Ion pairing in acetonitrile

The photochemistry of diphenyl- and bis(4-methylphenyl)iodonium salts has been investigated481,482. Diphenyliodonium halides (140, X = Cl, Br, I) exist as tight ion pairs in acetonitrile. Their photolysis gives almost exclusively iodobenzene by a homolytic cleavage from a charge-transfer excited state. In aqueous acetonitrile, however, the ion pairs are solvent-separated and substantial amounts of 2-, 3- and 4-iodobiphenyls (141) are formed in addition to iodobenzene (142), benzene (143), acetanilide (144) and biphenyl (145) (equation 126). In this medium the photodecomposition occurs via initial heterolysis of the molecule in its excited state, leading to iodobenzene and phenyl cation. 

Several mechanisms for the catalytic action of Cu(I) and Ag(I) have been considered6. Among these, the metal-assisted addition-elimination sequence shown in equation 85 and illustrated with cuprous triflate was deemed most consistent with various control studies. A mechanism not discussed but equally plausible is the metal-assisted MC sequence depicted in equation 86. The greater separation of iodonium-sulfonate ion pairs in acetonitrile versus benzene should provide the tosylate (or mesylate) ions with sufficient mobility to add to the /7-carbon atom of the alkynyliodonium ion. 

Fig. 4. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile. In all cases cyanoanthracenes served as electron acceptors in their excited states, (a) Methylated benzene derivatives [60b] as donors (one-ring compounds), V = 11.5 cm"1, As = 1.63 eV. (b) Methylated biphenyls or naphthalenes [60b] as donors (two-ring compounds), V = 8.5 cm-1, /, = 1.48 eV. (c) Methylated phenanthrenes [60b] as donors (three-ring compounds) V = 8.0 cm-1, Xs = 1.40 eV. (dl Diphenylbutadienes [61] as donors, V = 8 cm-, X, = 1.55 eV. In all cases X, = 0.25 eV and V= 1500 cm-1...

Fig. 5. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile (a) 9,10-Dicyanoanthracene in its excited state served as the acceptor. Aryl, alkyl, methoxy and amino benzene derivatives as well as aliphatic amines served as donors [62] (V = 23 cm-1, 2, = 0.97 eV, Aj = 0.64 eV). (b) Perylene, pyrene, benzperylene, and aromatic amines served as donors. Tetracyanoethylene (TCNE), pyromellitic dianhydride (PMDA), phthalic anhydride (PA), maleic anhydride, pyrene and perylene served as electron acceptors [63], Various combinations of donors or acceptors were excited (V = 20 cm , As = 1.45 eV, A, = 0.07 eV). The parabolas drawn are different from those offered in the original analysis. The parameters that were used were selected to emphasize the similarity to Fig. 4 (in all cases v = 1500 cm-1)...

Cations also affect other solvent bands for AcN including the combination band at 2293 cm (V3 -I-V4), the C —C stretching mode at 918 cm (V4), and the C —C=N deformation mode at 746 cm (2vg). Changes in the intensity of these bands with electrolyte concentration may also be used to assess ion pairing in acetonitrile solutions [35, 36]. 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

Fig. 3. Bimolecular electron transfer of photoexcited donors and acceptors. The parabolas (a) and (i>) represent Eq. 1 with l = 0.42 eV. Curve (a) is flattened at the top due to the diffusional limit [27]. The Rehm-Weller equations (Eq. 6) with identical A is presented by lines (e) and (/), where the former includes a diffusional limit of kd = 2 x 1010 M 1 s"The filled circles represent the Rehm-Weller data for neutral aromatic donors and acceptors in acetonitrile [25]. The squares denote similar data for inorganic (charged) acceptors and organic (neutral) donors also in acetonitrile [28], The data are fitted into Eq. 6 with X = 0.69 eV and kd = 9 x 109 M 1 s "1 (line d). The open squares represent the forward ET to the excited inorganic complexes the filled squares depict the bimolecular BET within the photogenerated ion pairs of the same systems [28], The triangles represent forward electron transfer between organic borates [29] and cyanines (filled triangles) or pyrylium cations (open triangles) within contact ion pairs in benzene. Even in this case, without diffusional interference, the data seem to fit better the Rehm-Weller equation (g) than the Marcus equation (c)...

A. Tilly Melin, Y. Askemark, K.-G. Wahlund, and G. Schill, Retention behaior of carboxylic acids and their quaternary ammonium ion pairs in reversed phase chromatography with acetonitrile as organic modifier in the mobile phase, Anal. Chem. 51 (1979), 976-983. 

Tropenyl Chloride. Tropenyl chloride is thought to exist completely in the form of an ion pair in methylene chloride (69) (dielectric constant of 8.9 at 25°C) (75). Charge-transfer spectra of some tropenyl halides (69) are shown in Figure 9. In a solvent such as acetonitrile (dielectric constant of 37.5 at 20°C) (75), these absorptions are absent, and the tropenyl halides are assumed to be dissociated into solvent-separated ion pairs which are in equilibrium with free ions (as determined by conductance measurements) (69). Thermodynamic data (Appendix 2) allow the following gas-phase energy change (between tropenylium cation and cycloheptatriene) to be estimated ... 

They also follow expected effects of added ions (e.g. Li N03") via the reduction of the water activity ("salting-out effect"). The apparent discrepancy comes from our choice to model uranyl by its neutral U02(N03)2 salt instead of U02, in order allow for its possible extraction. In ct, it is very difficult to predict the status of ion pairs in pure solutions (see e.g. ref (53) for lanthanide salts in acetonitrile) and, a fortiori, at liquid-liquid interfaces. The formation of neutral salts may also be too slow to occur at the simulated time scales. Thus, this facet cannot be addressed by our simulations. In this context, the spontaneous formation of uranyl complexes with TBP is remarkable, and points to the importance of (micro)-heterogeneities of the systems and metastable conditions to form the complexes and promote their extraction to organic or supercritical phases. 

Since it has been demonstrated by eonductimetry that both K" " and BU4N phen-oxides are in the form of ion pairs in dioxane and are largely dissociated in acetonitrile [33], the inereased reaetivity of ammonium phenoxide in dioxane can be ascribed to the greater interionie distanee. 

Photoreactions of [Ru(bipy)3]Cl2 and of [Ru(phen)3]Cl2 in acetonitrile proceed more slowly as pressure increases. The activation volumes increase dramatically with temperature, from +12 and +9 cm mol respectively at 288 K to +22 and +27 at 333 K. These increases are ascribed to a key role for chloride ion pairs in these dissociatively activated reactions. Rate constants for base-catalyzed isomerization of the y to the p form of [Ru(azpy)2Cl2], azpy = (22), are not linear in hydroxide concentration, prompting speculation on the detailed role of hydroxide in this conversion.Rate constants for replacement of coordinated water in cis- and tra 5-[Ru(LL)2X(OH2)]" by acetonitrile span a range of nearly 2 X 10" times. Several factors have to be invoked to explain the observed trend in the effects of ligand X on the ease of replacement of the aqua-ligand. " The... 

Conductivity measurements demonstrated that both K" and BiVjN" phenoxides exist practically as an ion pair in dioxane while they are largely dissociated in acetonitrile Therefore, the fact that quaternary ammoniiun phenoxide in dioxane is more reactive than the corresponding potassium salt is mainly due to the greater interionic distance and, accordingly, to the lower cation-anion interaction energy. 

In the case of As, only one /( As- H) (555 Hz) value related to the (AsH4) cation, has been reported. The Avi/2 values for arsenic compounds, im-like As chemical shifts, are generally strongly dependent on the type of solvent employed, partly surely owing to ion pairing in fact, the linewidths for (AsEt4) Br are reported to be 168, 455, 670, and 700 Fiz in water, acetonitrile, dimethylformammide, and dimethylsulfoxide, respectively. 

The complications due to ion pairing in non-aqueous solvents can be avoided to some extent by the use of dipolar aprotic solvents which have been much investigated recently. These solvents are unable to act as hydrogen bond donors (i.e., they have at most very weak acid properties), but they have large dipole moments and polarizabilities, and hence moderately high dielectric constants. Examples are acetonitrile (e = 38), dimethyl sulphoxide (s = 49), sulpholane s = 38), nitrobenzene (e = 35), nitromethane (6 = 36), dimethylformamide (e = 38), and propylene carbonate (6 = 65). Some of them can act as bases and hydrogen bond acceptors, but this is not true of nitrobenzene and nitromethane. Unlike the solvents which we have considered so far, they show little specific tendency to solvate anions, which has important consequences for the reactivity of many anions towards organic compounds. ... 

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