Dipolar dissociation

Direct Dissociative Processes Electronic Excitation, Nonresonant Dipolar Dissociation, and Dissociative Ionization... 

As in the case of EED processes, nonresonant dipolar dissociation (DD) involves a dissociative electronic excited state of the molecule AB (Figure 16.3), which brings the spontaneous scission of the molecule into ionic fragments (Abdoul-Carime et al. 2000, Herve du Penhoat et al. 2001, Sanche 2003) ... 

Hoffman, A., A. Laikhtman, S. Ustaze et al. 2001. Dissociative electron attachment and dipolar dissociation of H electron stimulated desorption from hydrogenated diamond films. Phys Rev B Condens Matter 63 045401. 

One final reaction in which 23 has been considered an intermediate is the formation of phthalic anhydride (73) from the plasmolysis of thiophene-2,3-dicarboxylic acid anhydride (229) in the presence of acetylene. It was suggested that a 1,3-dipolar dissociation of 229 gives thioketene (CjHjS) and presumably 23, which reacts with cyclobutadiene (known to be present during the plasmolysis of CjHj) to give phthalic anhydride (73) via 230. Naturally, no proof for this scheme exists. For that matter, no convincing proof for the existence of any oxaaryne can be considered available at this time. 

Zhou Y and Stell G 1993 Analytic approach to molecular liquids V. Symmetric dissociative dipolar dumb-bells with the bonding length o/3 = L = al2 and related systems J. Chem. Phys. 98 5777... 

Fig. 15. Ion movements in the electro dialysis process. Courtesy U.S. Agency for International Development, (a) Many of the substances which make up the total dissolved soHds in brackish water are strong electrolytes. When dissolved in water, they ionize ie, the compounds dissociate into ions which carry an electric charge. Typical of the ions in brackish water are Cl ,, HCO3, , and. These ions tend to attract the dipolar water molecules...

Active carbonyl compounds such as benzaldehyde attack the electron-rich double bond in DTDAFs to give a dipolar adduct, which immediately undergoes dissociation with formation of two molecules of 146 (64BSF2857 67LA155).Tlie existence of by-products such as benzoin led to the synthetic application of thiazolium salts in the acyloin condensation. For example, replacement of the classic cyanide ion by 3-benzyl-4-methyl-5(/3-hydroxyethyl) thiazolium salts allowed the benzoin-type condensation to take place in nonaqueous solvents (76AGE639) (Scheme 57). 

Figure 10.6 Dipolar or zwitterionic form of an amino acid. Amino acids exist in a charged form in aqueous solution, the carboxyl group being dissociated and the amino group associated. Some amino acids also have an extra ionizable group present in their side chain (R group). The ionization of each group is pH-dependent and for each amino acid there is a pH at which the charges are equal and opposite and the molecule bears no net charge. This is called the iso-ionic pH (p/).

The convolution analysis is based on the use of convolution data and further manipulation to obtain information on the ET mechanism, standard potentials, intrinsic barriers, and also to detect mechanism transitions. It is worth noting that the general outlines of the methodology were first introduced in the study of the kinetics of reduction of terf-nitrobutane in dipolar aprotic solvents, under conditions of chemical stability of the generated anion radical. For the study of concerted dissociative ET processes, linear scan voltammetry is the most useful electrochemical technique. 

Izutzu, K. (1990). Acid-Base Dissociation Constants in Dipolar Aprotic Solvents, Chemical Data Series No. 35 Blackwell Scientific Publications, Oxford... 

Tetracycline antibiotics are closely related derivatives of the polycyclic naphtha-cenecarboxamide. They are amphoteric compounds with characteristic dissociation constants corresponding to the acidic hydroxyl group at position 3 (pK about 3.3), die dimethylamino group at position 4 (pK, about 7.5), and the hydroxyl group at position 12 (pK about 9.4). In aqueous solutions of pH 4-7, tetracyclines exist as dipolar ions, but as the pH increases to 8-9 marked dissociation of the dimethylamine cation occurs. They are soluble in acids, bases, and alcohols but are quite insoluble in organic solvents such as chloroform. Their ultraviolet spectra show strong absorption at around 270 and 360 nm in neutral and acidic solutions. Tetracyclines are readily transformed into fluorescent products in the presence of metal ions or under alkaline conditions. 

Water has high permittivity and moderate acidity and basicity. Thus, in water, many cations and anions are easily solvated (hydrated) and many electrolytes are highly soluble and dissociate into ions. Water has fairly wide pH and potential ranges and a convenient liquid temperature range. Of course, water is an excellent solvent. However, as in Table 1.7, the reaction environment can be expanded much wider than in water by use of a solvent of weak acidity and/or basicity. This is the reason why dipolar aprotic solvents, which are either protophilic or protophobic, are used in a variety of ways in modern chemistry. 

The characteristics of acid-base reactions in dipolar aprotic solvents, compared to those in dipolar amphiprotic solvents, are the easy occurrence of homo- and heteroconjugation reactions [2, 3, 5]. However, before discussing the homo- and heteroconjugations, we first discuss the solvent effects on the acid dissociation constants in dipolar aprotic solvents. 

From Izutsu, K. Acid-base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Science, Oxford 1990, and others. 

In dipolar aprotic solvents, the dissociation process of some acids (HA) is complicated by homo- and heteroconjugation reactions. In the homoconjugation reaction, some part of the A ions formed by the dissociation of HA [Eq. (3.18)] reacts with undissociated HA to form HA2, as shown by Eq. (3.19) ... 

Potentiometry with a glass electrode is simple and convenient. We can use it to study the dissociation equilibria of very weak acids, for which spectrophotometry and conductimetry are inapplicable. Many data for acid dissociations in dipolar... 

As described in Section 4.1.2, electrophilic organic compounds are reducible at the electrode. Some reducible organic compounds are listed in Table 8.5 with the potentials of the first reduction step in dipolar aprotic solvents. As described in Ref. [47], organic compotinds undergo various complicated electrode reductions. Here, however, only simple but typical cases are considered they are reductions of the outer sphere type and the dissociative electron transfer reactions. 

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