Alkali dimers, reactions

In the preceding discussion of alkaline earth atom reactions, attention has been drawn to analogies with the corresponding alkali atom reactions, particularly in Section III.A. However, a still more revealing comparison might be anticipated with alkali dimer reactions, since both alkaline earth atoms A and alkali dimers M2 have two valence electrons in a totally symmetric orbital giving a singlet state of low ionization potential. 

The cross-dimerization reaction is very commonly employed for the manufacture of intermediates for synthetic musks, which have become an important class of perfumery chemicals. Synthetic musks have been the target of extensive research over the years due to a conservation order placed on the musk deer. Nitro musks are being steadily replaced by non-nitro polycyclic musks becau.se of technical drawbacks and health aspects of the former, which are explosive, sensitive, and virtually nonbiodegradable. Non-nitro musks, on the other hand exhibit better stability to light and alkali, and more nearly duplicate the odour of the macrocyclic musks occurring in nature. Indian musk odorants are easily soluble in alcohol and perfume compositions. They have the added advantage of non-discoloration in soap and domestic products. In view of the low price, their future in the perfume industry appears very promising. 

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

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. 

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... 

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 dimer reduction product of 2-oxopurine also quantitatively regenerated the parent monomer in an alkali catalyzed reaction at room temperature . It is of interest that similar alkaline lability is exhibited by the tram-anti cyclobutane photodimer of thymine The alkaline lability of the purine dihydrodimer may be interpreted as follows the ionized form of the dimer (in alkaline medium) is formally analogous to adducts of two radicals generated by ionizing radiation by the addition of an electron and a proton to pyrimidines or purines ionized in alkaline medium. It was shown by Rao Hayon that ketone radicals are in the enol form and are strong reducers, i.e. they may undergo rapid oxidation. In the case of the ionized 2-oxopurine hydrodimer, this may lead to transfer of two electrons to some acceptor, with concomitant conversion of the dimer to monomers, as follows ... 

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. 

As Fig. 10 indicates, the alkali dimer plus halogen molecule reactions exemplify highly energetic motion over the K2XY potential hypersurface. A... 

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 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). 

The previous examples (Section II.A) of facile four-centre reactions appear to be enabled by the intrusion of ionic interactions into the electronic structure of the potential energy surfaces. The alkali halide-alkali halide exchange reactions involve purely electrostatic interactions. The alkali dimer-halogen molecule reactions involve an electron jump in the entrance valley of the potential surface and thereafter proceed as ion recombination reactions. The present alkali halide-halogen molecule reactions also appear to be permitted by virtue of ionic interactions. The reaction complex, as illustrated in Fig. 26, has the structure of an alkali trihalide molecule. The observed... 

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