Two gas phase deuterium/hydrogen exchange reactions are described utilizing a simple inexpensive glass catalyst tube containing 0.5% Pd on alumina through which gas mixtures can be passed and products collected for analysis. The first of these exchange reactions involves H 2 + D 2 , which proceeds at temperatures as low as 77 K yielding a mixture that includes HD. Products are analyzed by 1 H NMR spectrometry. At low temperatures, this reaction requires a catalyst, but it proceeds without a catalyst at high temperature of a gentle flame. The second deuterium/hydrogen exchange reaction involves CH 4 + D 2 producing a series of isotopologues, methane-d x , x = 0−4, with product analysis by GC−MS and 1 H NMR spectrometry. This reaction only takes place in the presence of a catalyst at elevated temperatures due to the large energy of activation of the sp 3 -carbon-to-hydrogen bond. Two outcomes have been observed in the literature regarding D/H exchange and methane. Some catalysts and temperature conditions yield a single-exchange result, methane-d 1 . Others yield multiple exchange results, such as we observe with our catalyst. The single exchange outcome is associated with lower temperatures. Two mechanisms, one by Kemball (1959) and one by Frennet (1974), have been put forth to explain single and multiple exchange outcomes. We discuss our results in the context of these mechanisms. Interested readers could develop a research experience for undergraduate chemistry students based on the openended experiments presented here.
Complexes of lithium atoms with ethylene have been identified as potential hydrogen storage materials. As a Li atom approaches an ethylene molecule, two distinct low-lying electronic states are established; one is the A electronic state (for C geometries) that is repulsive but supports a shallow van der Waals well and correlates with the Li 2s atomic state, and the second is a B electronic state that correlates with the Li 2p atomic orbital and is a strongly bound charge-transfer state. Only the B charge-transfer state would be advantageous for hydrogen storage because the strong electric dipole created in the Li-(CH) complex due to charge transfer can bind molecular hydrogen through dipole-induced dipole and dipole-quadrupole electrostatic interactions. Ab initio studies have produced conflicting results for which electronic state is the true ground state for the Li-(CH) complex. The most accurate ab initio calculations indicate that the A van der Waals state is slightly more stable. In contrast, argon matrix isolation experiments have clearly identified the Li-(CH) complex exists in the B state. Some have suggested that argon matrix effects shift the equilibrium toward the B state. We report the low-temperature synthesis and IR characterization of Li-(CH) (n = 1, m = 1 and 2) complexes in solid parahydrogen which are observed using the C═C stretching vibration of ethylene in the complex. These results show that under cryogenic hydrogen storage conditions the Li-(CH) complex is more stable in the B electronic state and thus constitutes a potential hydrogen storage material with desirable characteristics.
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