Para-deuterium

As in the case of hydrogen and tritium, deuterium exhibits nuclear spin isomerism (see Magnetic spin resonance) (14). However, the spin of the deuteron [12597-73-8] is 1 instead of S as in the case of hydrogen and tritium. As a consequence, and in contrast to hydrogen, the ortho form of deuterium is more stable than the para form at low temperatures, and at normal temperatures the ratio of ortho- to para-deuterium is 2 1 in contrast to the 3 1 ratio for hydrogen. 

Similar principles apply to ortho- and para-deuterium except that, as the nuclear spin quantum number of the deuteron is 1 rather than as for the proton, the system is described by Bose-Einstein statistics rather than the more familiar Eermi-Dirac statistics. Eor this reason, the stable low-temperature form is orriio-deuterium and at high temperatures the statistical weights are 6 ortho 3 para leading to an upper equilibrium concentration of 33.3% para-deuterium above about 190K as shown in Eig. 3.1. Tritium (spin 5) resembles H2 rather than D2. 

Notice from Equation 4.113 for I = 1 (deuterium) it is the combination of the symmetric nuclear spin with the symmetric rotational functions which has the higher statistical weight. At high temperature (ortho/para)DEUTERiuM = 2/1, and at low temperature the ortho form predominates. 

Ortho-para deuterium, 27 25, 50 Ortho-para hydrogen conversion, 27 23 Oscillatory catalytic reactions, 37 213-215, 271-272 see also Platinum catalytic CO oxidation on Pt(l 11) and Pt(llO) surfaces COj formation, 37 216-217 kinetic oscillation mechanism, 37 220-228... 

The static reactor method used for absolute rate determinations, and almost always for ortho -para deuterium studies, is generally that based on the micro-Pirani gauge analysis chamber as adapted by Ashmead et al. (3). The time necessary for a single determination of the extrinsic field effect by this method is unfortunately likely to be measured in hours or days rather than in seconds as for the flow reactor. To date the only application of this method to the extrinsic field effect appears to be that of Eley et al. (4). Van Cauwelaert and Hall (5) have described a recirculating adaptation of the static reactor that would seem to be useful for studying the field effect. 

Not shown in Fig. 22 or Table IV are the results of Eley et al. (4) on the ortho -para deuterium conversion over Nd203 pretreated in vacuum for 150 hr at 673 K. The pattern of activity found most closely resembles that of Lu203. 

For a fixed temperature So.Do.p.Hs and the ortho-para separation factors Sp-D2.o Doy S0.H2.p-H2 are given by Equation 29 and are constants for a given surface. The dependence of the isotope separation factor on the gas phase ortho-para concentrations is shown in Figure 5. The maximum isotope separation factor is obtained when para-hydrogen is separated from para-deuterium. The minimum is obtained when ortho-hydrogen is separated from ortho-deuterium. 

Figure 10. Percentage of para-deuterium as a function of fraction of surface stripped for two rates of desorption...

Deuterium also exists in ortho and para forms, but the ortho form is the more stable at low temperatures. The equilibrium mixture at elevated temperatures contains 33.3% of para deuterium. 

Like hydrogen, deuterium yields ortho and para forms. At room temperature the ordinary gas comprises 66-6 per cent of ortho. Low temperature favours the production of para-deuterium, and the transformation of ortho to para is catalysed by charcoal at low temperatures exactly as for ordinary hydrogen. For comparative purposes, some constants may be noted —... 

The infrared absorption intensities of the translational optic modes of cubic ortho-hydrogen and of cubic para-deuterium (below 1.5°K) were measured by Hardy et al. (1968). These authors also calculated the predicted quadrupole-induced intensities from the theory of Schnepp (1967). 

Designating the form of a diatomic molecule in which both nuclei have opposite spin directions e.g. parahydrogen, para-deuterium. 

Properties of Light and Heavy Hydrogen. Vapor pressures from the triple point to the critical point for hydrogen, deuterium, tritium, and the various diatomic combinations are Hsted in Table 1 (15). Data are presented for the equiUbrium and normal states. The equiUbrium state for these substances is the low temperature ortho—para composition existing at 20.39 K, the normal boiling point of normal hydrogen. The normal state is the high (above 200 K) temperature ortho—para composition, which remains essentially constant. 

The experimental and theoretical aspects of the radiation and self-induced conversion kinetics and equiUbria between the ortho and para forms of hydrogen, deuterium, and tritium have been correlated (17). In general, the radiation-induced transitions are faster than the self-induced transitions. 

J. W. Pyper and C. K. Briggs, The Ortho-Para Forms of Hydrogen, Deuterium and Tritium Radiation and Self-induced Conpersion Kinetics and Equilibrium, Eawrence Eipermore Eaboratoy Report UCRE-52278, 1977. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

Charton has recently examined substituent effects in the ortho position in benzene derivatives and in the a-position in pyridines, quinolines, and isoquinolines. He concludes that, in benzene derivatives, the effects in the ortho position are proportional to the effects in the para position op). However, he finds that effects of a-sub-stituents on reactions involving the sp lone pair of the nitrogen atoms in pyridine, quinoline, and isoquinoline are approximately proportional to CT -values, or possibly to inductive effects (Taft s a ). He also notes that the effects of substituents on proton-deuterium exchange in the ortho position of substituted benzenes are comparable to the effects of the same substituents in the a-position of the heterocycles. 

Scholten and Konvalinka (9) in 1966 published the results of their studies on the kinetics and the mechanism of (a) the conversion of para-hydrogen and ortho-deuterium and (b) hydrogen-deuterium equilibration. At first the a-phase of the Pd-H system was used as catalyst, and then the results were compared with those obtained when the palladium had previously been transformed into its /3-hydride phase. 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

The importance of the inductive effect in controlling the reaction rates was further shown by Streitweiser and Humphrey596, who measured the rates of dedeuteration of toluene (a, a-d2), (a, 2,4,6-g 4), and (a, 2,3,4,5,6-g 6) by lithium cyclohexylamide at 50 °C and found the rate to be reduced by 0.4 %, 0.4 %, and 1.8 % for a deuterium atom in the ortho, meta and para positions respectively. The retardation is consistent with the +1 effect of deuterium but the differential positional effect could not be rationalised in simple and general terms. 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

TABLE 1.2 Effects of para Substituents during Deuteriumation of Styrenes... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

The study was subsequently extended to include the radiofluorodemetallation of organogermanium compounds31. Both fluorine-18 and 18F-acetylhypofluorite were used to displace the trimethylgermyl moiety from a series of para-substituted aryltrimethyl-germanes (equation 26). The 18F was produced in a cyclotron by bombarding Ne atoms with deuterium ions (equation 27). 

Three mechanisms have been proposed for this reaction (Scheme 21). The reaction is first order in each of the reactants. In another study, Reutov and coworkers159 found a large primary hydrogen-deuterium kinetic isotope effect of 3.8 for the reaction of tri-(para-methylphenyl)methyl carbocation with tetrabutyltin. This isotope effect clearly demonstrates that the hydride ion is transferred in the slow step of the reaction. This means that the first step must be rate-determining if the reaction proceeds by either of the stepwise mechanisms in Scheme 21. The primary hydrogen-deuterium kinetic isotope effect is, of course, consistent with the concerted mechanism shown in Scheme 21. 

TABLE 10. The primary hydrogen-deuterium kinetic isotope effects for the reactions of a series of para-substituted benzyl radicals with tributyltin hydride3... 

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