Nuclear spin isomers

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

For a given isotope, the nuclear spin isomer (ortho-para species) consisting of rotationally excited molecules—i.e, primarily / = 1 at low temperature—is always preferentially adsorbed on the surface. Thus,... 

All homonuclear diatomic molecules having nuclides with non-zero spin are expected to show nuclear spin isomers. The effect was first detected in dihydrogen where it is particularly noticeable, and it has also been established for D2, T2, N2, N2, Oj, etc. When the two nuclear spins are parallel (ort/jo-hydrogen) the resultant nuclear spin quantum number is 1 (i.e. 5 -b j) and the state is threefold degenerate (2S -(-... 

The separation of nuclear spin isomers and isotopes of hydrogen was the first prominent example of a practical application of capillary adsorption chromatography (Figure 1). Later, it was shown that the time required for the separation of protium and deuterium could be shortened by decreasing the column temperature to 47 K. 

Rgure 1 Chromatogram of the separation of nuclear spin isomers and isotopes of hydrogen in an open tubular adsorption glass capillary column. The 80 m long open tubular capillary column contained a 20 mm thick silica layer as an adsorbent 7= 77.4 K neon was used as the carrier gas. 1, helium 2, p-prolium 3, o-prolium 4, DH 5, o-deuterium 6, p-deuterium. 

Fig. 1-2. Chromatogram of separation for hydrogen nuclear-spin isomers and isotopes on a capillary adsorption glass column [16].

With ortho-hydrogen (0-H2 parallel nuclear spin) and para-hydrogen (p-Hj anti-parallel nuclear spin) hydrogen disposes of two nuclear spin isomers. At room temperature, the equihbrium fraction of p-Hj is 25%. This mixture is... 

H2 is in fact a mixture of two different species (0H2 and PH2) known as nuclear spin isomers. Several other symmetric molecirles also have two or more nuclear spin isomers in the gas phase. However, H2 is rmique among such molecules with regard to the energy separation between the two lowest energy states, namely, the ij = 0,1 = 0) and (/= ,/ = ) states of pH2 and 0H2, respectively. Small molecular mass implies a relatively small moment of inertia for H2, which makes the energy separation anomalously large (about 170 K). For H2 at room temperature, the o p ratio is very close to 3 1 (normal H2), that is, is essentially statistical as the 7=1 nuclear spin state is triply degenerate. However, already at 77 K the o p equilibrium value shifts to about 2 2, and near the H2 liquefaction temperature of about 20 K the equilibrium mixture contains 99.8% of PH2. 

As PHIP effects in the products are the direct measure of the deviation of the o p ratio of the nuclear spin isomers of H2 from the statistical 3 1 value, the influence of the substrate on the reversibility of oxidative hydrogen addition to the metal center can be studied, as demonstrated for Wilkinson s catalyst and a cationic Rh complex in the presence of phenylacetylene [12,45]. 

Pure para-hydrogen was first prepared in 1929. Although hydrogen seems to be the molecule that has been studied extensively, in principle all polyatomic molecules can exist as nuclear spin isomers. 

Another advanced NMR technique is para-hydrogen-induced polarization (PHIP) spectroscopy. Dihydrogen consists of two nuclear spin isomers, one of which has a total spin of zero (I = 0) and is called para-hydrogen. The other spin isomer has a total spin of one (/ = 1) and is called ortho-hydrogon. At room temperature, dihydrogen is a mixture of about 25% para- and 75% ortho-hydrogen. 

Electron spin resonance, nuclear magnetic resonance, and neutron diffraction methods allow a quantitative determination of the degree of covalence. The reasonance methods utilize the hyperfine interaction between the spin of the transferred electrons and the nuclear spin of the ligands (Stevens, 1953), whereas the neutron diffraction methods use the reduction of spin of the metallic ion as well as the expansion of the form factor [Hubbard and Marshall, 1965). The Mossbauer isomer shift which depends on the total electron density of the nucleus (Walker et al., 1961 Danon, 1966) can be used in the case of Fe. It will be particularly influenced by transfer to the empty 4 s orbitals, but transfer to 3 d orbitals will indirectly influence the 1 s, 2 s, and 3 s electron density at the nucleus. 

These two isomeric forms of R2 are physically different because of the pairing of the nuclear spins on the hydrogen nnclei in the molecule. They can be physically separated by chromatography at very low temperatures. Generally to convert one isomer to the other requires a cleavage of the R-R bond. As with the hydrogen-isotope-exchange reactions, no external acceptor or donor is required. 

For Re2(02CC Hs)4Cl2 the first reduction wave near —0.3 V is quasi-reversible. The product of this one-electron oxidation was not isolated as a solid, but ESR spectra were obtained on frozen dichloromethane solutions at 77°K at both X- and Q-band frequencies. Both naturally occurring rhenium isotopes have a nuclear spin of 5/2 with slightly different nuclear magnetic moments. For dimeric rhenium species there are three distinct isotopic isomers and each generates a unique ESR spectrum as a result of differing hyperfine interactions. The observed ESR spectrum is a superposition of these lines with the intensity... 

In detail the time distribution of the 83.5 keV line at early times is not reproduced by the fit, due to feeding from the 11 isomer. However, the distance to the next pronounced structure, at which all nuclear spins are in phase again is practically independent of this. Feeding from 13" into 11" is weak and spread out in time and therefore uniiqportant. Variations of the fitted range and other parameters changed the results for the frequencies by <1 %. Results of Dafni et al. [3] are also shown. Their experimental conditions were less clean, in particular the combined effect of two isomers had... 

The isomerization of donor olefins is illustrated by the reaction of chloranil with a pair of geometric isomers, cis- and trans-1 -phenylpropene. The irradiation of the quinone in polar solvents in the presence of either isomer results in nuclear spin polarization for both isomers. The key to understanding these effects lies in two observations (Fig. 9) (a) the polarization of the regenerated parent olefin is stronger than that of the rearranged olefin (b) the reaction of the ds-isomer results in stronger overall effects than does that of the trans-isomer [157, 158],... 

In this mechanistic scheme, the CIDNP intensities of reactant and product are determined by the competition of key steps at each stage of the reaction. For the system discussed here, the qualitative features of the observed polarization suggest that nuclear spin lattice relaxation during the lifetime of the olefin triplet state is negligible, that singlet and triplet pairs recombine with similar efficiencies, and that the triplet state decays to each of the isomers with equal efficiency. 

The first case of a nuclear isomer was found in 1921 by Hahn, who proved by chemical methods the existence of two isomeric states of Pa which were called UX2 and UZ. The decay scheme of Pa is plotted in Fig. 5.13. Both nuclear isomers are produced by decay of Th. 234mp ( i/2 = T17m) changes at nearly 100% directly into Later, the production of artificial radionuclides by nuclear reactions led to the discovery of a great number of nuclear isomers. In the case of °Br, for instance, two isomeric states were found (Fig. 5.14), and chemical separation of somBr and °Br is also possible. From the change of nuclear spin and of parity half-lives can be assessed by application of the selection rules (eq. (5.40)) and of eqs. (5.37) and (5.38). The half-lives of nuclear isomers may vary between seconds and many years. 

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