Organic molecule dissociation

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

In connection with the adsorption of organic molecules at the surface of an electrode it is possible to distinguish two types (a) adsorption of undissociated molecules and (b) adsorption of intermediates formed by dissociation of the original molecule. The variation of coverage of the surface of a... 

Tables 2.3 and 2.4 list a selection of typical dissociation energies. The values given in Table 2.4 are average dissociation energies for a number of different molecules. For instance, the strength quoted for a C—O single bond is the average strength of such bonds in a selection of organic molecules, such as methanol (CH3—OH), ethanol (CH,CH2—OH), and dimethyl ether (CH,—O—Cl l5). The values should therefore be regarded as typical rather than as accurate values for a particular molecule.

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

The acid dissociation of neutral molecules is such a highly endothermic reaction that the acid dissociation of nitromethane can hardly take place. The results of the calculations presented here provide a theoretical support for nitromethane as an ideal model of aprotic solvent in the acid-base theory of organic molecules. 

The dissociation and ionisation products react with one another in lower layers of the atmosphere to give more complex compounds such as benzene and similar organic molecules with masses between 100 and 350 Da, which are converted to organic anions (20-8,000 Da), which then in turn lead to tholins. 

In the case of CH3CD3, various other dissociation modes exist, including C—C and C—H bond breakage, either separately or in various combinations (Freeman, 1968). A few other examples follow of dissociation of excited inorganic and organic molecules ... 

The electrical conductivity of BaS04 is closest to that of C6Hi206, an organic molecule, which does not dissociate this observation further supports the previous evidence of the weak-electrolyte properties of BaS04. 

A. C6H1206, glucose, is an organic molecule. It would not be expected to dissociate into ions that would conduct electricity. The reported electrical conductivity for glucose supports this. 

Diacyl peroxides are, however, also electron transfer oxidants, which according to a theoretical analysis should possess standard potentials, °[(ArCOO)2/RCOO RCOO ) of around 0.6 V in water, provided that the electron transfer process is of the dissociative type (50) (Eberson, 1982c). Such a value brings thermal ET steps involving DBPO within reach for redox-active organic molecules, as for example suggested by the so-called CIEEL mechanism of chemiluminescence (Schuster, 1982). 

Table 2.4 Recommended R—bond dissociation enthalpies for some selected organic molecules (kJ/mol) ...

Case 3 There are two interfacial rate-determining steps, consisting of 1) formation of an interfacial complex between the interfacially adsorbed molecules of the extractant and the metal ion and (2) transfer of the interfacial complex from the interface to the bulk organic phase and simultaneous replacement of the interfacial vacancy with bulk organic molecules of the extractant. For this mechanism, we distinguish two possibilities. The first (case 3.1) describes the reaction with the dissociated anion of the extracting reagent, B"(ad). The second (case 3.2) describes the reation with the undissociated extractant, BH(ad). 

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