Isotopically substituted hydrogen molecules

Hydrogen/deuterium isotopic substitution coupled with neutron diffraction studies have been used to show that the addition of an alkali metal to liquid NH3 disrupts the hydrogen bonding present in the solvent. In a saturated lithium-ammonia solution (21 mole percent metal), no hydrogen bonding remains between NH3 molecules. Saturated L1-NH3 solutions contain tetrahedrally coordinated Li, whereas saturated K-NH3 solutions contain octahedrally coordinated K. 

Isotope substitution experiments can detect hydrogen tunnelling. Isotope substitution experiments can sometimes be used to determine whether hydrogens are cleared from molecules through mechanisms that involve tunnelling. To test this, two isotopes are substituted for hydrogen (1) deuterium (D, mass = 2) is substituted and the ratio of rate coefficients kn/ko is measured, and (2) tritium (T, mass = 3) is substituted and kn/k i is measured. 

Garret, B. C. and Truhlar, D. G. Generalized transition state theory. Quantum effects for collinear reactions of hydrogen molecules and isotopically substituted hydrogen molecules, JPhys.Chem., 83 (1979), 1079-1112... 

In this paper, we apply the three algorithms to the determination of the V effect of the torsional levels of hydrogen peroxide. The size of this molecule is optimum for the comparison of the three methods. In addition, a previous study shows that torsion of the central bond can be separated from the rest of the vibrational modes, thereby simplifying the problem. Finally, the torsional pseudopotential ofD202 is also calculated for determining the isotopic substitution effects. The experimental data are from Camy-Peiret et al [15]. 

This treatment is based on the following assumptions (a) all differences, except for zero point vibrational energy are negligible (b) the hydrogen isotope is attached to a heavy polyatomic molecule, such that isotopic substitution affects only the vibrations of... 

A more subtle application of the same technique involves isotopic substitution. Because most C02 frequencies are reproducible from crystal to crystal to within 0.07 cm ", it is possible to demonstrate small, but significant shifts of v3 when certain hydrogen atoms are replaced by deuterium. Analogous orientation effects have been reported for mixed crystals of isotopically substituted benzene, where isotopic differences in adjacent molecules gave rise to splittings as large as 4.7 cm "1 [91],... 

Normal vibration calculations, if based on a correct structure and correct potential field, would supposedly permit a unique correlation to be made between predicted and observed absorption bands. In most cases this ideal situation is far from being achieved in the study of high polymer spectra. More usually the structure and force field are to some extent unknown, or normal mode calculations are not available, so that other methods must be used in order to establish the origin of bands in the spectrum. Even if complete calculations were available it would be desirable to check their predictions by means other than a comparison of observed and predicted frequency values. One method of doing so is by studying isotopically substituted molecules, and the most useful case is that in which deuterium is substituted for hydrogen. 

The isotopic substitution of hydrogen atoms with deuterium in fulleranes do not alter the electronic transitions. This has already been observed in other molecules... 

A physical implication of this assumption is that the occurrence of isotopic substitution in one of the positions of XLm has no effect on the exchange equilibrium of any of the other positions, i.e. the exchange behaviour of the molecule XLm is equivalent to m molecules of a hypothetical solute X L containing one hydrogen nucleus per molecule (Block and Gold, 1959). In other words, thermodynamic secondary hydrogen isotope effects are assumed to be absent. 

The most detailed structures have been obtained from neutron diffraction data, when isotopic substitution methods have been possible to use. Neutrons are strongly scattered by hydrogen atoms, making it possible to determine not only bond lengths and coordination number of the metal ion, but also the orientation of the water molecules in its first coordination sphere. A summary of results is given in Table II (19-31). 

This denotes a plane through the molecule, about which a reflection operation may be carried out. The symbol a originates from the German word Spiegel, which means a mirror. In H2O (Fig. 6.1.1), when one of the hydrogen atoms is substituted by its isotope D, the C2 axis no longer exists. However, the molecular plane is still a symmetry plane. 

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