Hydrogen nuclear spin states

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. 

The isotope of carbon with seven neutrons, l3C, composes about 1.1% of carbon atoms. It is similar to hydrogen in that it has two nuclear spin states of different energy when it is in an external magnetic field. The spectroscopy that is done using this nucleus, l3C-NMR, provides direct information about the carbon chains in the compound, information that is often complementary to that obtained from H-NMR spectroscopy. 

As the ortho-para conversion involves the forbidden triplet-singlet transition in the nuclear spin state, spontaneous transformation is very slow. Thus, samples of hydrogen that have been cooled exhibit the proportion of the two forms corresponding to the equilibrium composition at room temperature. The conversion of the ortho molecules to para molecules and vice versa requires the simultaneous breaking of both the spin and orbital symmetries, and the equilibrium ratio is therefore established slowly. 

Hydroboration-oxidation, 227-233, 250, 582 Hydroformylation, 661, 732 Hydrogen. See also Hydrogenation Nuclear magnetic resonance spectroscopy covalent bonding in, 12 formation of, 6 molecular orbitals, 34-35 nuclear spin states, 490-491 Hydrogenation. See also Heat of... 

At 0 K, J = 1 for ortho-hydrogen, and 2J + 1 = 3. These three rotational states combine with the three nuclear spin states to yield nine quantum states available to the 0-H2 molecule at 0 K. In normal hydrogen, the f mole fraction of 0-H2 is evenly distributed over all nine of these states. On the other hand, for P-H2 at 0 K, J = 0 and 2J + 1 = 1. This one rotational state combines with the one nuclear spin state to yield a single quantum state that is available to the P-H2 molecule. The entire mole fraction of P-H2 in n-H2 occupies this single quantum state at 0 K. 

In the hydrogen-decoupled mode, the sample is irradiated with two different radio frequencies. The first radio frequency is used to excite the C nuclei. The second is a broad spectrum of frequencies that causes all hydrogens in the molecule to undergo rapid transitions among their nuclear spin states. On the time scale of a C-NMR spectrum, each hydrogen is in a time average of the two states, with the... 

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