Solvent stabilization effect

Such solvent stabilization effects on the reactivity of singlet carbenes in equilibrium with their triplet ground states have also been observed experimentally in other arylcarbene derivatives. ... 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

The reason the adamantyl system is much more sensitive to the substitutions of CH3 for H is that its cage structure prevents solvent participation whereas the i-propyl system has much stronger solvent participation. The internal stabilizing effect of the methyl substituent is therefore more important in the adamantyl system. 

Saturated hydrocarbons show a slight stabilizing effect on the initial state but a very destabilizing effect on the transition state, consistent with arguments based on solvent polarity. 

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

Iborra and co-workers (Entry 8) examined the transesterification of N-acetyl-i-tyrosine ethyl ester in different ionic liquids and compared their stabilizing effect relative to that found with 1-propanol as solvent [36]. Despite the fact that the enzyme activity in the ionic liquids tested reached only 10 to 50 % of the value in 1-propanol, the increased stability resulted in higher final product concentrations. Fixed water contents were used in both studies. 

A further important feature of HMPA is its stabilizing effect on the Redox potential of [Fe(CO)4]2 by ion solvation. In less polar solvents, electron-transfer reactions take place and [Fe(CO)4]2 is oxidized to [HFe3(CO)iThis redox reaction is suppressed in HMPA. 

Lozano and co-workers reported an interesting stabilization effect of IL for lipase-catalyzed reaction the authors discovered that the presence of an appropriate substrate was essential for stabilization of enzyme in an IL solvent. The half lifetime of native CAL was only 3.2 h in [emim][PFg] solvent, while it lengthened remarkably to 7500 h in the presence of the substrate. The authors succeeded in demonstrating an efficient lipase-recyclable use system based on SCCO2 solvent (Fig. 9). - ... 

In the reaction, it was essential to use an IL as a co-solvent. Lozano, Iborra and co-workers recently reported an interesting stabilizing effect of two types of water-immiscible ILs ([emim][TFSI] and [BuMe3N][TFSI]) for CAL-B-catalyzed transesterification of vinyl butyrate. The synthetic activity and the stability of the enzyme in these IL solvent systems were markedly enhanced as compared to those in hexane. CAL-B maintained its activity higher than 75% after 4 days of incubation in [emim][TFSI] solvent, while it showed an activity of only 25% when incubated in both water and hexane media at 50°C. Comparison of the ratio of a-helix and (3-sheet by CD spectra showed the activity was closely related with a-helix content which reduced to 31% immediately after lipase was added to hexane and had reached only 2% after 4 days in hexane. On the contrary, no significant reduction of a-helix content was... 

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

One method of overcoming the detrimental solvent dewetting effects is to use liquid C02 as the solvent for nanoparticle dispersions [52], since C02 does not experience the dewetting instabilities due to its extremely low surface tension [53]. In this case, nanoparticles must be stabilized with fluorinated ligands [30, 33, 54—65] or other C02-philic ligands [60,66-76], such that they will disperse in the C02 prior to dropcasting. These fluorinated ligands tend be toxic and environmentally persistent and, typically, only very small nanoparticles can be dispersed at low concentrations. 

The apparent lowering of the rotational barrier in triafulvenes is open to interpretation either by substituent or solvent stabilization of ground-state polarity leading to a decrease of C3/C4 double bond character or by stabilization of a more polar - probably perpendicularly orientated184 — transition state by substituent or solvent effects. 

To test the validity of this mechanism, chromium carbonyl (1.0 g) was photolyzed under Ar at ambient temperature in a solution of methanol and hexamethylphosphoramide in the apparatus shown in Figure 5. The lamp was turned off periodically to check for the disappearance of slightly soluble Cr(C0) . Several photolyzing cycles were necessary to effect nearly complete conversion to the solvent-stabilized coordinately unsaturated species (equivalent to in Figure 4),... 

The intramolecular 1,2-H shifts of alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example the ku values of Me2CHCCl, PhCHMeCCl, and EtCCl exceed 108 s 1 in hydrocarbon solvents at 25°C (Table 4).60 86 87 However, due to the stabilizing effect of the oxa spectator substituent, acetoxycarbenes react at much reduced rates relative to their chlorocarbene analogues,90,91 thus providing kinetically accessible results for a wide array of bystander-substituted alkylacetoxycarbenes.81 92... 

Convincing evidence was found that the majority of acyclic aldo-nitrones exist in the Z-form, by investigating the ASIS-effect (aromatic solvent induced shift effect) (399). However, in some cases, specified by structural factors and solvent, the presence of both isomers has been revealed. Thus, in C -acyl-nitrones the existence of Z -and -isomers was detected. Their ratio appears to be heavily dependant on the solvent polar solvents stabilize Z-isomers and nonpolar, E-isomers (399). A similar situation was observed in a- methoxy-A-tert-butylnitrones. In acetone, the more polar Z-isomer was observed, whereas in chloroform, the less polar E-isomer prevailed. The isomer assignments were made on the basis of the Nuclear Overhauser Effect (NOE) (398). /Z-Isomerization of acylnitrones can occur upon treatment with Lewis acids, such as, MgBr2 (397). Another reason for isomerization is free rotation with respect to the C-N bond in adduct (218) resulting from the reversible addition of MeOH to the C=N bond (Scheme 2.74). The increase of the electron acceptor character of the substituent contributes to the process (135). 

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

In the Onsager s SCRF model, the solute is placed in a cavity immersed in a continuous medium with a dielectric constant e. The molecular dipole of the solute induces a dipole in the solvent, which in turn interacts with the molecular dipole, leading to a net stabilization effect. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

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