Noble gases, frozen

The simplest molecular solids are the frozen noble gases—for example, solid neon. In this case, the molecules are single atoms and the intermolecular interactions are London forces. These forces are nondirectional (in contrast to covalent bonding, which is directional), and the maximal attraction is obtained when each atom is surrounded by the largest possible number of other atoms. The problem, then, is simply to find how identical spheres can be packed as tightly as possible into a given space. 

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

The application of matrix isolation to organometallic chemistry has been extensively described elsewhere (4,5,6,7). Two methods have generally been employed. In the first, based on G.C. Pimentel s original development, the solid matrix environment is a frozen noble gas - usually Ar - at 10-20K and the unstable fragment is generated either by photolysis of a parent molecule already trapped in the matrix, or by cocondensation from the gas phase. In the... 

One specific advantage of the frozen noble gas is that it does not absorb IR and thus it is possible, in principle, to examine the spectrum of any coordinated ligand within the matrix. In practice, it is more difficult because, although the intensity of v(N-N) and V(N-O) bands are comparable with V(C-O) bands, most of the vibrations of other ligands give rather weak IR bands. However, as an example, in recent important work, Perutz (8) has demonstrated that photolysis of CpRh(C2H ) in Ar matrices at 20K leads to reversible loss of C2H to form CpRh(C2H ) identified by IR... 

In solutions or in frozen matrices the effect of the environment is not negligible any more. The NIR spectra of Cgg were measured in various frozen noble gas matrices [34] and or T)sd distortions were found. The result was the same in the apolar methylcyclohexane matrix, while the distortion was H2h in the polar 2-methyltetrahydrofuran (2-MeTHF) matrix [35]. One might expect the same polarity dependence in solutions, but electrochemically generated Cgg ions showed a... 

Significant progress has been made in the development of noble gas hydrides, with 23 neutral species reported by 2009. Typically prepared by UV photolysis of precursors in frozen noble gas matrices, hydrides are known for the elements argon (HArF, mentioned previously), krypton, and xenon and include both a dihydride (HXeH) and compounds with bonds between noble gases and F, Cl, Br, I, C, N, O, and S. 

Nickel is the only metal to react directly with carbon monoxide at room temperature at an appreciable rate, although iron does so on heating under pressure. Cobalt affords HCo(CO)4 with a mixture of hydrogen and carbon monoxide (p. 387). In general, therefore, direct reaction does not provide a route to metal carbonyls. The metal atom technique (p. 313) has been used to prepare carbonyls of other metals in the laboratory e.g. Cr(CO)g, but it offers no advantages over the reduction method discussed below. When metal vapours are cocondensed with carbon monoxide in frozen noble gas matrices at very low temperatures (4-20K) the formation of carbonyl complexes is observed. These include compounds of metals which do not form any stable isolable derivatives e.g. Ti(CO), Nb(CO) and Ta(CO)g as well as Pd(C0)4 and Pt(C0)4. Vibrational spectra of the matrix show that coordinatively unsaturated species such as Ni(CO) n = 1-3) or Cr(CO) (n = 3-5) are also formed under these conditions. 

Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas (p. 626). They have the ideal composition, [Gg(H20)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way (p. 358). Samples containing up to 20% by weight of Ar have been obtained. 

The third component to the electrostatic interaction is caused by the motion of the electron cloud, which creates an oscillating field. It couples to the oscillating field of the neighboring molecules, which gives an attractive contribution to the total energy. This should be obvious, from the following example. Consider the frozen electron distribution of a nonpolar molecule (e.g., a noble gas). The instantaneous distribution possesses a dipole moment, which for the same reason as described above, induces a dipole in neighboring molecules, which in turn act on the first molecule, etc. This contribution is denoted the dispersion term or the London term [9]. Note that this contribution is only approximately pairwise additive. 

Despite the fact that a transition metal-noble gas complex has been isolated only very recently, the study of nohle gas coordination of transition metals actually has a long history. Early experiments used the technique of matrix isolation 18). Under the cryogenic conditions of frozen inert matrices, highly reactive photoproducts become sufficiently long-lived to allow their detection at leisure by conventional spectroscopic techniques such as UV/visible, IR, and EPR spectroscopy. 

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