Isotopically substituted hydrogen

Predict whether normal or inverse isotope effects will be observed for each reaction below. Explain. Indicate any reactions in which you would expect > 2. The isotopically substituted hydrogens are marked with asterisks. 

Garret, B. C. and Truhlar, D. G. Generalized transition state theory. Quantum effects for collinear reactions of hydrogen molecules and isotopically substituted hydrogen molecules, JPhys.Chem., 83 (1979), 1079-1112... 

Table I. Go and togXe/4 (both in cm. ) for Isotopically Substituted Hydrogen Molecules...

Garrett B. C. and Truhlar D. G. (1979) Generalized Transition State Theoiy. Quantum Effects for Collinear Reactions of Hydrogen Molecules and Isotopically Substituted Hydrogen Molecules, J. P/zys. Chem. 79, 1079-1112. 

Another consequence of Fig. 3 (and of the harmonic oscillator approximation as well) is that the H—X vibration will have a slightly greater maximum amplitude than the D—X vibration. The space required by the hydrogen atom will thus be somewhat greater than that occupied by the deuterium atom. One could then imagine a steric secondary isotope effect in reactions which either increased or decreased crowding of isotopically substituted hydrogens. 

Generalized Transition State Theory. Quantum Effects for Collinear Reactions of Hydrogen Molecules and Isotopically Substituted Hydrogen Molecules. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

Studies of the kinetic effects of isotopic substitutions can provide support for a certain type of mechanism. The kie can be most helpful to settle whether a particular bond to hydrogen or another light element is broken in the activation process. 

Vibrational spectra are often so complicated that assignment of a particular absorption to a given bond is difficult. One way to confirm that an assignment is correct is to carry out selective isotopic substitution. For example, we can replace a hydrogen atom with a deuterium atom. If an iron-hydride (Fe—H) stretch occurs at 1950 cm-1, at what energy will this stretch occur, approximately, for a compound that has deuterium in place of the hydrogen Refer to Major Technique 1, which follows these exercises. 

In terms of its steric impact, fluorine is the smallest substituent that can replace a hydrogen in a molecule, other than an isotope of hydrogen. Table 1.1 provides insight as to the comparative steric impact of various fluorinated substituents on the equilibrium between axial and equatorial substitution in cyclohexane.14... 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Part I. The experimental basis, 39, 27 Isotope effects, hydrogen, in aromatic substitution reactions, 2, 163... 

Microwave spectroscopy is probably the ultimate tool to study small alcohol clusters in vacuum isolation. With the help of isotope substitution and auxiliary quantum chemical calculations, it provides structural insights and quantitative bond parameters for alcohol clusters [117, 143], The methyl rotors that are omnipresent in organic alcohols complicate the analysis, so that not many alcohol clusters have been studied with this technique and its higher-frequency variants. The studied systems include methanol dimer [143], ethanol dimer [91], butan-2-ol dimer [117], and mixed dimers such as propylene oxide with ethanol [144]. The study of alcohol monomers with intramolecular hydrogen-bond-like interactions [102, 110, 129, 145 147] must be mentioned in this context. In a broader sense, this also applies to isolated ra-alkanols, where a weak Cy H O hydrogen bond stabilizes certain conformations [69,102]. Microwave techniques can also be used to unravel the information contained in the IR spectrum of clusters with high sensitivity [148], Furthermore, high-resolution UV spectroscopy can provide accurate structural information in suitable systems [149, 150] and thus complement microwave spectroscopy. 

Since the quark is a fermion with an atomic weight somewhere between 0.5 and 10, its adduct with an electron, the simplest quarkonium species, is essentially an isotopic substitution of a hydrogen atom having the solutions of the Schrodinger equation as wave-functions. 

The BO approximation, which assumes the potential surface on which molecular systems rotate and vibrate is independent of isotopic substitution, was discussed in Chapter 2. In the adiabatic regime, this approximation is the cornerstone of most of isotope chemistry. While there are BO corrections to the values of isotopic exchange equilibria to be expected from the adiabatic correction (Section 2.4), these effects are expected to be quite small except for hydrogen isotope effects. 

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