Methanol solvation effect

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

B3LYP/6-31G //HF/6-31G energies, including aqueous solvation effects, predicted this reaction to be exothermic by 5.1 kcalmoG with an a of just 4.4 kcalmoG, which is lower than the experimental values for substitution by iV-methylanilme in methanol With the likely degree of charge separation in the transition state, it is reasonable to suppose that a in aqueous solution (s = 80) would be lower than the in methanol (8 = 33). 

Aromoline and thalicberine have been isolated from Thalictrum lucidum,51 and berbamine and oxyacanthine from Berberis julianeae Schneid.52 The biosynthesis of bisbenzylisoquinoline alkaloids has been reviewed.53 An X-ray structure determination of ( + )-tubocurarine dibromide methanol solvate has been reported,54 and neuromuscular sensitivity to tubocurarine55 and the cardiovascular effects of the alkaloid56 have been further studied. Highly selective biological AT-demethylation of tetrandrine to iV(2 )-nortetrandrine by Streptomyces griseus has been described.29... 

MDH active site models were geometry minimized at the BLYP/DNP level with no constrains, and further tested upon methanol oxidation as explained in the following Section. The reaction mechanisms are tested in gas-phase only for Model A, since the inclusion of solvation effects (for protein environment) generally creates a pronounced effect on the calculated energetics for this model as observed from the literature because of the incomplete representation of the first-coordination shell of Ca. Model B, which considers the complete coordination sphere of the ion, was used for testing the mechanism with and without the pres-... 

Table 59 presents activation parameters for the decarboxylation of trichloroacetic acid in various basic solvents. Presumably the acid is in the form of its anion in these solvents. The activation parameters fall into a fairly narrow range and the differences presumably represent specific solvation effects. In an acidic solvent, decanoic acid, the activation parameters for the decomposition of potassium trichloroacetate are increased considerably. The values are A/f = 41.4 kcal.mole" and A5 = 27.7 eu . The activation parameters presumably reflect a composite of a prior equilibrium between decanoic acid and the trichloroacetate anion along with decarboxylation of the latter anion. The rate of decarboxylation of sodium nitroacetate is about five times faster in methanol than in water . This effect was attributed to dispersion of the negative charge at the transition state , a process which is more favorable in the less polar methanol solvent. Similarly, the decar-... 

It is a serious drawback that it is not possible to determine the transfer activity coefficient of the proton (or of any other single-ion species) directly by thermodynamic methods, because only the values for both the proton and its counterion are obtained. Therefore, approximation methods are used to separate the medium effect on the proton. One is based on the simple sphere-in-continuum model of Born, calculating the electrostatic contribution of the Gibb s free energy of transfer. This approach is clearly too weak, because it does not consider solvation effects. Different ex-trathermodynamic approximation methods, unfortunately, lead not only to different values of the medium effect but also to different signs in some cases. Some examples are given in the following log yH+ for methanol -1-1.7 (standard deviation 0.4) ethanol -1-2.5 (1.8), n-butanol -t-2.3 (2.0), dimethyl sulfoxide -3.6 (2.0), acetonitrile -1-4.3 (1.5), formic acid -1-7.9 (1.7), NH3 -16. From these data, it can be seen that methanol has about the same basicity as water the other alcohols are less basic, as is acetonitrile. Di-... 

Some nucleophilic tendencies towards carbon are shown in Table 21. They vary by a factor of only ca. 10 within any one solvent, from the least reactive (2,4-dinitrophenoxide in MeOH) to the most reactive (e.g. CeHsS" in MeOH). If, as shown, solvation effects can produce changes of 10 in nucleophilic tendencies then it is clearly pointless, unless solvent is specified and its effect taken into account, to discuss rate data, for reactions in methanol and in other protic solvents, in terms of intrinsic properties of the nucleophile, such as structure, charge type, polarizability, hardness and softness, size, a-e fects, ability to adjust valence shells to transition state requirements, bond strength, and so on. Solvation of the nucleophile is a major factor in determining nucleophilic tendencies. 

Table IV compares for a series of dienes the yields of 1,2 addition products obtained with Rh(NBD)(dppe)+ as the catalyst precursor under intercalated and homogeneous reaction conditions. The yields of terminal olefins are consistently higher for the intercalated catalyst. The deviation from solution yields are larger when the intercalated catalyst is solvated with methanol than with acetone.0 Methanol swells the interlayers to an average thickness of 12 A, whereas acetone swells the interlayers to w 15 A. Since the more constricted methanol solvated interlayers provide the higher yields of terminal olefins, spacial factors as well as polarization effects induced by the charged silicate sheets may be contributing to the deviations from solution behavior. In this reaction system polarization effects may well be more important than spacial factors in directing hydrogenation transfer because the spacial requirements of the transition states derived from or r 3 allyl intermediates should be very similar.

Extensive studies have been made of solvent effects on atom transfer reactions involving ions [12]. In the case of reaction (7.3.23), the rate constant decreases from 250M s in A-methylpyrrolidinone to 3 x 10 M s in methanol. This effect can be attributed to solvation of the anionic reactant Cl and the anionic transition state [12]. Since the reactant is monoatomic, its solvation is much more important. It increases significantly with solvent acidity leading to considerable stabilization of the reactants. As a result the potential energy barrier increases and the rate decreases with increase in solvent acidity. As shown in fig. 7.7, this leads to an approximate linear relationship between the logarithm of the rate constant and the solvent s acceptor number AN, an empirical measure of solvent acidity (see section 4.9). Most of the results were obtained in aprotic solvents which have lower values of AN. The three data points at higher values of AN are for protic solvents. 

For a toluene/methanol solution, figure 6, the hyperfine structure is well resolved, but the spectrum is broadened as a result of dipole-dipole interactions. As with the THF solution, both isolated and associated ions are present, but in toluene/-methanol the isolated species is predominant. Figure 6 compares the solution spectra of a 1.15 mole % Mh-SPS in toluene with various concentrations of methanol co-solvent. These materials are insoluble in toluene and some methanol is needed in order to dissolve them. At the lowest methanol concentration studied, 2% (v/v), a single broad line spectrum was observed. At higher concentrations of methanol, the six hyperfine structure became evident. These results indicate that the methanol is effective at solvating the cations, which is consistent with the conclusion made... 

The polarizable continuum model (PCM) by Tomasi and coworkers [77-79] was selected to describe the effects of solvent, because it was used to successfully investigate the effect of solvent upon the energetics and equilibria of other small molecular systems. The PCM method has been described in detail [80]. The solvents and dielectric constants used were benzene (s = 2.25), methylene chloride (g = 8.93), methanol (g = 32.0), and water (g = 78.4). Full geometry optimizations were carried out for the discrete and PCM models. To simultaneously account for localized hydrogen bonding and bulk solvation effects, PCM single-point energy calculations have been conducted on stationary points of the acrolein and butadiene reaction with two waters explicitly... 

In the first reaction, intramolecular attack by the hydroxyl on the carboxyl carbonyl proceeds 5 X 108 times faster than the corresponding inter-molecular process (2). In the second reaction, the rate increases 107-fold upon switching the solvent from methanol to dimethylformamide (3). Obviously, the huge rate increases in these organic systems do not necessarily prove that similar effects are at work in enzymes. But to be suspicious is quite natural, and many people, too numerous to mention, have pointed out the possible relationship between enzyme catalysis and intramolecularity or solvation effects. 

---