Atomic data, hydrogen isotopes

A hydrogen isotope with a nucleus consisting of one proton and two neutrons (the nucleus is referred to as the triton). Tritium, a radioisotope symbolized by iH or T, decays by negative beta emission (0.01860 MeV) with a half-life of 12.32 years. The atomic weight of tritium is 3.01605 amu. It is frequently used in metabolic and kinetic experiments. The following decay data indicate the time followed by the fraction of original amount at the specified time 0, 1.000 1 month, 0.995 2, 0.991 3,... 

The most detailed structures have been obtained from neutron diffraction data, when isotopic substitution methods have been possible to use. Neutrons are strongly scattered by hydrogen atoms, making it possible to determine not only bond lengths and coordination number of the metal ion, but also the orientation of the water molecules in its first coordination sphere. A summary of results is given in Table II (19-31). 

The two most important nuclei for NMR in polymer chemistry, viz. the hydrogen isotope 1H (protonium) and the carbon isotope 13C. Proton-NMR gives information on the skin of the molecule (since the skin consists mainly of hydrogen atoms in different combinations) whereas 13C-NMR provides information on the carbon skeleton of the molecule. Other isotopes nowadays frequently used in NMR are 19F, 29Si and 31P. Important data on of several nuclei are given in Table 12.5. We shall restrict ourselves from now on to H and 13C NMR. 

It should be noted that majority of the available data for processes discussed in Sects. 19.2 and 19.3 are pertinent to the light H/H2 isotopes of hydrogen. In a D-T fusion reactor plasma, the processes enumerated in Sects. 19.2 and 19.3 will take place for D,T atoms and D2,DT and T2 molecules and their ions. The vibrational energy levels in the molecular species of heavier hydrogen isotopes are different than those in H2 this introduces differences in the corresponding cross-sections for the same types of processes (even after an appropriate mass scaling of vibrational energies). More importantly, there are certain types of molecular reactions that proceed via formation of an intermediary complex (such as (Hj/), (H, ), Hg, and... 

Isotope exchange is a technique which gives reliable comparative data on the electrophilic and protophilic reactivity of the five-membered heterocycles furan, thiophene, and selenophene, revealing how the activity of their hydrogen atoms varies with the position of the hydrogen in the ring and with the number and position of substituents. The results from kinetic data on isotopic exchange in deuterated selenophenes, thiophenes, and furans, may be summarized as follows. 

Table 4.1 Atomic data of hydrogen isotopes, from (Ref [i])...

With Robert s data one should consider the criticism of his results by Deno According to the latter the equalization of carbon and hydrogen atoms that precedes the reaction of the carbonium ion with the solvent results from intramolecular rearrangements so it should be regarded neither as a proof of the equivalence of carbon atoms in carbonium ion structures nor as a confirmation of resonance structures. Hence, Robert s evidence that on conversion of 2-norbornyl tosylate into norbomyl acetate all the atoms except C become nearly equivalent does not favour the structure of the nonclassical ion 5 because the experimental data on isotopic equalization indicate only relative rates of carbonium ion rearrangements. 

To first order, the 7 moments are equal to the equilibrium moments of inertia 7 . The above procedure is repeated for another parent species. Once a sufficient number of 7 for different parent isotopic species have been determined, the moment-of-inertia equations may be solved to give the structure. The structure for SO2 is given in Table XVIII. This measure of the molecular structure has limited applicability because of the large amount of precise isotopic moment-of-inertia data required and because the first-order approximation 7 = 7 is not sufficient especially for light atoms. Thus, hydrogen bond lengths cannot be determined by this method. 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

These data led to the model already described several times above. The enzyme executes a search for a tunneling sub-state, apparently 13 kcaFmol in energy above the principal state from this state the hydrogen atom tunnels with no further vibrational excitation. Probably motion of the secondary center is coupled into the tunneling coordinate. The result is large, temperature-independent primary and secondary isotope effects in the context of an isotope-independent activation energy. 

Hydrogen is the most abundant chemical element in the universe, and in its various atomic and molecular forms furnishes a sensitive test of all of experimental, theoretical and computational methods. Vibration-rotational spectra of dihydrogen in six isotopic variants constituting all binary combinations of H, D and T have nevertheless been recorded in Raman scattering, in either spontaneous or coherent processes, and spectra of HD have been recorded in absorption. Despite the widely variable precision of these measurements, the quality of some data for small values of vibrational quantum number is still superior to that of data from electronic spectra [106], almost necessarily measured in the ultraviolet region with its concomitant large widths of spectral lines. After collecting 420... 

Hydrogen abstraction from propan-2-ol and propan-2-ol- /7 by hydrogen and deuterium atoms has been studied by pulsed radiolysis FT-ESR. A secondary kinetic isotope effect was observed for H (D ) abstraction from the C—H (C—D) bonds. The results were compared with ab initio data. In similar work, the kinetic isotope effects in H and D abstraction from a variety of other alcohols in aqueous solvents have been measured. It was found that, compared with the gas phase, the reactions exhibit higher activation energies in agreement with the ability of solvation to decrease the dipole moment from the reactant alcohol to the transition state. 

D-xylose was converted into 2-furaldehyde in acidified, tritiated water, no carbon-bound isotope was detected. This suggested that the 1,2-enediol (2) reacted immediately, as otherwise, tritium would have been detected at the aldehydic carbon atom of 2-furaldehyde, as a result of aldose-ketose interconversion.An acidic dehydration performed with d-[2- H]xylose showed that an intramolecular C-2-C-1 hydrogen transfer had actually occurred. Thus, these data indicated that an intramolecular hydride shift is more probable than the previously accepted step involving a 1,2-enediol intermediate. 

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