Noble gases, energy comparisons

The first transition metal-nohle gas complex to be observed in liquefied noble gas solution was CrCCOlsXe (38). Continuous UV photolysis of Cr(CO)6 dissolved in liquid Xe at 175 K or liquid Kr doped with 5% Xe at 151K produced a new species. This new species had v(C—O) IR bands (Fig. 7) which could be assigned to CrlCOsXe by comparison with those of CrCCOlsXe in a Xe matrix [Eq. (2)]. When the photolysis was halted, Cr(CO)5Xe decayed Avith a lifetime of 2 s. The activation energy for the thermal decay of CrCCOlsXe in hquid Kr + 5% Xe was determined to be Ea = 15 2 kJ mol . The surprisingly long lifetime of CrCCOlsXe in solution at this temperature was attributed to the high concentration of Xe and the extremely low concentration of the other reactants. 

The molecular orbital description of He 2 predicts two electrons in a bonding orbital and two electrons in an antibonding orbital, with a bond order of zero—in other words, no bond. This is what is observed experimentally. The noble gas He has no significant tendency to form diatomic molecules and, like the other noble gases, exists in the form of free atoms. He2 has been detected only in very low pressure and low temperature molecular beams. It has a very low binding energy, approximately 0.01 J/mol for comparison, H2 has a bond energy of 436 kJ/mol. 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

Table 4.1 Exchange-only ground-state energies from ROPM and RHF calculations for noble gas atoms Coulomb (C) and Coulomb-Breit (C + B) limit in comparison with complete transverse exchange (C + T) (Engel et al. 1998a). For the RHF approximation the energy difference with respect to the ROPM is given, AE = tot(RHF) — tot(ROPM), providing results from (a) finite-differences calculations (Dyall et al. 1989) and (b) a basis-set expansion (Ishikawa and Koc 1994). All energies in mHartree. uext and c as in Ishikawa and Koc (1994).

Another noble gas dimer in which the coupling constant (the FC term only) has been calculated is the Xe dimer (DFT calculations with a local functional) The result suggests that the reduced indirect spin-spin coupling in Xe2, close to the minimum of energy, is one order of magnitude larger than in He2 However, rather unexpectedly, the sign is opposite. Obviously, the computational approach is widely different in Refs. and a direct comparison is probably not possible. 

The potential energy of an explosive depends on (1) The volume of gas generated—calculated for purposes of comparison purposes at 760 mm. and 0° and (2) on the temp, developed on explosion, whereby the gases are expanded enormously. A. Noble and F. Abel estimate the total work theoretically performable is 332,000 gram-metres per gram, or 486 foot-tons per lb. of powder. 

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