analysis of kinetic isotope effects as a measure of the rate of a particular step relative to the rate of all preceding steps in the reaction sequence. This factor indicates the fate of the intermediate that immediately precedes the step in which the change in bonding to the isotopic atom occurs. If the key step is described by the constant ki (and its isotopically altered value ki ), then... 

An isotope of hydrogen a stable, non-radioactive isotope atomic number 1 atomic mass 2.014 molecular weight (for the diatomic heavy hydrogen molecule) 4.028. 

Isotope Atomic bomb tests Chernobyl accident Chernobyl 30 km zone3-5 Western Europe ... 

Isotopes Atoms of an element that differ in the number of neutrons in the nucleus. 

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