Deuterium-bond

A feature, noted SiGa—SiAs, is also observed in Fig. 17a near 393 cm-1. It is ascribed to two Si atoms on adjacent sites SiGa—SiAs (Theis and Spitzer, 1984). The intensity of this LVM also decreases after deuteration (Fig. 17b). While this donor-acceptor pair should be electrically neutral, it seems, however, it can bind a D atom, resulting in the observation of a D-related line at 704 cm-1 (972 cm-1 in the H-treated sample) this line is not related to deuterium bonded to isolated SiAs as it is not observed in every sample. It is tentatively attributed to the wagging motion of an... 

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261... 

Some care must be exercised in setting up the crossover equipment to account for kinetic isotope effects associated with abstraction or insertion into a carbon-hydrogen (deuterium) bond. In general, abstraction is expected to exhibit a larger isotope effect than insertion, and this appears to be the case (see below). To accommodate this, and to increase the sensitivity of the experiment, it is often necessary to employ a smaller amount of the hydrocarbon than of its deuteriated analog. 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

The basic differences between hydrogen and deuterium bonding have been... 

Carbon-deuterium bonds normally are broken more slowly than carbon-hydrogen bonds. This so-called kinetic isotope effect provides a general method for determining whether particular carbon-hydrogen bonds are broken in slow reaction steps. 

D. Carbon hydrogen bond is stronger than the corresponding carbon deuterium bond. 

B is correct. The question stem presents a mechanism for an elimination reaction (the product gains a double bond) that relies on a rapid C—H bond dissociation as the rate-limiting step. When the heavier deuterium (D) is used instead of a pure hydrogen atom, the reaction rate decreases because of a stronger carbon—deuterium bond. 

Because deuterium is heavier than hydrogen, the carbon-deuterium bond has a lower vibrational frequency than the carbon-hydrogen bond. This difference in frequency makes the carbon-deuterium bond slower to react. 

AC — D (carbon-deuterium) bond is electronically much like a C — H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however. 

More recendy, O Konski and Flautt have made quadrupole resonance studies of solid ammonia (1543). They observe a 12.6 percent decrease of the coupling constant upon condensation of NHj gas, and attribute the change to an increase of ionic character upon formation of H bonds. The even larger shift for ND (21 percent) is hence interpreted as evidence that deuterium bonds are stronger than the hydrogen bonds. 

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