Reactions of alkali dimers

Despite an extensive study of the reactions of alkali dimers (see the reviews of Grice [208] and Herm [216]), there have been few direct measurements of energy disposal in these systems. The chemiluminescent reactions of oxygen and halogen atoms with alkali dimers have already been described [Sects. 3.1.7(c) and 3.1.8(b)], as have the alkali atom-alkali dimer exchange reactions [Sect. 3.1.2(b)], In this section we describe the chemiluminescent reactions of alkali diatomic molecules with halogen molecules. 

Emission has been observed from the electronically excited alkali halide product in the four-centre reactions [612, 613] 

No emission is observed from reactions involving Na2 or F2 despite these processes being energetically allowed. This is attributed to either these reactions having very small cross-sections (Na2) or to the fact that the electronically excited alkali halide product is unstable (MF ). In the reactions with the interhalogens C1F and IX, emission was only observed from MCI and MI, respectively, the more stable of the two possible products being in the ground state. Atomic D-line emission was also noted and attributed to the process [613] 

In the last step, the MX molecule acts as a chaperone to bring the M and Y together to form the electronically excited halide MY. The chemiluminescent cross sections are about two orders of magnitude smaller than the total reaction cross sections. For the K2 reactions, the cross sections are 0.3—3 A2 for M emission and smaller for MX emission for Rb2 and Cs2, the MX emission becomes stronger. 

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The reactions of alkali dimers with halogen molecules also exhibit a multiplicity of reaction pathways with production of electronically excited atoms, electronically excited molecules, and ions all apparently possible [364, 367-370], In these reactions, however, the dominant pathway, with a reactive cross section >150 A2, leads to products in their electronic ground... 

The alkaline earth atoms Ca, Sr, Ba have low ionization potentials, / 5-6 eV [however /(Mg) = 7-5 eV], which do not greatly exceed those of the alkali atoms. Indeed, the ionization potential of barium, /(Ba) = 5-2 eV, is less than that of lithium, /(Li) = 54 eV. Thus we might anticipate that the reactions of alkaline earth atoms would resemble those of alkali atoms, particularly in exhibiting electron jump transitions. However, the alkaline earth atoms are divalent and two valence electrons can potentially be transferred. This should introduce interesting new features into the reaction dynamics. A particularly close analogy might thus be expected with the reactions of alkali dimers discussed in Section II. 

The second electron jump occurs with very high probability in the reactions of alkali dimers with polyhalide molecules (see Section II.B), resulting in the capture of both alkali atoms as bound alkali halides MX. Thus, it would be very interesting to see whether the dihalides AX2 become prominent reaction products in the alkaline earth atom reactions with polyhalide molecules. Unfortunately, angular distribution measurements are presently equivocal on this point (see Section III.A). 

At the origin of modem reaction dynamics are the scattering experiments on alkali-halogen systems which Herschbach and his school pioneered. Very rich chemical branching was observed in reactions of alkali dimer molecules with halogens [1,2], e.g. 

Two particularly interesting chemiluminescent reactions have been discovered using beams of alkali dimer molecules (M2). These species are formed in high yield in an alkali metal nozzle beam source and they can be separated from the remaining atoms by means of an inhomogeneous magnetic field. This technique has made it possible to study reactions of these diatomic species by molecular beam methods for the first time [364] and as a result it has been demonstrated conclusively that reactions of the type... 

Chemiluminescent channels have also been observed in this type of reaction [113]. Reactions of alkali metal dimers with oxygen molecules have also been reported [114-116],... 

Double alkoxides of zirconium with alkali metals of the type MZi (OR) have been obtained by reaction of alkali metal alkoxides with zirconium alkoxides (220). Although these usually are monomeric derivatives, the reaction between zirconium tetra-/-butoxide [1071-76-7] and sodium Abutoxide was found (221) to form dimeric [NaZr(OC(CH2)3)j2-... 

The reactions44-45 of alkali dimers with halogen molecules offer an opportunity to study the dynamics of four-centre reactions. Three-centre reactions of an atom plus diatomic molecule have been extensively studied and also four-centre reactions46 of an atom plus triatomic molecule. However, there are few examples known47 of reactive four-centre diatomic plus diatomic systems. 

Chemiluminescence studies51,55 of alkali dimer (Na2,K2,Rb2,Cs2) reactions with halogen atoms (Cl, Br, 1) have discovered large cross sections, Q 10-100 A2, for formation of alkali atoms in the lowest excited state. 

The reactions of alkali-metal dimers have been investigated, but to the authors knowledge the reactions of mixed dimers have not been studied, e.g. NaK, and any effects due to altering the reaction partner investigated. 

Struve et al. [81] have measured the sodium D line chemiluminescence produced in the reaction of Na2 and Cl atoms. Evans and Polanyi [264] suggested two possible mechanisms for the chemiluminescence, namely the direct reaction of alkali-metal dimer with a halogen atom to produce electronically excited sodium Na2 + Cl NaCl + Na(3 P)... 

A brief report has appeared on the crossed-beam reaction between Ba and LiCl, using laser-induced fluorescence detection of the BaCl product, which gives a lower bound for the bond-dissociation energy of BaCl of 110 3 kcal mol-1.242 Similar studies on the reactions of alkali-metal dimers with hydrogen atoms and molecules 243 and the photodissociation of alkali-metal iodides244 have been reported. 

In solution the electrochemical reduction or the reaction of alkali metals in THF lead to mono- and dianions without any evidence for 7t-dimer formation. The ESR signal of the mono-anion reveals f = 2.0046 for didodecylsexithiophene which is substantially different from the cation and the free electron. It can be concluded that, in the case of the anion, the sulphur orbitals contribute to a large extent to the singly occupied molecular orbital of the anion [192]. 

Metal ketyls form in reactions of alkali metals with aromatic ketones in some polar solvents, like dioxane or tetrahydrofuran. These ketyls exist in equilibrium mixtures of monomeric anion radicals and dimeric dianions.Originally, there was some controversy about the mechanism of initiation of monomers like acrylonitrile or methyl methacrylate by sodium benzophenone. The following mechanism was derived from spectral evidence. The initiation is by transfer of the electron and the ion charge " ... 

The 1 1 molar reaction of alkali and titanium isopropoxide also yielded a crys-tallizable derivative of composition M Ti(OPr )5. Interestingly, a similar reaction of MOBu and Zr(OBu )4 also resulted in a crystallizable volatile dimeric product [ KZr(OBu )5 2]. The basic information on the more dominant was thus set out in nona-alkoxodimetallate M2(OR)9 , penta-alkoxometallate M(OR)5 , and hexa-alkoxometaUate M(OR)6 derivatives were thus laid in this early publication. 

The reactions of alkali metal dimers with X2 and RX are quite interesting because of the many reaction paths including chemiluminescence and chemi-ionization/ Based upon measurement of the alkali halide product, the angular distributions show a peak in the forward direction, as well as some scattering in the backward direction. In most cases three products, M + MX + X (or R), are probably formed. The potential surfaces for these systems must involve some deep wells and, as data become available, the energy disposal for this class of reaction should be most interesting. 

In the reactions of 10.13a with alkali metal terr-butoxides cage expansion occurs to give the sixteen-atom cluster 10.15, in which two molecules of MO Bu (M = Na, K) are inserted into the dimeric structure. The cluster 10.13a also undergoes transmetallation reactions with coinage metals. For example, the reactions with silver(I) or copper(I) halides produces complexes in which three of the ions are replaced by Ag" or Cu" ions and a molecule of lithium halide is incorporated in the cluster. ... 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

Ionic polysulfides dissolve in DMF, DMSO, and HMPA to give air-sensitive colored solutions. Chivers and Drummond [88] were the first to identify the blue 83 radical anion as the species responsible for the characteristic absorption at 620 nm of solutions of alkali polysulfides in HMPA and similar systems while numerous previous authors had proposed other anions or even neutral sulfur molecules (for a survey of these publications, see [88]). The blue radical anion is evidently formed by reactions according to Eqs. (5)-(8) since the composition of the dissolved sodium polysulfide could be varied between Na2S3 and NaaS with little impact on the visible absorption spectrum. On cooling the color of these solutions changes via green to yellow due to dimerization of the radicals which have been detected by magnetic measurements, ESR, UV-Vis, infrared and resonance Raman spectra [84, 86, 88, 89] see later. 

Succinic anhydride is dimerised to 1,6-dioxaspiro [4.4] nonane-2,7-dione by heating with sodium hydroxide. Modification of an existing procedure by adding further sodium hydroxide after the initial reaction led to a severe exothermic reaction after heating for some 30 h which fused the glass flask to the heating mantle, probably at a temperature approaching 550°C. The reason for this was not known [1], At elevated temperatures and under influence of alkali, succinic acid condenses decarboxylatively beyond the dimeric spiroacetal, sometimes explosively. Contamination of the anhydride with base is to be avoided [2],... 

Cg.Me(.)2Sm(THF)2 also shows high reactivity in its reactions with C,H C=CCgH and CgHj-NzNC H- and the subsequent reactions of these products With CO 5 The clH CECC.H reaction (65), which forms [(C Me )2Sm]2C2(CgH )2 (Equation 8 may have a parallel in alkali metal chemistry, except that in the heterogeneous alkali metal systems, polymerization or dimerization of the intermediate radical... 

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