Eyring Analysis Research Articles

Molecular friction dissipation and mode coupling in organic monolayers and polymer films

The impact of thermally active molecular rotational and translational relaxation modes on the friction dissipation process involving smooth nano-asperity contacts has been studied by atomic force microscopy, using the widely known Eyring analysis and a recently introduced method, dubbed intrinsic friction analysis. Two distinctly different model systems, i.e., monolayers of octadecyl-phosphonic acid (ODPA) and thin films of poly(tert-butyl acrylate) (PtBA) were investigated regarding shear-rate critical dissipation phenomena originating from diverging mode coupling behaviors between the external shear perturbation and the internal molecular modes of relaxation. Rapidly (ODPA) versus slowly (PtBA) relaxing systems, in comparison to the sliding rate, revealed monotonous logarithmic and nonmonotonous spectral shear rate dependences, respectively. Shear coupled, enthalpic activation energies of 46 kJ∕mol for ODPA and of 35 and ∼65 kJ∕mol for PtBA (below and above the glass transition) were found that could be attributed to intrinsic modes of relaxations. Also, entropic energies involved in the cooperative backbone mobility of PtBA could be quantified, dwarfing the activation energy by more than a factor of five. This study provides (i) a material specific understanding of the molecular scale dissipation process in shear compliant substances, (ii) analyses of material intrinsic shear-rate mode coupling, shear coordination and energetics, (iii) a verification of Eyring's model applied to tribological systems toward material intrinsic specificity, and (iv) a valuable extension of the Eyring analysis for complex macromolecular systems that are slowly relaxing, and thus, exhibit shear-rate mode coupling.

Kinetic study of the reaction of dimethyl carbonate with trialkylamines

AbstractQuaternary ammonium compounds are produced worldwide in hundreds of millions of pound volume annually for a plethora of end‐uses from fabric‐care formulations to asphalt emulsifiers, typically from nongreen alkylating reagents. The kinetics of a reaction employing dimethyl carbonate (DMC) as a green alkylating agent was investigated using three trialkylamines (tributylamine, trihexylamine, and trioctylamine) at several temperatures. Arrhenius and Eyring analysis of the data showed that values of Ea (79 kJ/mol), ΔH‡ (75 kJ/mol), and ΔS‡ (220 J/(mol K)) were the same for all three amine reactants, consistent with a report that Ea is independent of alkyl chain length when the chain length is greater than three carbons. Although rates are significantly slower with DMC than with other alkylating reagents, the resulting methyl carbonate anion has advantages for clean anion metathesis, which is important for some applications, especially those involving ionic liquids. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 221–225, 2010

Detailed Study of C−O and C−C Bond-Forming Reductive Elimination from Stable C2N2O2−Ligated Palladium(IV) Complexes

This paper describes the synthesis of a series of Pd(IV) complexes of general structure (N~C)(2)Pd(IV)(O(2)CR)(2) (N~C = a rigid cyclometalated ligand; O(2)CR = carboxylate) by reaction of (N~C)(2)Pd(II) with PhI(O(2)CR)(2). The majority of these complexes undergo clean C-O bond-forming reductive elimination, and the mechanism of this process has been investigated. A variety of experiments, including Hammett plots, Eyring analysis, crossover studies, and investigations of the influence of solvent and additives, suggest that C-O bond-forming reductive elimination proceeds via initial carboxylate dissociation followed by C-O coupling from a 5-coordinate cationic Pd(IV) intermediate. The mechanism of competing C-C bond-forming reductive elimination from these complexes has also been investigated and is proposed to involve direct reductive elimination from the octahedral Pd(IV) centers.

C−H Bond Cleavage with Reductants: Re-Investigating the Reactivity of Monomeric MnIII/IV−Oxo Complexes and the Role of Oxo Ligand Basicity

The thermodynamic properties of structurally similar Mn(III) and Mn(IV) complexes have been reinvestigated to understand their reactivity with substrates having C-H bonds. The complexes have the general formula [MnH(3)buea(O)](n-), where [H(3)buea](3-) is the tripodal ligand, tris[(N'-tert-butylureaylato)-N-ethylene]aminato. These complexes are unique because of the intramolecular hydrogen-bonding (H-bond) network surrounding the Mn-oxo units. The redox potentials for the Mn(III/IV)(O) couple was incorrectly assigned in earlier reports: the corrected value is -1.0 V vs Cp(2)Fe(+)/Cp(2)Fe in DMSO, while the Mn(IV/V)(O) process is -0.076 under the same conditions. The oxo ligand in the Mn(III)(O) complexes is basic with a pK(a) of 28.3; the basicity of the terminal oxo ligand in the Mn(IV)(O) complex is estimated to be approximately 15. These values were used to re-evalulate the O-H bond dissociation energy (BDE(OH)) of the corresponding Mn(II/III)-OH complexes: BDE(OH) values of 89 and 77 kcal/mol were determined for [Mn(III)H(3)buea(OH)](-) and [Mn(II)H(3)buea(OH)](2-), respectively. Both Mn(O) complexes react with 9,10-dihydroanthracene (DHA) to produce anthracene in nearly quantitative yields. This is surprising based on the low redox potiental of the complexes, suggesting the basicity of the oxo ligand is a major contributor to the observed reactivity. In contrast to the thermodynamic results, a comparative kinetic investigation found that the Mn(III)(O) complex reacts nearly 20 times faster than the Mn(IV)(O) complex. Activation parameters, determined from an Eyring analysis, found that the entropy of activation is significantly different between the two systems (DeltaDeltaS(++) = -35 eu, where DeltaDeltaS(++) = DeltaS(++)(Mn(IV)(O)) - DeltaS(++)(Mn(III)(O)). This unusual kinetic behavior can be explained in the context of the basicity of the oxo ligands that leads to different mechanisms: for [Mn(III)H(3)buea(O)](2-) a proton transfer-electron transfer mechanism is proposed, whereas for [Mn(IV)H(3)buea(O)](-) a hydrogen-atom transfer pathway is likely.

Unprecedented rearrangement during the formation of P–P homoatomic N-phosphino formamidine complexes

A variety of homoatomic P–P donor–acceptor homoleptic (R = R′) and heteroleptic (R ≠ R′) N-phosphino formamidine complexes [ iPr 2N–C(H) N–PR 2–PR′ 2]Cl were synthesized from the addition of N-phosphino formamidine (phosfam) donor reagent iPr 2N–C(H) N–PR 2 on halogenophosphane compounds R′ 2PCl which are synthetic sources for the corresponding phosphenium derivatives R 2P +. We have demonstrated that the dynamic equilibrium observed between the different species is shifted either completely to the side of the free species or to the side of the donor–acceptor adduct [ iPr 2N–C(H) N–PPh 2–PPh 2]Cl by changing the solvent or by varying the temperature. Activation parameters of Δ S ≠ = (−130 ± 7.2) J mol −1 K −1, Δ H ≠ = (8.4 ± 0.6) kJ mol −1 and Δ G ≠ (298.15 K) = (53.6 ± 2.3) kJ mol −1 were determined by an Eyring analysis over the temperature range of 193–293 K. The negative entropy of activation is consistent with an associative pathway and the low value of Δ H ≠ suggests that the energy barrier for this reaction is entropically controlled. Phosphine–phosphenium adducts is the most appropriate term to describe the dynamic process observed at variable temperature for complexes [ iPr 2N–C(H) N–PR 2 → PR′ 2] +, but the 31P NMR chemical shift and the calculated electronic charges are more in favor of a phosphinophosphonium Lewis drawing [ iPr 2N–C(H) N–PR 2–PR′ 2] +. Formation of the homoatomic P–P heteroleptic formamidine complexes [ iPr 2N–C(H) NPR′ 2PR 2]Cl (R = Ph, R′ = Et, iPr) results in the formal insertion of the phosphino group of the corresponding alkyl chlorophosphanes R′ 2PCl into the N–P bond of the starting phosfam ligand iPr 2N–C(H) N–PR 2. Computed data are in agreement with the transient formation of a heteroatomic N–P intermediate [ iPr 2N–C(H) N(PR 2)PR′ 2]Cl, which then rearranges to the more thermodynamically favored homoatomic P–P compound [ iPr 2N–C(H) N–PR 2–PR′ 2]Cl.

Synthesis, Characterization, and cis–trans Isomerization Studies of cis‐[PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2‐P,P′}] and trans‐[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2‐P,P′}] Metallacrown Ethers

AbstractThe metallacrown ether cis‐[PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2‐P,P′}] (cis‐1) has been kinetically trapped via precipitation during the reaction of PdCl2 and Ph2P(CH2CH2O)3CH2CH2PPh2 in an acetonitrile/THF mixture. The kinetics of the cis–trans isomerization and oligomerization equilibria of cis‐1 have been followed using quantitative 31P{1H} NMR spectroscopy. The reaction follows reversible first‐order kinetics, and an Eyring analysis was carried out to yield ΔH‡f (81 ± 2 kJ mol–1), ΔH‡r (46 ± 2 kJ mol–1), ΔS‡f (117 ± 4 J mol–1 K–1), and ΔS‡r (20 ± 6 J mol–1 K–1) for the isomerization reaction. The ΔH‡f is consistent with a proposed associative transition state involving partial cleavage of a Pd–P bond and partial formation of Pd–O(ether) bond to an adjacent ether oxygen. For comparison, trans‐[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2‐P,P′}] (trans‐2) was isolated from a mixture of cis and trans isomers by column chromatography. An analysis of the isomerization process of trans‐2 to cis‐2 using quantitative 31P{1H} NMR spectroscopy revealed that no isomerization occurs at 348 K in the absence of an acid catalyst and that the acid‐catalyzed trans–cis isomerization is zero order in complex. Crystal structures of cis‐1, cis‐2, and trans‐2 are also presented, and the conformations of the metallacrown ether rings in these structures have been related to the data presented above. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Phosphaalkenes as Long-Lived Phosphorus Cluster Surface Functional Groups: Intramolecular P═C Addition to a Niobium-Supported P7 Cage

The diniobium octaphosphorus complex (P8)[Nb(OC[(2)Ad]Mes)3]2 (1) (Ad = adamantylidene, Mes = 2,4,6-Me3C6H2) contains a reactive niobium phosphinidene moiety that can be exploited for metathetical scission of the NbP bond. When 1 is treated with aryl ketones, loss of ONb(OC[(2)Ad]Mes)3(OEt2) (2) is observed along with concomitant formation of the corresponding phosphaalkene (RC6H4)2CPP7Nb(OC[(2)Ad]Mes)3 (3-R). Complexes 3-R rearrange to incorporate the (RC6H4)2CP unit into the phosphorus cage, thereby generating a saturated organo-phosphorus cluster complexed to the niobium tris-enolate platform, (RC6H4)2CP8Nb(OC[(2)Ad]Mes)3 (4-R). The structure of one such rearranged cluster 4-H, as determined by X-ray crystallography, is briefly discussed. An Eyring analysis of the first-order rearrangement of 3-H to 4-H gives activation parameters of DeltaH(double dagger) = 16.7 kcal/mol and DeltaS(double dagger) = -20.4 eu. A Hammett analysis of the phosphaalkene rearrangement, 3-R to 4-R, with substitution varying at the para positions of the aryl rings, reveals a linear relationship between the sigma values and the rearrangement rate constants. A concerted, asynchronous mechanism for the least-motion rearrangement of 3-H to 4-H is presented. When 1 is treated with alkyl ketones, similar loss of 2 and formation of the corresponding phosphaalkene is observed; however, the phosphaalkene complexes have considerably greater stability and are readily isolated.

P2 Addition to Terminal Phosphide M≡P Triple Bonds: A Rational Synthesis of cyclo-P3 Complexes

The diphosphaazide complex (Mes*NPP)Nb(N[Np]Ar)3 (Mes* = 2,4,6-tri-tert-butylphenyl, Np = neopentyl, Ar = 3,5-Me2C6H3), 1, has previously been reported to lose the P2 unit upon gentle heating, to form (Mes*N)Nb(N[Np]Ar)3, 2. The first-order activation parameters for this process have been estimated here using an Eyring analysis to have the values Delta H(double dagger) = 19.6(2) kcal/mol and Delta S(double dagger) = -14.2(5) eu. The eliminated P2 unit can be transferred to the terminal phosphide complexes P[triple bond]M(N[(i)Pr]Ar)3, 3-M (M = Mo, W), and [P[triple bond]Nb(N[Np]Ar)3](-), 3-Nb, to give the cyclo-P3 complexes (P3)M(N[(i)Pr]Ar)3 and [(P3)Nb(N[Np]Ar)3](-). These reactions represent the formal addition of a P[triple bond]P triple bond across a M[triple bond]P triple bond and are the first efficient transfers of the P2 unit to substrates present in stoichiometric quantities. The related complex (OC)5W(Mes*NPP)Nb(N[Np]Ar)3, 1-W(CO)5, was used to transfer the (P2)W(CO)5 unit in an analogous manner to the substrates 3-M (M = Mo, W, Nb) as well as to [(OC)5WP[triple bond]Nb(N[Np]Ar)3](-). The rate constants for the fragmentation of 1 and 1-W(CO)5 were unchanged in the presence of the terminal phosphide 3-Mo, supporting the hypothesis that molecular P2 and (P2)W(CO)5, respectively, are reactive intermediates. In a reaction related to the combination of P[triple bond]P and M[triple bond]P triple bonds, the phosphaalkyne AdC[triple bond]P (Ad = 1-adamantyl) was observed to react with 3-Mo to generate the cyclo-CP2 complex (AdCP2)Mo(N[(i)Pr]Ar)3. Reactions of the electrophiles Ph3SnCl, Mes*NPCl, and AdC(O)Cl with the anionic, nucleophilic complexes [(OC)5W(P3)Nb(N[Np]Ar)3](-) and [{(OC)5W}2(P3)Nb(N[Np]Ar)3](-) yielded coordinated eta(2)-triphosphirene ligands. The Mes*NPW(CO)5 group of one such product engages in a fluxional ring-migration process, according to NMR spectroscopic data. The structures of (OC)5W(P3)W(N[(i)Pr]Ar)3, [(Et2O)Na][{(OC)5W}2(P3)Nb(N[Np]Ar)3], (AdCP2)Mo(N[(i)Pr]Ar)3, (OC)5W(Ph3SnP3)Nb(N[Np]Ar)3, Mes*NP(W(CO)5)P3Nb(N[Np]Ar)3, and {(OC)5W}2AdC(O)P3Nb(N[Np]Ar)3, as determined by X-ray crystallography, are discussed in detail.

Folding of a LysM Domain: Entropy-Enthalpy Compensation in the Transition State of an Ideal Two-state Folder

Protein-engineering methods (Φ-values) were used to investigate the folding transition state of a lysin motif (LysM) domain from Escherichia coli membrane-bound lytic murein transglycosylase D. This domain consists of just 48 structured residues in a symmetrical βααβ arrangement and is the smallest αβ protein yet investigated using these methods. An extensive mutational analysis revealed a highly robust folding pathway with no detectable transition state plasticity, indicating that LysM is an example of an ideal two-state folder. The pattern of Φ-values denotes a highly polarised transition state, with significant formation of the helices but no structure within the β-sheet. Remarkably, this transition state remains polarised after circularisation of the domain, and exhibits an identical Φ-value pattern; however, the interactions within the transition state are uniformly weaker in the circular variant. This observation is supported by results from an Eyring analysis of the folding rates of the two proteins. We propose that the folding pathway of LysM is dominated by enthalpic rather than entropic considerations, and suggest that the lower entropy cost of formation of the circular transition state is balanced, to some extent, by the lower enthalpy of contacts within this structure.

Mechanism of Reversible Alkyne Coupling at Zirconocene: Ancillary Ligand Effects

The mechanism of reversible alkyne coupling at zirconium was investigated by examination of the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings studied possess trimethylsilyl substituents in the alpha-positions, and the ancillary Cp2, Me2C(eta(5)-C5H4)2, and CpCp* (Cp* = eta(5)-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] with PMe3, to produce Cp2Zr(eta(2)-PhC[triple bond]CSiMe3)(PMe3) and free PhC[triple bond]CSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming phosphine (k(obs) = 1.4(2) x 10(-5) s(-1) at 22 degrees C), and the activation parameters determined by an Eyring analysis (DeltaH(double dagger) = 28(2) kcal mol(-1) and DeltaS(double dagger) = 14(4) eu) are consistent with a dissociative mechanism. The analogous reaction of the ansa-bridged complex Me2C(eta(5)-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] is 100 times faster than that for the corresponding Cp2 complex, while the corresponding CpCp* complex reacts 20 times slower than the Cp2 derivative. These rates appear to be largely influenced by the steric properties of the ancillary ligands.

Self‐Assembly of N2‐Modified Guanosine Derivatives: Formation of Discrete G‐Octamers

In the presence of Na(+) ions, two N(2)-modified guanosine derivatives, N(2)-(4-n-butylphenyl)-2',3',5'-O-triacetylguanosine (G1) and N(2)-(4-pyrenylphenyl)-2',3',5'-O-triacetylguanosine (G2), are found to self-associate into discrete octamers that contain two G-quartets and a central ion. In each octamer, all eight guanosine molecules are in a syn conformation and the two G-quartets are stacked in a tail-to-tail fashion. On the basis of NMR spectroscopic evidence, we hypothesize that the pi-pi-stacking interaction between the N(2)-side arms (phenyl in G1 and pyrenyl in G2) can considerably stabilize the octamer structure. For G1, we have used NMR spectroscopic saturation-transfer experiments to monitor the kinetic ligand exchange process between monomers and octamers in CD(3)CN. The results show that the activation energy (E(a)) of the ligand exchange process is 31 +/-5 kJ mol(-1). An Eyring analysis of the saturation transfer data yields the enthalpy and entropy of activation for the transition state: DeltaH(not =)=29 +/-5 kJ mol(-1) and DeltaS(not =)=-151 +/-10 J mol(-1) K(-1). These results are consistent with an associative mechanism for ligand exchange.

Electronic structure control of the nucleophilicity of transition metal-thiolate complexes: an experimental and theoretical study.

New metal(II)-thiolate complexes supported by the tetradentate ligand 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)py(2)) have been synthesized and subjected to physical, spectroscopic, structural, and computational characterization. The X-ray crystal structures of these complexes, [L(8)py(2)M(S-C(6)H(4)-p-CH(3))]BPh(4) (M = Co, Ni, Zn), reveal distorted square-pyramidal divalent metal ions with four equatorial nitrogen donors from L(8)py(2) and axial p-toluenethiolate ligands. The reactions of the complexes with benzyl bromide produce isolable metal(II)-bromide complexes (in the cases of Co and Ni) and the thioether benzyl-p-tolylsulfide. This reaction is characterized by a second-order rate law (nu = k(2)[L(8)py(2)M(SAr)(+)][PhCH(2)Br]) for all complexes (where M = Fe, Co, Ni, or Zn). Of particular significance is the disparity between k(2) for M = Fe and Co versus k(2) for M = Ni and Zn, in that k(2) for M = Ni and Zn is ca. 10 times larger (faster) than k(2) for M = Fe and Co. An Eyring analysis of k(2) for [L(8)py(2)Co(SAr)](+) and [L(8)py(2)Ni(SAr)](+) reveals that the reaction rate differences are not rooted in a change in mechanism, as the reactions of these complexes with benzyl bromide exhibit comparable activation parameters (M = Co: DeltaH() = 45(2) kJ mol(-)(1), DeltaS() = -144(6) J mol(-)(1) K(-)(1); M = Ni: DeltaH() = 43(3) kJ mol(-)(1), DeltaS() = -134(8) J mol(-)(1) K(-)(1)). Electronic structure calculations using density functional theory (DFT) reveal that the enhanced reaction rate for [L(8)py(2)Ni(SAr)](+) is rooted in a four-electron repulsion (or a "filled/filled interaction") between a completely filled nickel(II) d(pi) orbital and one of the two thiolate frontier orbitals, a condition that is absent in the Fe(II) and Co(II) complexes. The comparable reactivity of [L(8)py(2)Zn(SAr)](+) relative to that of [L(8)py(2)Ni(SAr)](+) arises from a highly ionic zinc(II)-thiolate bond that enhances the negative charge density on the thiolate sulfur. DFT calculations on putative thioether-coordinated intermediates reveal that the Co(II)- and Zn(II)-thioethers exhibit weaker M-S bonding than Ni(II). These combined results suggest that while Ni(II) may serve as a competent replacement for Zn(II) in alkyl group transfer enzymes, turnover may be limited by slow product release from the Ni(II) center.

An experimental and theoretical study on the substituent effect of the permanganate oxidation of styrenes.

The kinetic data obtained for the cycloadditions of the permanganate ion to a series of styrene derivatives in dichloromethane solution in the presence of a quaternary ammonium ion were examined with two theoretical approaches, on the assumption that the reactions proceed via a concerted [3 + 2] mechanism. The semi-quantitative frontier molecular orbital analysis of the kinetic data shows a linear free energy relationship with better correlation than the Hammett plot with a values when the point for p-NO2 group is omitted. Further examination of the results of the FMO analysis reveals that the deviation of the point for p-nitrostyrene is attributed to the transition structure being more reactant-like than that of the other derivatives. The plot of log k2 vs. -deltaG++ calculated by the density functional theory (Becke3LYP) follows a straight line with the desired correlation for all the substituents. A marked tendency was observed for the MO calculations to underestimate the deltaG++ value for electron-withdrawing substituents when the calculation was carried out excluding the quaternary ammonium ion. This inconsistency was much improved by the calculations incorporating the quaternary ammonium ion. The actual values of deltaG++ obtained from the Eyring analysis are in good agreement with those calculated at the B3LYP/6-311 +G(d,p)//B3LYP/ LanL2DZ level.

Catalytic Isomerization and Disproportionation of Olefins Promoted by Group 4/d0 Benzamidinate Complexes

The group 4/d0 benzamidinate complexes cis-[p-CH3-C6H4C(NSiMe3)2]2Zr(CH3)2 (1), C3-tris([N-trimethylsilyl][N‘-myrtanyl]benzamidinate)zirconium chloride (2) and cis-[p-CH3C6H4C(NSiMe3)2]2Ti(CH3)2 (3) were found to be active catalytic precursors, in the presence of MAO (methylalumoxane), for the isomerization of aliphatic olefins (1-octene, 2-E-octene, 3-methyl-1-butene, 2-methyl-1-butene, allylbenzene) and the disproportionation of 1,4- and 1,3-cyclohexadienes. The active species was found to be the cationic hydride complex obtained for aliphatic olefins by the insertion of the methyl ligand with concomitant β-hydrogen elimination, and for the alicyclic dienes by an activation of the allylic hydrogen and accompanying elimination of methane. Kinetic studies and Eyring analysis for the isomerization of allylbenzene show that the reaction is first order in both complex and olefin with ΔH† = 17.8(6) kcal·mol-1 and ΔS† = −25.1(2) eu. These findings support the epimerization mechanism proposed for these types of...

Transient kinetics and thermodynamics of anthroylouabain binding to Na/K-ATPase

The Na/K-ATPase is an integral membrane protein enzyme which uses energy derived from hydrolysis of ATP to pump Na + out of and K + into the cell. Ouabain belongs to a class of drugs known as cardiac glycosides, which are useful for treating congestive heart failure. Therapeutic value is achieved when these drugs bind to and inhibit the Na/K-ATPase of cardiac muscle. We gain insight into this important interaction by measuring the thermodynamics of the interaction of anthroylouabain (AO), a fluorescent derivative of ouabain, with the Na/K-ATPase. AO has the useful property that its fluorescence intensity is greatly enhanced (∼10×) when it binds to the enzyme. Using this enhancement, we measure temperature dependence of transient kinetics for the association and dissociation of AO interacting with membrane fragments of Na/K-ATPase purified from dog kidney. Using a standard Eyring analysis, we find that the overall association of AO with the enzyme is driven by substantial contributions from both enthalpy and entropy, and that in an energy diagram for the association pathway, the free energy change is quite similar to that of ouabain deduced from previously published results [E. Erdmann, W. Schoner, BBA 307 (1973) 386]. However, in the transition state, there are substantial differences for the enthalpy and entropy, presumably due to the presence of the anthracene moiety.

Tensile mechanical properties of PEEK films over a wide range of strain rates. II

Tensile mechanical properties of poly(aryl ether ether ketone) (PEEK) films showing different thermal histories have been investigated at room temperature to point out the main key microstructural features governing properties over a wide strain rate range, i.e., from 10−5 to 300 s−1. The strain rate sensitivity of the mechanical properties of amorphous PEEK films significantly depends on the analyzed strain rate range: i.e., 1) from 10−5 to 10 s−1, the strain rate dependence of both apparent Young's modulus and yield stress is weak; and 2) from 10−1 to 200 s−1, both parameters significantly increase. Thus, based on the definition of the relationships between temperature, strain rate, and frequency respectively used for tensile tests and dynamic mechanical spectrometry, it was shown that the mechanical behavior of PEEK films at room temperature could be governed by similar molecular mechanisms as those giving rise to the β1 and β2 transitions. The Eyring analysis shows that motions of five or six monomers are implied at the beginning of the plastic deformation of amorphous and semi-crystalline PEEK films, while at higher strain rates, shorter chain segments are concerned. Thus, the crystalline phase only induces an increase in the stress level because of the reinforcement effect but does not modify the molecular mechanisms governing the plastic deformation of PEEK films at room temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1053–1059, 1997

Tensile mechanical properties of PEEK films over a wide range of strain rates. II

Tensile mechanical properties of poly(aryl ether ether ketone) (PEEK) films showing different thermal histories have been investigated at room temperature to point out the main key microstructural features governing properties over a wide strain rate range, i.e., from 10−5 to 300 s−1. The strain rate sensitivity of the mechanical properties of amorphous PEEK films significantly depends on the analyzed strain rate range: i.e., 1) from 10−5 to 10 s−1, the strain rate dependence of both apparent Young's modulus and yield stress is weak; and 2) from 10−1 to 200 s−1, both parameters significantly increase. Thus, based on the definition of the relationships between temperature, strain rate, and frequency respectively used for tensile tests and dynamic mechanical spectrometry, it was shown that the mechanical behavior of PEEK films at room temperature could be governed by similar molecular mechanisms as those giving rise to the β1 and β2 transitions. The Eyring analysis shows that motions of five or six monomers are implied at the beginning of the plastic deformation of amorphous and semi-crystalline PEEK films, while at higher strain rates, shorter chain segments are concerned. Thus, the crystalline phase only induces an increase in the stress level because of the reinforcement effect but does not modify the molecular mechanisms governing the plastic deformation of PEEK films at room temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1053–1059, 1997

Enantioselective catalysis. Part 102 . Epimerization of glucose and mannose in the presence of nickel(II) complexes of optically active ligands

The epimerization reaction glucose ⇌ mannose, catalyzed by nickel(II) complexes, was investigated. The achiral ligand N,N,N′,N′-tetramethyl-1,2-diaminoethane 1 and the chiral ligands ( R,R)- and (S,S)-N,N,N′,N′-tetramethyl-1,2-diamino-1,2-diphenylethane 2 and 3 were used. The standard epimerization of glucose and mannose, respectively, was carried out in methanol at 60°C using a stoichiometric amount of nickel(II) complex. The influence of substoichiometric and overstoichiometric amounts of the catalyst and the temperature and time dependence were studied. Some of the kinetic and thermodynamic parameters turned out to be dependent on whether the ( R,R)- or ( S,S)-ligand 2 or 3 was used in the nickel(II) complex. The data were subjected to a Michaelis-Menten and Eyring analysis.

The effect of respiration on the permeability of the mitochondrial membrane to ions

1. The rates of cation uptake, for either organic cations such as tetrapropylammonium, TPA +, at variable tetraphenylboron concentrations, TPB −, or inorganic cations such as Mn 2+, or K + plus valinomycin, have been measured in mitochondria either respiring, under uncoupler titrations, or non-respiring, under variable K + diffusion potentials. 2. The flow-force relationship for the respiration-coupled ion fluxes during titrations with uncouplers is almost identical to that obtained for the K +-diffusion driven fluxes. Similar results are obtained when TPA + is replaced with inorganic cations, either monovalent such as K + (+ valinomycin), or divalent such as Mn 2+. 3. By applying the Eyring analysis, as developed by Garlid et al. (Garlid, K.D., Beavis, A.D. and Ratkje, S.K. (1989) Biochim. Biophys. Acta 976, 109–121), from the flux-voltage relationships the values for the permeability coefficients and for the energy barriers have been obtained for the transport of the ion pair TPA +-TPB −, of Mn 2+ and of K + plus valinomycin, in non-respiring and in respiring, coupled and uncoupled, mitochondria. 4. The findings that the rates of respiration-coupled ion fluxes, at all values of membrane potential, are similar to the rates of the K + diffusion potential-coupled ion fluxes and the similar pattern of the flux-voltage relationships during the titrations with uncouplers and artificial gradients indicate that the membrane permeability for ions is not modified by respiration.

Structural variations affecting the eyring analysis of macro deformation in polymers

The stress-activation volume is affected by the testing method. Determining Δσ by straining many samples at different rates leads to larger and more erroneous activation volumes than using a method where sudden rate changes in a single sample are employed. To explain this, the effective structure is assumed to depend on the applied strain rate.