Mol Deg Research Articles

Ring-Opening Polymerization of Trimethylene Carbonate Using Aluminum(III) and Tin(IV) Salen Chloride Catalysts

Aluminum and tin salen complexes have been shown to effectively catalyze the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) to polycarbonate. The most active salen derivative in each instance contained a phenylene backbone with chloro substituents in the 3,5-positions of the phenolate rings, with the aluminum derivatives being significantly more active than their tin(IV) counterparts. Importantly, the resultant polycarbonate was shown by 1H NMR to be void of ether linkages. The reaction was demonstrated to proceed via a mechanism first order in both [catalyst] and [monomer] and to involve TMC ring-opening by way of acyl oxygen bond cleavage. Consistent with a reaction pathway involving an insertion of the monomer into the metal−nucleophile bond (e.g., Al−Cl or Sn−Cl), the activation parameters were determined to be ΔH⧧ = 51 kJ/mol and ΔS⧧ = −141 J/(mol deg).

2-Pyridinethiol/2-pyridinethione tautomeric equilibrium. A comparative experimental and computational study.

The gas phase and solvent dependent preference of the tautomerization between 2-pyridinethiol (2SH) and 2-pyridinethione (2S) has been assessed using variable temperature Fourier transform infrared (FTIR) experiments, as well as ab initio and density functional theory computations. No spectroscopic evidence (nu(S)(-)(H) stretch) for 2SH was observed in toluene, C(6)D(6), heptane, or methylene chloride solutions. Although, C(s)() 2SH is 2.61 kcal/mol more stable than C(s)() 2S (CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPE), cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the FTIR observations and in quantitative agreement with the -2.6 kcal/mol solution (toluene or C(6)D(6)) calorimetric enthalpy for the 2S/2SH tautomerization favoring the thione. As the intramolecular transition state for the 2S, 2SH tautomerization (2TS) lies 25 (CBS-Q) to 30 kcal/mol (CCSD/cc-pVTZ) higher in energy than either tautomer, tautomerization probably occurs in the hydrogen bonded dimer. The B3LYP/6-311+G(3df,2p) optimized C(2) 2SH dimer is 10.23 kcal/mol + ZPE higher in energy than the C(2)(h)() 2S dimer and is only 2.95 kcal/mol + ZPE lower in energy than the C(2) 2TS dimer transition state. Dimerization equilibrium measurements (FTIR, C(6)D(6)) over the temperature range 22-63 degrees C agree: K(eq)(298) = 165 +/- 40 M(-)(1), DeltaH = -7.0 +/- 0.7 kcal/mol, and DeltaS = -13.4 +/- 3.0 cal/(mol deg). The difference between experimental and B3LYP/6-311+G(3df,2p) [-34.62 cal/(mol deg)] entropy changes is due to solvent effects. The B3LYP/6-311+G(3df,2p) nucleus independent chemical shifts (NICS) are -8.8 and -3.5 ppm 1 A above the 2SH and 2S ring centers, respectively, and the thiol is aromatic. Although the thione is not aromatic, it is stabilized by the thioamide resonance. In solvent, the large 2S dipole, 2-3 times greater than 2SH, favors the thione tautomer and, in conclusion, 2S is thermodynamically more stable than 2SH in solution.

Temperature compensation of postsynaptic currents from the extraocular muscle of temperate teleost fishes.

Miniature end plate currents were recorded from white inferior oblique extraocular muscle fibres of one temperate marine teleost (Aldrichetta forsteri, Family Mugilidae) and two temperate freshwater teleosts (Galaxias fasciatus, Family Galaxiidae and Oncorhynchus mykiss, Family Salmonidae). Miniature end plate currents were digitised and averaged over a temperature range of 5-25 degrees C. For each species, decay of miniature end plate currents was exponential and exhibited a strong temperature dependence. Lower temperatures resulted in prolonged decay phases, which decreased exponentially as a function of absolute temperature. Although values of the exponential time constant tau (tau) obtained for each species at 5 degrees C, 15 degrees C and 25 degrees C were significantly different, at any given temperature, there were no significant differences between tau values for the three species, despite differences in phylogeny (different families) and habitat (marine versus freshwater). At their normal temperature of 15 degrees C, mean values of tau for the three species ranged from 840 micros to 940 micros, and apparent activation energies ranged from -41 kJ mol(-1) deg(-1) to 50 kJ mol(-1) deg(-1). These observations confirm earlier reports that teleost miniature end plate currents are consistently shorter than those of other vertebrates.

Ethidium bromide binding to native and denatured poly(dA)poly(dT)

Isotherms of the EtBr adsorption on native and denatured poly(dA)poly(dT) in the temperature interval 20–70°C were obtained. The EtBr binding constants and the number of binding sites were determined. The thermodynamic parameters of the EtBr intercalation complex upon changes of solution temperature 20–48°C were calculated: 1.0·106 M−1≤K≤1.4·106 M−1, free energy ΔGo=−8.7±0.3 kcal/mol, enthalpy ΔHo≅0, and entropy ΔSo=28±0.5 cal/(mol deg). UV melting has shown that the melting temperature (Tm) of EtBr-poly(dA)poly(dT) complexes (μ=0.022,4.16·10−5 M EtBr) increased by 17°C as compared with the ΔTm of free homopolymer, whereas the half-width of the transition (Tm) is not changed. It was shown for the first time that EtBr forms complexes of two types on single-stranded regions of poly(dA)poly(dT) denatured at 70°C: strong (K1=1.7·105 M−1; ΔGo=−8.10±0.03 kcal/mol) and weak (K2=2.9·103 M−1; ΔGo=−6.0±0.3 kcal/mol).The ΔGo of the strong and weak complexes was independent of the solution ionic strength, 0.0022≤μ≤0.022. A model of EtBr binding with single-stranded regions of poly(dA)poly(dT) is discussed.

Monochlorogallane: physical properties and structure of the gaseous molecule H2Ga(mu-Cl)2GaH2 as determined by vibrational, electron diffraction, and ab initio studies.

Monochlorogallane, synthesized by the metathesis of gallium(III) chloride with an excess of trimethylsilane at ca. 250 K, has been characterized by chemical analysis, by its IR, Raman, and 1H NMR spectra, and by the products of its reaction with trimethylamine. The vibrational spectra of the vapor species isolated in solid Ar, N2, or CH4 matrixes at ca. 12 K imply the presence of only one species, viz. the dimer with an equilibrium structure conforming to D2h symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements augmented by the results of ab initio molecular orbital calculations. An equilibrium structure with D2h symmetry has been assumed in the analysis of the electron diffraction pattern. However, as the molecule has a very low frequency Ga(mu-Cl)2Ga ring-puckering mode, a dynamic model was used to describe it with the aid of a set of pseudoconformers spaced at even intervals (deltadelta = 5 degrees, deltamax, = 20 delta) around the ring-puckering angle delta and Boltzmann-weighted according to a quartic potential V(delta) = V4delta4 + V2delta2. The differences in bond distances and angles between the different pseudoconformers were constrained to the values derived from the ab initio calculations employing second-order Moller-Plesset (MP2) methods (with all the electrons included in the correlation calculations) and a 6-311G(d) basis set. The results for the weighted average of the principal distances (ralpha) and angles (<alpha) (with estimated 1sigma uncertainties) from the combined GED/ab initio study are r(Ga-Cl) 2.350(2) A, r(Ga-H) 1.523(20) A, <Cl-Ga-Cl 89.7(5)degrees, <H-Ga-H 135.1 degrees, V4 3.0 x 10(-6) kcal mol(-1) deg(-4), and V2 6.0 x 10(-4) kcal mol(-1) deg(-2).

Kinetic and Thermodynamic Studies of Reaction of *Cr(CO)(3)C(5)Me(5), HCr(CO)(3)C(5)Me(5), and PhSCr(CO)(3)C(5)Me(5) with *NO. Reductive Elimination of Thermodynamically Unstable Molecules HNO and RSNO Driven by Formation of the Strong Cr-NO Bond.

Reaction of H-Cr(CO)(3)C(5)Me(5) with *NO at 1-2 atm pressure in toluene solution yields Cr(NO)(CO)(2)C(5)Me(5) as the sole metal-containing product in addition to N(2)O and HNO(2) as the principle nitrogen-containing products. N(2)O and HNO(2) are attributed to decomposition of the initial product HNO. Kinetic studies yield the rate law d[P]/dt = -k(2nd)( )(order)[HCr(CO)(3)C(5)Me(5)][*NO]; k(2nd)( )(order) = 0.14 M(-)(1) s(-)(1) at 10 degrees C, with DeltaH() = 11.7 +/- 1.5 kcal/mol and DeltaS() = -16.3 +/- 3.5 cal/(mol deg). The rate of reaction is not inhibited by CO. The kinetic isotope effect for reaction of D-Cr(CO)(3)C(5)Me(5) is k(H)/k(D) = 1.7. These observations are consistent with a first step involving direct H (D) atom transfer from the metal hydride to *NO, forming HNO. Also supporting this mechanism is the approximately 150-times slower reaction of H-Mo(CO)(3)C(5)Me(5) and failure to observe reaction for H-W(CO)(3)C(5)Me(5) in keeping with metal-hydrogen bond strengths Cr < Mo < W. Reaction of PhS-Cr(CO)(3)C(5)Me(5) with NO at 1-2 atm pressure in toluene solution also forms Cr(NO)(CO)(2)C(5)Me(5) as the sole metal-containing product. The initial product is the unstable nitrosothiol PhS-NO. Kinetic studies yield the rate law d[P]/dt = -k(1st)( )(order)[PhS-Cr(CO)(3)C(5)Me(5)]; k(1st)( )(order) = 3.1 +/- 0.3 x 10(-)(3) s(-)(1) at 10 degrees C, with DeltaH() = 21.6 +/- 1.2 kcal/mol, DeltaS() = + 3.9 +/- 1.5 cal/(mol deg). The rate of reaction is independent of both NO and CO pressure. The transition state in the first-order process is proposed to involve migration of bound thiolate to coordinated CO, forming Cr(CO)(2) (eta(2)-C(=O)SPh)C(5)Me(5). The enthalpy of reaction of *Cr(CO)(3)C(5)Me(5) and NO yielding Cr(NO)(CO)(2)C(5)Me(5) and CO has been measured by solution calorimetry: DeltaH degrees = -33.2 +/- 1.8 kcal/mol. The Cr-NO bond strength is estimated as approximately 70 kcal/mol and provides the net thermodynamic driving force for the proposed elimination of the unstable molecules HNO and PhSNO.

Global analysis of the thermal and chemical denaturation of the N-terminal domain of the ribosomal protein L9 in H2O and D2O. Determination of the thermodynamic parameters, deltaH(o), deltaS(o), and deltaC(o)p and evaluation of solvent isotope effects.

The stability of the N-terminal domain of the ribosomal protein L9, NTL9, from Bacillus stearothermophilus has been monitored by circular dichroism at various temperatures and chemical denaturant concentrations in H2O and D2O. The basic thermodynamic parameters for the unfolding reaction, deltaH(o), deltaS(o), and deltaC(o)p, were determined by global analysis of temperature and denaturant effects on stability. The data were well fit by a model that assumes stability varies linearly with denaturant concentration and that uses the Gibbs-Helmholtz equation to model changes in stability with temperature. The results obtained from the global analysis are consistent with information obtained from individual thermal and chemical denaturations. NTL9 has a maximum stability of 3.78 +/- 0.25 kcal mol(-1) at 14 degrees C. DeltaH(o)(25 degrees C) for protein unfolding equals 9.9 +/- 0.7 kcal mol(-1) and TdeltaS(o)++(25 degrees C) equals 6.2 +/- 0.6 kcal mol(-1). DeltaC(o)p equals 0.53 +/- 0.06 kcal mol(-1) deg(-1). There is a small increase in stability when D2O is substituted for H2O. Based on the results from global analysis, NTL9 is 1.06 +/- 0.60 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 5.8 +/- 3.6 degrees C in D2O. Based on the results from individual denaturation experiments, NTL9 is 0.68 +/- 0.68 kcal mol(-1) more stable in D2O at 25 degrees C and Tm is increased by 3.5 +/- 2.1 degrees C in D2O. Within experimental error there are no changes in deltaH(o) (25 degrees C) when D2O is substituted for H2O.

Significance of melanin binding and metabolism in the activity of 5-acetoxyacetylimino-4-methyl- Δ2-1,3,4,-thiadiazoline-2-sulfonamide

5-Acetoxyacetylimino-4-methyl- Δ 2-1,3,4,-thiadiazoline-2-sulfonamide (compound (1)) is an ester prodrug that lowered intraocular pressure (IOP) in albino New Zealand rabbits, but was found to be inactive in pigmented Dutch Belt rabbits. In order to explain the differences in pharmacological activity for the two rabbit species, metabolism and melanin binding were studied. Depending on the initial concentration, the binding of compound (1) to natural melanin ( Sepia officinalis) was 20–60%. The binding constant, K, at 37°C was 4.32×10 5 M −1 and the maximum moles bound to melanin, r max, was 4.5×10 −7 mol/mg of melanin. From a determination of binding at temperatures between 25°C and 47°C, a van't Hoff plot was constructed to determine enthalpy and entropy changes accompanying the binding process, Δ H and Δ S, respectively. Values calculated from the plot were −12.7 and −15.4 kcal/(mol deg), respectively. Negative values for these parameters are consistent with charge transfer interactions and therefore suggest that this may be an operative mechanism between compound (1) and melanin. The in vitro incubation of compound (1) was also studied with various ocular tissues from both albino and pigmented rabbits which were iris-ciliary body, intact cornea, stroma/endothelium and aqueous humor. A major metabolite, MET 1, was identified and also observed from in vivo analyses of the same tissues following topical application. The metabolite was isolated and subjected to mass spectroscopy and proton nuclear magnetic resonance spectroscopy analysis. From these analyses, it was hypothesized that the formation of MET 1 involved a GSH conjugation mechanism which displaced the sufonamide (-SO 2NH 2) group. The metabolism was found to be less extensive in the pigmented rabbit than in the albino rabbit and suggested that the binding affinity of compound (1) for melanin was a better explanation for the lack of IOP activity in the pigmented rabbit than differences in metabolism.

Why Does D2Bind Better Than H2? A Theoretical and Experimental Study of the Equilibrium Isotope Effect on H2Binding in a M(η2-H2) Complex. Normal Coordinate Analysis of W(CO)3(PCy3)2(η2-H2)

Vibrational data (IR, Raman and inelastic neutron scattering) and a supporting normal coordinate analysis for the complex trans-W(CO)3(PCy3)2(η2-H2) (1) and its HD and D2 isotopomers are reported. The vibrational data and force constants support the well-established η2-bonding mode for the H2 ligand and provide unambiguous assignments for all metal−hydrogen stretching and bending frequencies. The force constant for the HH stretch, 1.3 mdyn/Å, is less than one-fourth the value in free H2 and is similar to that for the WH stretch, indicating that weakening of the H−H bond and formation of W−H bonds are well along the reaction coordinate to oxidative addition. The equilibrium isotope effect (EIE) for the reversible binding of dihydrogen (H2) and dideuterium (D2) to 1 and 1-d2 has been calculated from measured vibrational frequencies for 1 and 1-d2. The calculated EIE is “inverse” (1-d2 binds D2 better than 1 binds H2), with KH/KD = 0.78 at 300 K. The EIE calculated from vibrational frequencies may be resolved into a large normal mass and moment of inertia factor (MMI = 5.77), an inverse vibrational excitation factor (EXC= 0.67), and an inverse zero-point energy factor (ZPE = 0.20), where EIE = MMI × EXC × ZPE. An analysis of the zero-point energy components of the EIE shows that the large decrease in the HH stretching frequency (force constant) predicts a large normal EIE but that zero-point energies from five new vibrational modes (which originate from translational and rotational degrees of freedom from hydrogen) offset the change in zero-point energy from the H2(D2) stretch. The calculated EIE is compared to experimental data obtained for the binding of H2 or D2 to Cr(CO)3(PCy3)2 over the temperature range 12−36 °C in THF solution. For the binding of H2 ΔH = −6.8 ± 0.5 kcal mol-1 and ΔS = −24.7 ± 2.0 cal mol-1 deg-1; for D2 ΔH = −8.6 ± 0.5 kcal/mol and ΔS = −30.0 ± 2.0 cal/(mol deg). The EIE at 22 °C has a value of KH/KD = 0.65 ± 0.15. Comparison of the equilibrium constants for displacement of N2 by H2 or D2 in the complex W(CO)3(PCy3)2(N2) in THF yielded a value of KH/KD = 0.70 ± 0.15 at 22 °C.

Reaction of Phenyl and Methyl Disulfide with ·Cr(CO)3C5Me5 and HCr(CO)3C5Me5. Metal Radical and Metal Hydride Reactivity at the Sulfur−Sulfur Bond. Different Mechanisms for Oxidative Addition of Alkyl and Aryl Disulfides

The enthalpies of reaction of the ·Cr(CO)3C5Me5 and disulfides (RSSR) forming RS−Cr(CO)3C5Me5 have been measured by solution calorimetry, ΔH = −13.3 + 1.5 kcal/mol (R = Ph) and −11.2 + 1.2 kcal/mol (R = Me). These data lead to Cr−SR bond strength estimates of 35 and 43 kcal/mol, respectively. The rates of oxidative addition have been investigated by FTIR spectroscopy. Phenyl disulfide reacts by a second-order mechanism whose rate-determining step is the attack of the chromium radical on the sulfur−sulfur bond, yielding chromium thiolate and a thiyl radical [k(298 K) = 1.3 M-1 s-1, ΔHΦ = +10.2 kcal/mol, ΔSΦ = −24.4 cal/(mol deg)]. Methyl disulfide reacts by a third-order mechanism going through a termolecular transition state in which the stronger sulfur−sulfur bond in the alkyl disulfide is attacked simultaneously by two chromium radicals [k(298 K = 393 M-2 s-1, ΔHΦ = −0.2 kcal/mol, ΔSΦ = −47 cal/(mol deg)]. The rates of reaction of MeSSMe and PhSSPh with the hydride H−Cr(CO)3C5Me5 have also been investig...

Effect of Sodium on the Energetics of Thrombin – Thrombomodulin Interaction and its Relevance for Protein C Hydrolysis

Measurements of the apparent affinity constant for thrombomodulin (TM) binding to human α-thrombin as a function of both Na +and temperature at constant ionic strength (0.15 M) showed that TM affinity increases in the presence of Na +and vice versa. Moreover, this experimental strategy allowed us to accurately split the free energy of sodium binding into its entropic and enthalpic components for both the TM-free and TM-bound enzyme. Namely, at 25°C, the value of Δ Gof sodium binding was found equal to −2.4 kcal/mol in the absence of TM and −3.6 kcal/mol for the thrombin-TM complex. The enthalpic contribution to the free energy of sodium binding is equal to −27 kcal/mol and −21 kcal/mol in the TM-free and TM-bound thrombin forms, respectively. Finally, the entropy change for sodium binding was also affected by TM, being equal to −83 cal/(mol deg) and −58 cal/(mol deg) in TM-free and TM-bound thrombin species, respectively. Moreover, the thermodynamic parameters for TM binding to Na +-free thrombin species were solved. TM binding is characterized by an enthalpy and entropy change equal to −10 kcal/mol and 2 cal/(mol deg), respectively, for Na +-free thrombin. It is well known that Na +binding to thrombin causes conformational transitions and functional activation of the enzyme molecule. The finding that binding of thrombomodulin enhances thrombin affinity for sodium and vice versaraises the question as to whether the change of Na +ligation induced by TM binding could contribute to the change in thrombin specificity for the hydrolysis of Protein C. Therefore, the effect of sodium binding to thrombin on the hydrolysis of human Protein C was extensively investigated. At both 25 and 37°C the value of k cat/ K mfor Protein C hydrolysis by thrombin in the absence of TM was found to be enhanced by Na +over a concentration ranging from 0 to 150 mM. Application of thermodynamic principles demonstrated that the Na +-thrombomodulin linkage contributes, under physiological conditions of sodium activity and temperature, to reduce significantly the tran- sition-state stabilization free energy for Protein C hydrolysis.

Kinetic Study of the Two-Electron Oxidation of Cobalt Tetraneopentoxyphthalocyanine by Thionyl Chloride

Electronic spectroscopy was used to study the kinetics of the reaction between Co{sup II}TnPc(2-) and SOCl{sub 2} in 1,2-dichlorobenzene. SOCl{sub 2} reacts rapidly with Co{sup II}TnPc(2-) to yield the two-electron-oxidized species Cl{sub 2}Co{sup III}-TnPc(1-) and a (SOCl{sub 2})Co{sup II}TnPc(2-) adduct; estimates of the rate constants for formation of these species are derived from stopped-flow data. The adduct further reacts with another molecule of SOCl{sub 2} (k{sub 4} = 1.51 {+-} 0.50 M{sup {minus}1} s{sup {minus}1} at 22.2{degrees}C, {Delta}H{sup {+-}} = 13.5 {+-} 1.1 kcal/mol, {Delta}S{sup {+-}} = -11.9 {+-} 3.3 cal/(mol deg)) to yield Cl{sub 2}Co{sup III}TnPc(1-). In the presence of HCl, oxidation to form only Cl{sub 2}Co{sup III}TnPc proceeds very slowly. Implications for Li/SOCl{sub 2} batteries are discussed.

Structure and dynamics of the urea-trioxane inclusion compound phases, studied by deuteron NMR spectroscopy

Deuterium NMR measurements are reported on the solid phases of the urea-trioxane inclusion compound (UTIC) prepared from isotopically normal urea and perdeuterated trioxane. The results are interpreted in terms of the dynamic state and orientational disorder of the trioxane sublattice in the various solid phases of the UTIC. At the low-temperature region of phase IV(<-140 o C) the trioxane molecules are static on the NMR time scale, but above this temperature range J-fold jumps about the molecule C 3 axis sets in with a rate equation k J (s -1 )=2.2'710 13 exp(-4.8/RT), where R is in kcal/(mol deg)

ChemInform Abstract: Solution Studies of Gold(I) Complexes of n‐Hexyldimethylphosphine, n‐Butyldiphenylphosphine, 1‐Dimethylphosphino‐6‐diphenylphosphinohexane, 1,6‐Bis(dimethylphosphino)hexane and 1,6‐Bis(diphenylphosphino)hexane

Abstract The 31 P{ 1 H} NMR spectra of the monodentate phosphines PMe 2 hex n and PPh 2 Bu n in the presence of Et 4 N [AuBr 2 ] in CDCl 3 solution have been studied at various phosphine/gold ratios and temperatures. At ca. 195 K and a P/Au ratio of 1, only [BrAu(phosphine)] is present. Increasing the amount of phosphine leads to the formation of [BrAu(phosphine) 2 ], [Au(phosphine) 3 ]Br and [Au(phosphine) 4 ]Br. When AuBr 2 − , PMe 2 hex n and PPh 2 Bu n are mixed in a 1:1:1 ratio [BrAu(PMe 2 hex n )], [BrAu(PMe 2 hex n ) 2 ], [BrAu(PPh 2 Bu n )], [BrAu(PPh 2 Bu n ) 2 ] and the mixed ligand [BrAu(PMe 2 hex n )(PPh 2 Bu n )] species are present. Addition of more PMe 2 hex n causes the disappearance of [BrAu(PMe 2 hex n )] with formation of [BrAu(PMe 2− hex n ) 2 ]; however this does not occur in the case of PPh 2 Bu n when [BrAu(PPh 2 Bu n )] is still present even at a Au:PMe 2 hex n :PPh 2 Bu n ratio of 1:1:2. Employing the unsymmetrical ditertiary phosphine Ph 2 P(CH 2 ) 6 PMe 2 enables the identification of the dimeric two- coordinate [BrAu(ligand)AuBr], the dimeric three- coordinate [BrAu(ligand) 2 AuBr] (two forms) and the monomeric three-coordinate [BrAu(ligand)]. Similar studies for the symmetrical ditertiary phosphines Ph 2 P(CH 2 ) 6 PPh 2 (dph) and Me 2 P(CH 2 ) 6 PMe 2 are reported. The rate constants for the exchange BrAu(PMe 2 hex n ) ⇌ BrAu(PMe 2 hex n ) 2 , BrAu(PPh 2 Bu n ) ⇌ BrAu(PPh 2 Bu n ) 2 and BrAu(dph)AuBr ⇌ BrAu(dph) 2 AuBr have been determined from the 31 P{ 1 H} NMR spectra by line shape analysis. Activation parameters have been calculated by a least-squares method. The activation entropies for the exchange processes range from −1.1 to 13.9 cal(mol deg) −1 depending on the ligand type. An associative mechanism is more probable although the possibility of a dissociative process cannot be ruled out. We also report here an improved synthetic method for unsymmetrical bisphosphines with chain length>(CH 2 ) 4 .

Solution studies of gold(I) complexes of n-hexyldimethylphosphine, n-butyldiphenylphosphine, 1-dimethylphosphino-6-diphenylphosphinohexane, 1,6-bis(dimethylphosphino)hexane and 1,6-bis(diphenylphosphino)hexane

The 31P{ 1H} NMR spectra of the monodentate phosphines PMe 2hex n and PPh 2Bu n in the presence of Et 4N [AuBr 2] in CDCl 3 solution have been studied at various phosphine/gold ratios and temperatures. At ca. 195 K and a P/Au ratio of 1, only [BrAu(phosphine)] is present. Increasing the amount of phosphine leads to the formation of [BrAu(phosphine) 2], [Au(phosphine) 3]Br and [Au(phosphine) 4]Br. When AuBr 2 −, PMe 2hex n and PPh 2Bu n are mixed in a 1:1:1 ratio [BrAu(PMe 2hex n)], [BrAu(PMe 2hex n) 2], [BrAu(PPh 2Bu n)], [BrAu(PPh 2Bu n) 2] and the mixed ligand [BrAu(PMe 2hex n)(PPh 2Bu n)] species are present. Addition of more PMe 2hex n causes the disappearance of [BrAu(PMe 2hex n)] with formation of [BrAu(PMe 2−hex n) 2]; however this does not occur in the case of PPh 2Bu n when [BrAu(PPh 2Bu n)] is still present even at a Au:PMe 2hex n:PPh 2Bu n ratio of 1:1:2. Employing the unsymmetrical ditertiary phosphine Ph 2P(CH 2) 6PMe 2 enables the identification of the dimeric two- coordinate [BrAu(ligand)AuBr], the dimeric three- coordinate [BrAu(ligand) 2AuBr] (two forms) and the monomeric three-coordinate [BrAu(ligand)]. Similar studies for the symmetrical ditertiary phosphines Ph 2P(CH 2) 6PPh 2 (dph) and Me 2P(CH 2) 6PMe 2 are reported. The rate constants for the exchange BrAu(PMe 2hex n) ⇌ BrAu(PMe 2hex n) 2, BrAu(PPh 2Bu n) ⇌ BrAu(PPh 2Bu n) 2 and BrAu(dph)AuBr ⇌ BrAu(dph) 2AuBr have been determined from the 31P{ 1H} NMR spectra by line shape analysis. Activation parameters have been calculated by a least-squares method. The activation entropies for the exchange processes range from −1.1 to 13.9 cal(mol deg) −1 depending on the ligand type. An associative mechanism is more probable although the possibility of a dissociative process cannot be ruled out. We also report here an improved synthetic method for unsymmetrical bisphosphines with chain length>(CH 2) 4.

Thermodynamics and kinetics of carbon dioxide chemisorption on calcium oxide

Equilibrium chemisorption of CO/sub 2/ on CaO was measured at 923-1013 K, and the kinetics of rapid adsorption and of slow dissolution of CO/sub 2/ in the CaO, perhaps as CO/sub 3//sup 2 -/ at grain boundaries, were measured in 1333 Pa of CO/sub 2/ at 983-1033 K. Surface areas were measured by the BET method. From the assumptions that only (100) surfaces of the CaO were exposed, it was calculated that approx. 55-99% of the surface O/sup 2 -/ ions reacted with CO/sub 2/ to form CO/sub 3//sup 2 -/ ions. The enthalpy of adsorption was -199 +/- 8 kJ/mol and the thermal entropy of adsorption was -153 +/- 8 J/(mol deg). Both values were somewhat less negative at > 80% coverage. The apparent activation enthalpy for adsorption was 61 +/- 10 kJ/mol. The apparent activation enthalpy of the slow dissolution process was 303 +/- 15 kJ/mol. 16 references, 4 figures, 1 table.

Follitropin binding to receptors in testis: Studies on the reversibility and thermodynamics of the reaction

Studies were designed to assess the extent to which binding of follitropin (FSH) to its receptor is a reversible process. [ 125I]hFSH was allowed to interact with membrane receptors from calf testis for 2 h at various temperatures by which time significant specific binding of [ 125I]hFSH had taken place in all instances. Unlabeled FSH was then added (delayed addition) and the amount of [ 125I]hFSH remaining bound was monitored as a function of time. In order to assess reversibility, [ 125I]hFSH binding in the latter samples was compared to that occurring when unlabeled FSH had not been added or when unlabeled FSH was present from the start of the incubation. Binding was essentially fully reversible at 4°C, but reversibility decreased with increasing temperature. Reversibility of FSH binding decreased markedly at temperatures greater than 26°C and was considered irreversible at temperatures above 30°C. At 4°C essentially full reversibility (> 90%) was observed when the unlabeled hormone was added after 7 h of incubation, but decreased when added after 12 h. At warmer temperatures (22 or 30°C) there was a progressive decrease in reversibility as the time of delay before addition of unlabeled hormone was lengthened. By determining the affinity constant (in separate experiments) at various temperatures, a thermodynamic analysis was possible. This analysis was restricted to the temperature range 4–26°C in order to minimize complications arising from irreversible binding. The reaction was endothermal at low temperatures ( T < 12.5° C) and exothermal at higher temperatures ( T > 12.5° C) and was associated with a decrease in heat capacity of 1800 cal [mol deg] −1 at 25°C. The results are consistent with the concept that the hydrophobic effect plays an important role in FSH binding, but that the reaction is complex and may be composed of more than one step.

Oxidation by aqueous fluoroxysulfate: catalysis by silver(I)

The oxidations of the ions Cr/sup 3 +/, CO/sup 2 +/, and VO/sup 2 +/ by the fluoroxysulfate ion, SO/sub 4/F/sup -/, in aqueous solution are catalyzed by Ag/sup +/. The rate-determining step for all three catalyzed reactions is the bimolecular oxidation of Ag/sup +/ by SO/sub 4/F/sup -/, which has a rate constant of (1.3 +- 0.2) x 10/sup 3/ M/sup -1/ s/sup -1/ at 17/sup 0/C. Activation parameters for this reaction are ..delta..H/sup + +/ = 6.1 +- 0.5 kcal/mol and ..delta..S = -23 +- 2 cal/(mol deg). In the absence of Ag/sup +/, Co/sup 2 +/, and VO/sup 2 +/ react very slowly with SO/sub 4/F/sup -/, while Cr/sup 3 +/ does not react at all. Despite its high thermodynamic oxidizing power, the fluoroxysulfate ion acts as a very selective oxidant.

Vapor-complex equilibriums in the cobalt(II) chloride-indium(III) chloride system

The vapor-phase equilibria of the cobalt(II) chloride-indium(III) chloride system have been investigated spectrophotometrically in the temperature range from 900 to 1100/sup 0/K and at total pressures from approx. 0.5 to approx. 40 atm. Thermodynamic considerations imply the reaction CoCl/sub 2/(s) + 2InCl/sub 3/(g) ..-->.. CoIn/sub 2/Cl/sub 8/(g) (..delta..H = -19.2 kcal/mol; ..delta..S = -24.0 cal/(mol deg)). Increasing the temperature and/or decreasing the indium chloride pressure tends to dissociate the blue CoIn/sub 2/Cl/sub 8/ complex according to the reaction CoIn/sub 2/Cl/sub 8/(g) ..-->.. InCl/sub 3/(g) + CoInCl/sub 5/(g) (..delta..H = 3.4 kcal/mol; ..delta..S = 33.4 cal/(mol deg)). The values of the thermodynamic functions relevant to the formaton of CoIn/sub 2/Cl/sub 8/ are compared with those of other cobalt chloride vapor complexes and are interpreted as indicating similarities in the vapor structures.

Thermodynamics of the interaction of insulin with its receptor.

Insulin binding to its cellular receptors is markedly dependent on the temperature. The thermodynamic parameters for the reaction of insulin with the high affinity state of its receptor have been evaluated with equilibrium studies at multiple temperatures between 5 degrees and 37 degrees C. The thermodynamics of the insulin-receptor interaction is not classical. The van't Hoff plot is not linear. Both the enthalpy and entropy changes, due to the formation of the hormone . receptor complex, decrease markedly with temperature, corresponding to a large heat capacity change of -766 cal/(mol deg) at 25 degrees C. The reaction is endothermic and entropically driven at low temperature and exothermic and enthalpically driven at higher temperature. This thermodynamic behavior is suggestive of a hydrophobic reaction and supports Blundell's concept that the loss of non-polar surface residues in the formation of the hormone . receptor complex is an important driving force of the reaction. Alternatively, this nonclassical behavior may indicate that the reaction of insulin with its receptor involves more than one step.