Activation parameters determination

Addition-elimination (for the chloro compound) and elimination-addition (via an intermediate haloalkyne, for the bromo and iodo compounds) mechanisms account for the activation parameters determined for reaction of 2-(/3,/3-dihalovinyl)-5-nitrothiophenes with MeONa-MeOH. °°... 

Kinetic studies of the trifluoromethylation of aniline with 5-salt 17, dinitro 5-salt 39, and nonheterocyclic 5-salt 29 were carried out and the activation parameters determined (Table I). 

Activation parameters determined for CgHsCCl insertions into 44 and 37 (in benzene solution) were = 2.9 and 3.2 kcal/mol, with AS = —25.6 and —24.2 e.u., respectively. These activation energies are slightly greater than those observed in additions of CeHsCCl to alkenes, but the very negative entropies of activation are comparable to those observed in CeHsCCl additions, and reflect the entropy decrease that occurs when two reactant molecules combine to form a single product molecule. ... 

The [MH(PF3)J complexes (M = Co, Rh, Ir) and the isoelectronic species [M H(PF3)4] (M = Fe, Ru, Os) are all stereochemically non-rigid, and detailed multinuclear NMR spectroscopic studies have been carried out and activation parameters determined. It has been proposed that the phosphorus environments are interchanged by a tetrahedral tunnelling rearrangement mechanism. Some typical temperature-dependent and 19F NMR spectra are shown in Fig. 7. 

Stock-Schweyer et al. [112] reported a high-pressure effect on molecular motions in the paraelectric phase of a (70/30) VDF and TrFE copolymer. Fluorine-19 NMR relaxation times (Ti and Tjp) were studied over a range of pressures from 0.1 to 200 MPa. Correlation times of the molecular motions, as functions of pressure and temperature, were obtained and the activation parameters determined. The experimental data confirmed the presence of a slow motion in the amorphous phase in addition to the fast anisotropic motion. The results indicated that the relaxation times of the copolymer are controlled by the effects of both temperature and volume. The authors concluded that 40-50% of the mobility increase of segments with increasing temperature under constant pressure results from volume expansion. 

A limited number of mechanistic studies of stoichiometric amide hydrolysis reactions promoted by Zn(II) complexes have been reported. Groves and Chambers reported studies of the zinc-mediated hydrolysis of an internal amide substrate wherein amide carbonyl coordination to the zinc center is not possible (Scheme 15).93 Examination of the rate of this reaction as a function of pH (6.5-10.5) yielded a kinetic pKa = 9.16. First-order rate constants at 70 °C (/i = 0.5 (NaC104)) for the Zn-OH2 and Zn-OH-mediated amide hydrolysis reactions differed by a factor of 100 when the data was fit to Equation (1) for the proposed mechanism shown in Scheme 16. The activation parameters determined for this reaction (Ai7 = 22(1) kcal mol-1 and AS = — 18(3) eu) are consistent with an intramolecular hydrolysis process wherein a zinc-bound hydroxide acts as an intramolecular nucleophile to attack the amide carbonyl in the ratedetermining step. 

Such work can be performed by either pulsed laser [32-37] or pulse radiolytic [38] excitation of the sensitizer molecule. Of particular importance [39] has been the very significant increase in the accuracy of activation parameter determinations that this development has allowed. 

Satterlee etal extended these studies to characterize the axial lability of various imidazoles and pyridines in bis-ligated low-spin Fe complexes of synthetic porphyrins, including tetraphenyl-, octaethyl-, and tetra-n-propylporphyrins. For L = A -methylimidazole, typical rate constants k are 35-100s for TPPFeCl and T-n-PrPFeCl, and 950 s for OEPFeCl typical activation parameters determined in this study are = 17-20kcalmol = 15 —19 J mol More recently it has been shown by... 

NO rate constants and activation parameters determined for nitric oxide binding to a five-coordinate monohydroxido species Fe (TMPS)(OH) (formed at pH > 6.9) (12) appear to be significandy smaller than those found for the (TMPS)Fe(H20)2 species (Table 1), clearly showing that NO coordination to Fe (TMPS)(OH) can no longer be controlled by the lability or accessibihty of the iron(III) center. In contrast to the mechanism of NO binding to iron(III) diaqua-ligated species, nitric oxide coordination to... 

In view of the finding that some ferriheme proteins in insects (37) are able to carry nitric oxide, a NO carrier role for the metmyogiobin in vivo can be considered. As was shown, NO binds reversibly to the six-coordinate, high-spin iron(III) center of metmyogiobin (38) possessing a water and histidine molecule as distal and proximal ligand, respectively. Activation parameters determined for NO interaction with metMb (Table 4) clearly... 

The reactivity pattern of the second reaction step observed at pH >3, fe2(obs). is very similar to that observed for the reaction between the [Co(P)(N02 )(H20)] complex and NO under similar conditions. Importantly, the observed rate constant for this reaction does not depend significandy on the NO2 concentration, but it depends linearly on the NO concentration. This finding together with the dissociative nature of the activation parameters determined for this reaction leads to the conclusion that in the presence of NO the [Co(P)(N02 )(H20)] complex must exist in equilibrium with small amounts of the [Co(P)(N02 )(NO)] intermediate (see lower part of Scheme 12). The last intermediate slowly decomposes through an inner-sphere electron-transfer reaction to generate the final products [Co (P) (NO )] and NO2. Since the latter reaction represents the rate-determining step for the second reaction, the proposed mechanism seems to be consistent with the observed NO and N02 concentration dependences, as well as with the activation parameters determined in this study (126). 

Since the cis isomer cyclizes to the cyclohexadiene with an activation energy roughly 12 kcal/mol lower (see Section 3.5), it is reasonable to assume that the activation parameters determined are those for the geometric isomerization recognizing that only the cis isomer can cyclize. The activation volume for the cyclization of the cis material is -9.8 cc/mol. ... 

Reactivity trends for D substitution at [Fe(CN)sL] anions will depend on solvent effects on the starting anion and on the separating moieties [Fe(CN)s] and L (L ) in the transition state. The activation parameters determined for a series of anions of this type with L = ammonia, amine, or aniline have been held to indicate the importance of transfer of the leaving group L to the solvent, an importance seemingly not recognized in some other studies. Small differences in reactivity between various complexes with L=NH2(CH2) NH2 or NHa-(CH2) NH3+, 3< <6, have also been attributed to solvation effects at the... 

A comparison of the activation parameters determined by two different methods is indicative of their satisfactory agreement. For example, the values of the free activation energy (AG ) are virtually the same within the deviations found. This is not surprising, however, since is slightly affected by the systematic errors in contrastto AS and A/7 [16, p. 108]. Analysis of our data shows that both methods ( F DNMR and DNMR) lead to comparable results and can be used independently of one another when studying the dynamic behavior of fluoro oximato complexes. 

Activation parameters determined at / = 0.4 mol Reductant is Reductant is [Cr(phen)3] +. Reductant isy,y -bipyridylchromium(ii) complex. 

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