Protium compounds

After he had succeeded In enriching deuterium by the Raleigh distillation of liquid hydrogen, Urey undertook both theoretical and experimental investigations of the differences in the chemistry of protium and deuterium compounds. On the theoretical side Urey and Rittenberg (17) utilized the methods developed for the calculation of the partition function and the free energy of a diatomic molecule from spectroscopic data. For an ideal gas 

The internal partition function is a sum of Boltzmann factors, multiplied by their degeneracies, Pj, over the vibrational and rotational states of the molecule. In the summation over the rotational states it is necessary to consider symmetry selection rules. This leads to a factor S in and + RlnS 

AE electronic is identically zero under the Born-Oppenheimer approximation. The only contribution to AEj comes from the molecular vibrations. 

Thus once the spectroscopic rotational and vibrational constants are known for a pair of isotopic molecules it is possible to calculate the partition function ratio of a pair of isotopic molecules 

ACS Symposium Series American Chemical Society Washington, DC, 1975. 

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In hydrogen sulfide, phosphine, and the hydrides of their heavier congeners, in which the bonds are nearly normal covalent bonds, the bond angles are observed to be close to 90° (Table 4-1). The values given in the table apply to the deuterium compounds as well as to the protium compounds. 

B. Location of Kinetically Significant Hydrogen Westheimer and Nicolaides62 have utilized the deuterium isotope effect to determine whether the C—H or O—H bond rupture is rate controlling in the oxidation of isopropanol by chromic acid. Isopropanol-2-di was synthesized and found to react at approximately one-sixth the rate of the all protium compound. This is conclusive evidence that C—H rather than O—H bond rupture is rate determining. 

A. Differentiation between S 2- and S -Mechanisms Kinetic isotope effects had a fundamental importance, however, for the elucidation of the substitution proper. Melander (1950) showed that a series of benzene derivatives containing tritium were nitrated and brominated at the same rate as the corresponding ordinary protium compound. This is the most important and elegant experimental support for the Sjj2- or two-stage mechanism (reactions 1-2, the... 

Larger deviations from semi-classical behaviour have been observed in the reaction of o-methylacetophenone and p-methoxyacetophenone with hydroxide ions, in which the protium compound was studied by bromination and the tritium compound by detritiation. For these two reactions E -E had values of 3.6 0.4 and 3.3 0.9 kcal mol" respectively, compared with the expected value of 1.7 kcal mol" while the values of lg(/4XM ) were 1.5 0.3 and 1.1 0.6. Although the precision of these last values is not high, they certainly lie outside the semi-classical limits. ... 

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... 

Isotopically Labeled Compounds. The hydrogen isotopes are given special names H (protium), H or D (deuterium), and H or T (tritium). The superscript designation is preferred because D and T disturb the alphabetical ordering in formulas. 

Labeled compounds should prove valuable in studying liquid-phase oxidations, as they have in vapor-phase research, especially at low conversions where scrambling of protium and deuterium is avoided or held to a minimum. 

This requirement could best be satisfied by formation of a complex— perhaps of the donor-acceptor type—or an actual compound, a phenyl-cyclohexadiene (13), between two molecules of benzene or other aromatic compound, in which protium and deuterium atoms readily move and exchange. 

Isotope effects in the range 0-97-T26 have been observed (Table 3) but according to Shiner (1970) an a-deuterium isotope effect close to unity (range 0-97-1-06) indicates a classical SN 2 reaction. The isotope effect is theoretically related to the molecular vibrational frequencies of the initial and transition states for the protium and deuterium compounds. Thus, nucleophilic attack must reduce the change in vibrational frequencies. 

Of all the elements, isotope effects see Isotope Effect) are greatest for hydrogen, justifying the use of distinctive names for the hydrogen isotopes protium, deuterium, and tritium. However, the chemical properties of H, D, and T are essentially identical, except in matters such as equilibrium constants. The isotopes of hydrogen are extensively used as tracers in deuterium- or tritium-labeled compounds. 

Hydrogen is by far the most abundant element in the universe (and on the sun) and, primarily in its compounds, is the third most abundant element in the Earth s crust. The element occurs as three isotopes ordinary hydrogen or protium, H deu-... 

Tritium is popular as a tracer because hydrogen occurs in so many different compounds. For example, suppose a scientist wants to trace the movement of water through soil. The scientist can make up a sample of water made with tritium instead of protium. As that water moves through the soil, its path can be followed by means of the radioactivity the tritium gives off. 

The answer is found in a series of studies begun by Lars Melander (of the Nobel Institute of Chemistry, Stockholm) and extended by many other workers. A variety of aromatic compounds labeled w ith deuterium or tritium were subjected to nitration, bromination, and Friedel-Crafts alkylation. It was found that in these reactions deuterium or tritium is replaced at the same rate as protium there is no significant isotope effect. 

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