Ortho deuterium

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. 

Now consider D2. The nuclear spin of a deuteron is 1, and we can have 7=0, 1, or 2. The T— 2 and r=0 nuclear spin functions are symmetric and the T— 1 functions are antisymmetric (Problem 4.24). The ground electronic state must be 2+ (as in H2), and the nuclei are bosons. Hence the 7=0 and T=2 spin functions go with the 7=0,2,4,... rotational levels. Cooling D2 to low temperature in the presence of a paramagnetic catalyst gives molecules with 7=0 and T=0 and 2. Warming this gas in the absence of catalyst gives 7 = 0,2,4,... molecules this modification of D2 is called ortho deuterium. (The convention is to use ortho for the modification with symmetric nuclear spin functions.)... 

The reactivity varies within very wide limits. Thus, an ortho deuterium atom in fluorobenzene is exchanged faster than corresponding atoms in toluene by seven orders of magnitude. For all the... 

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. 

Structural proof for the orthometallated species has not always been convincing. Parshall reported the ortho-deuterium incorporation in phosphite ligands [56]. In agreement with this, Coolen showed that the hydrogenation catalyst HRh[P(OPh)3]4 provided ortho-deuterated product when stirred under deuterium atmosphere [55], Both researchers suggested that the deuterium exchange proceeded via orthometallation. The intermediate orthometallated complex was characterized hy P NMR [55],... 

The N-donor ligand complexes (XCIV) undergo arene carbon-metal bond cleavage by metallic hydrides. Use ofdeuteride reagents enables one to effect specific ortho-deuterium incorporation. Treatment of complexes such as XCVII with FeCla results in carbonylation to give 1-exo-lV-phenyl-benzisoindoline (XCVIII) (Flannigan et ai, 1969). 

In addition to H2, D2, and molecular tritium [100028-17-8] the following isotopic mixtures exist HD [13983-20-5] HT [14885-60-0] and DT [14885-61-1]. Table 5 Hsts the vapor pressures of normal H2, D2, and T2 at the respective boiling points and triple points. As the molecular weight of the isotope increases, the triple point and boiling point temperatures also increase. Other physical constants also differ for the heavy isotopes. A 98% ortho—25/q deuterium mixture (the low temperature form) has the following critical properties = 1.650 MPa(16.28 atm), = 38.26 K, 17 = 60.3 cm/mol3... 

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. 

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. 

When one of the ortho hydrogens is replaced by deuterium, the rate drops from 1.53 X 10 " s to 1.38 X lO s. What is the kinetic isotope effect The product from such a reaction contains 60% of the original deuterium. Give a mechanism for this reaction that is consistent with both the kinetic isotope effect and the deuterium retention data. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

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... 

Data refer to H2 of normal isotopic composition (i.e. containing 0.0156 atom % of deuterium, predominantly as HD). All data refer to the mixture of ortho- and puru-forms that are in equilibrium at room temperature. 

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. 

H2 is calculated to be 0.24 K above that of P-H2. Similar differences are found for the bps which occur at the following temperatures normal-H2 20.39 K, 0-H2 20.45 K. Eor deuterium the converse relation holds, 0-D2 melting some 0.03 K below normal -D2 (66.7% ortho) and boiling some 0.04 K below. The effects for other elements are even smaller. 

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. 

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 deuterium isotope effect for each hydrogen atom ortho to the diazonio group ( H/ D = 1.22, Swain et al., 1973b) is the largest secondary aromatic hydrogen isotope effect yet observed. It is comparable to those observed for a-deuterium in reactions involving carbocation formation from secondary aliphatic esters. Ob-... 

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. 

It was of course not possible to introduce the deuterium atoms closer than three bonds away from the center of reaction , but the attenuation of the inductive effect with distance was not expected to affect the validity of the conclusions, especially considering the work of Streit-wieser and Klein (1964), who found that the isotope effect per deuterium in the solvolysis of benzhydryl chloride only decreased from 1 9% for deuterium in the ortho positions to l-5% for deuterium in the meta positions. 

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