Sodium energy transfer processes

The beam of KBr now meets a beam of atomic sodium, and the radiation emitted turns out to be of the potassium resonance lines, so that the energy transfer process must be written... 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

The results obtained so far with collisional energy-transfer spectroscopy are restricted to excited sodium atoms A = Na(32/,3/2) and quenching by a variety of simple polar and nonpolar molecules. The technique is applicable to any vaporizable molecule and will be available for a number of other atoms as well in due course with the progress of laser technology. The E-V-R transfer processes from and to sodium atoms have a number... 

Two modes of operation are used. When energy-transfer spectra are recorded, the selector frequency, that is, the final sodium velocity c a- is swept several times with a period of some minutes from around 1500 m/sec to 4000 m/sec. Alternatively, at a fixed selector velocity the polarization direction may be rotated to observe the influence of atomic alignment on the scattering process. 

The sodium D-line radiation dominates the system because the Nad is long-lived, the vibrational-electronic energy transfer is efficient and the excited atom radiates in 10-B seconds. The multistep process bleeds off the excitation energy. This behavior probably is common in systems containing atoms with low-lying energetically accessible electronic states29,55. 

Sonochemical homopolymerization of dichlorosilanes in the presence of sodium is successful at ambient temperatures in nonpolar aromatic solvents (toluene or xylenes) only for monomers with a-aryl substituents. Dialky 1-dichlorosilanes do not react with dispersed sodium under these conditions, but they can be copolymerized with phenylmethyldichlorosilane. Copolymers with a 30-45% content of dialkylsilanes were formed from equimolar mixtures of the corresponding comonomers. Copolymerization might indicate anionic intermediates. A chloroterminated chain end in the polymerization of phenylmethyldichlorosilane can participate in a two-electron-transfer process with sodium (or rather two subsequent steps separated by a low-energy barrier). The resulting silyl anion can react with both dichlorosilanes. The presence of a phenyl group in either a or P position in chloroterminated polysilane allows reductive coupling, in contrast to peralkyl species, which do not allow the reaction. Therefore, dialkyl monomers can copolymerize, but they cannot homopolymerize under sonochemical conditions. 

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

The true nature of homogeneous anionic polymerization only became apparent through studies of the soluble aromatic complexes of alkali metals, such as sodium naphthalene. These species are known to be radical anions [154-158], with one unpaired electron stabilized by resonance and a high solvation energy, and are therefore chemically equivalent to a soluble sodium. They initiate polymerization by an electron transfer process [145,148], just as in the case of the metal itself, except that the reaction is homogeneous and therefore involves a much higher concentration of initiator. The mechanism... 

Pd-Pb/C catalysts with different amounts of Pb were prepared using NaBH4 chemical reduction method in the presence of sodium citrate. Pd-Pb (4 1)/C showed better activity towards ethanol electrooxidation in alkaline electrolyte than Pd/C catalyst. The Arrhenius equation was used to calculate the activation energy, which showed a smaller value, thus implying a faster charge transfer process. The enhanced activity of Pd-Pb/C was explained by a bifunctional mechanism and the d-band theory [56]. Pd4-Au/C and Pd2.5-Sn/C catalysts prepared by He et al. [72] showed lower activity for ethanol electrooxidation in alkaline electrolyte than commercial Pt/C but were more tolerant to poisoning. 

The simplest way to get chemical energy on this scale is by the combination of radicals, by the recombination of ion pairs, or by extremely exothermic electron transfer processes. All these three types of reaction can give rise to chemiluminescence under suitable conditions. A simple example is the oxidation of extremely energetic anion radicals formed by reduction of neutral molecules. As Fig. 6.38 indicates, an electron transfer from such an ion to a strong electron acceptor, i.e., one with an AO of low energy, can leave the resulting molecule in an excited state. A good example is the reaction of sodium 9,10-diphenylanthracene with radicals. This is a — E (reverse -type) reaction. 

The value of the lattice energy is a large negative number. The formation of the crystalline NaCl lattice from sodium cations and chloride anions is highly exothermic and more than compensates for the endothermicity of the electron transfer process. In other words, the formation of ionic compounds is not exothermic because sodium wants to lose electrons and chlorine wants to gain them rather, it is exothermic because of the large amount of heat released when sodium and chlorine ions coalesce to form a crystalline lattice. 

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