Hydrogen nuclear spin

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

In the following pages, only hydrogen and the carbon 13 isotope, both of which have a nuclear spin of 1/2 will be discussed. 

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. 

Ortho-Para Tritium. As in the case of molecular hydrogen, molecular tritium exhibits nuclear spin isomerism. The spin of the tritium nucleus is S, the same as that for the hydrogen nucleus, and therefore H2 and T2 obey the same nuclear isomeric statistics (16). Below 5 K, molecular tritium is... 

Nuclear Magnetic Resonance. AH three hydrogen isotopes have nuclear spins, I 7 0, and consequently can all be used in nmr spectroscopy (Table 4) (see Magnetic spin resonance). Tritium is an even more favorable nucleus for nmr than is H, which is by far the most widely used nucleus in nmr spectroscopy. The radioactivity of T and the ensuing handling problems are a deterrent to widespread use for nmr. Considerable progress has been made in the appHcations of tritium nmr (23,24). 

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. 

Most physical properties are but little affected by nuclear-spin isomerism though the thermal conductivity of P-H2 is more than 50% greater than that of 0-H2, and this forms a ready means of analysing mixtures. The mp of P-H2 (containing only 0.21% (3-H2) is 0.15 K below that of normal hydrogen (containing 75% 0-H2), and by extrapolation the mp of (unobtainable) pure... 

Nuclear magnetic resonance, NMR (Chapter 13 introduction) A spectroscopic technique that provides information about the carbon-hydrogen framework of a molecule. NMR works by detecting the energy absorptions accompanying the transitions between nuclear spin states that occur when a molecule is placed in a strong magnetic field and irradiated with radiofrequency waves. 

In effect, the division by two is the result of the molecular symmetry, as specified by the character table for the group 0. In general it is useful to define a symmetry number a (= 2 in this case), as shown below. The well-known example of the importance of nuclear spin is that of ortho- arid para-hydrogen (see Section 10.9.5). 

The rotational temperature of H2O in the molecular beam is quite low, about 10 K. As in the hydrogen molecule, the water molecule has para and ortho rotational levels with nuclear spin-statistics of 1 3 respectively. Since the para and ortho rotational levels have different nuclear wavefunc-tions, the conversion between the para and ortho levels is extremely slow, as in the hydrogen molecule. In H2O, the nuclear spin-statistics for the lowest rotational levels are as follows ... 

For those new to the field of fluorine NMR, there are a number of convenient aspects about fluorine NMR that make the transition from proton NMR to fluorine NMR relatively easy. With a nuclear spin of j and having almost equal sensitivity to hydrogen along with sufficiently long relaxation times to provide reliable integration values, 19F nuclei... 

In very pure hydrogen, there can be hardly any permanent chemical change produced by irradiation. However, the ion-molecule reaction (5.1) does occur in the mass spectrometer, and it is believed to be important in radiolysis. The H2 molecule can exist in the ortho (nuclear spin parallel) or para (antiparallel) states. At ordinary temperatures, equilibrium should favor the ortho state by 3 1. However, the rate of equilibration is slow in the absence of catalysts but can be affected by irradiation. Initially, an H atom is produced either by the reaction (5.1) or by the dissociation of an excited molecule. This is followed by the chain reaction (H. Eyring et al, 1936)... 

The impurity interacts with the band structure of the host crystal, modifying it, and often introducing new levels. An analysis of the band structure provides information about the electronic states of the system. Charge densities, and spin densities in the case of spin-polarized calculations, provide additional insight into the electronic structure of the defect, bonding mechansims, the degree of localization, etc. Spin densities also provide a direct link with quantities measured in EPR or pSR, which probe the interaction between electronic wavefunctions and nuclear spins. First-principles spin-density-functional calculations have recently been shown to yield reliable values for isotropic and anisotropic hyperfine parameters for hydrogen or muonium in Si (Van de Walle, 1990) results will be discussed in Section IV.2. 

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