Alkali + halogen collisions

Bandrauk (1969) used a distorted wave Born approximation to calculate the inelastic cross sections of atomic alkali halogen collisions. He found a forwardly peaked differential cross section without oscillations. His total cross section decreases with E 1/2 which is the high energy asymptotic behaviour of the LZ cross section (37). The magnitude of the cross section is much larger than the LZ-result, however. His results were not essentially different if instead of a Coulombic Hl2(R) he used a constant or screened Coulombic interaction. 

Concluding, it seems appropriate first to discuss the total alkali-halogen ionization cross sections, next to extend the discussion to differential cross sections and finally to discuss other collision partners. A special section will be devoted to the determination and interpretation of the molecular electron affinities. 

Catalogue of available data on alkali-halogen charge transfer collisions... 

The electron jump forming Kj BrJ in the K2 + Bf2 collision occurs at a 7A distance. The resulting complex has the biradical structure K K+Br Br [108, 109, 111]. Its principal decay mode is the formation of the di-alkali metal complex K K+Br because, as in alkali metal-halogen reactions, the excess electron of the Bfj moiety induces its dissociation. Most reactive trajectories lead to further dissociation of the complex to K + K+Br, which corresponds to the dominant channel of the reaction, i.e. process 11. The two other reaction channels require a second electron jump, hence forming temporarily K" K Br Br . The hent geometry of the latter complex tends to produce two KBr molecules as in reaction 12, whereas its linear geometry leads to reaction 13. 

Soon after the first experiments on halogen molecules, it became clear that alkali collisions with many other, electronegative and even electropositive, molecules lead to ionization. Many of these results have been published now, especially on relatively small inorganic molecules, whereas experiments on large organic molecules have been announced. Moreover the history of the chemical reaction beam research seems to repeat itself after an alkali-age of about five years the first non-alkali experiments have been reported, which will be discussed in the next section. 

John C. Polanyi (1929- ) was born in Berlin to parents of Hungarian descent. His father, Michael Polanyi, developed transition state theory (see chapter 4) and also studied collisions between alkali metals and halogen molecules. The Polanyi family left Germany in 1933 for England where John received his education. He joined the University of Toronto in 1956 and used IR chemiluminescence, pioneered by his father, to study molecular collisions. In 1986, Dudley R. Herschhach, Yuan T. Lee, and John C. Polanyi shared the 1986 Nobel Prize in chemistry for the contributions to reaction dynamics. 

Even in its more complex form, collision theory does not accurately predict the reaction between metal atoms such as sodium or potassium and halogen molecules such as bromine, for example K(g) + Brjfg) KBr(g) + Br(g). As the alkali metal atom approaches the bromine molecule its valence electron moves to the bromine molecule (thus providing a harpoon ). There are then two ions with an electrostatic attraction between them. As a result the ions move together and the reaction takes place. This mechanism, which has been worked out quantitatively, explains why the reaction occurs far more readily than might be expected taking into account only mechanical collisions between the alkali metal atoms and halogen molecules. 

The d that measures the reaction cross section may be substantially different from a d determined from gas-phase transport experiments. Ion-molecule and alkali metal-halogen reactions often have cross sections substantially larger than what would be expected from analysis of transport data for the corresponding species. As we have defined Q( ), it is a function of molecular energy however, experimental data usually present Q E), a function of molar energy. At 300 K the mean collision per colliding pair is 6 X10 J (equivalent to 0.04 eV or, in molar units, to 4 kJ mol ). 

Even larger low-energy cross sections have been observed. Studies of reactions of alkali metals with halogens are even more dramatic, as long as there is no barrier. For the K + Br2 system a cross section of 2 nm has been measured assuming each collision to be reactive,... 

Consider a collision between K and Br2. The first stage of the reaction is envisaged as the transfer of the valence electron of the alkali metal atom to the halogen molecule. Such a transfer is shown by Eq. (3.18) below to be possible even when reactants are quite a few Angstrom apart. Once the transfer takes place and a temporary ion-pair (e.g., K+Br ) is formed, the strongly attractive coulomb force accelerates the two ions toward each other. This is followed by formation of... 

---