Cal Mol Research Articles

Thermochemical Properties of Hydroxycyclohexadienyl Peroxy Isomers from Reaction of O2 with the Benzene-OH adduct

Abstract The addition of OH radical to benzene and other aromatic species to form a cyclohexadienyl radical ( C•HD-OH ), is the first step for conversion (oxidation) of aromatic species in atmospheric chemistry. Reaction of this C•HD-OH intermediate, with ground-state molecular oxygen forms a number of hydroxyl-cyclohexadienyl peroxy ( CHD-OH -OO•) isomers, which further react to form aerosols. Ab initio and density functional calculations were used to study the structures and thermochemical properties (Δ f H o 298, S o(T) and C p (T) of the hydroxyl-cyclohexadienyl peroxy isomers ( CHD-OH -OO•) ((i) para-cis-(Z) and para-trans-(E) (OO• relative to the OH group); (ii) ortho-cis-(Z) and ortho-trans-(E) at pseudo-equatorial (e') and pseudo-axial (a') positions). The thermochemical properties are important for modeling studies on the kinetics of the aerosol formation. Molecular structures and vibration frequencies were determined at the B3LYP/6-31G(d,p) level. Energy calculations were performed at the CBS-lq, CBS-4, G3(MP2)//B3LYP/6-31G(d,p), and BLYP/6-311++G(2d,p)//BLYP/6-31G(d,p) levels. Enthalpies of formation (Δ f H o 298) were calculated at each level using group balance isodesmic reactions. An analysis is performed for entropy from inclusion of internal rotor analysis versus use of torsion frequencies and conformer statistics. Standard entropy, S o(T), and heat capacity, C p (T), from vibrational, translational, and external rotation contributions were calculated using statistical mechanics based on the vibration frequencies and structures. Hindered rotational contributions to S o(T) and C p (T) were calculated from the energy levels, where the internal rotation potential was calculated at the B3LYP/6-31G(d) level. This analysis of the hindered internal rotor contributions yields higher entropy than the values calculated when the rigid-rotor-harmonic-oscillator approximation (RRHO) applied for the calculated torsion frequencies. Differences between the uses of RRHO for torsions with correction for statistical mix of conformations, versus hindered internal rotor analysis are as high as 3.5 cal mol –1 K –1 in entropy and 3.6 cal mol –1 K –1 in heat capacity at 50 ≤ T/K ≤ 5000. A number of intramolecular interactions are found to stabilize the CHD-OH -OO• isomers: (i) intramolecular hydrogen bonding (between the OH and the peroxy OO• groups, OO–H–O), (ii) a negative hyperconjugation where the lone pair on oxygen atom in OH group is hyperconjugated with the antibonding σ * orbital of adjacent C=C bond, (iii) an anomeric interaction in which antibonding σ * orbital of C–O bond in a pseudo-axial position with a neighboring double bond π C–C. The eclipsing steric repulsion between OH and OO• groups counteract the stabilizing hydrogen bonding interaction. The importance of these effects are evaluated in determining the thermodynamic stability of the CHD-OH -OO• isomers. Evaluations of data from the isodesmic reaction at G3(MP2)//B3LYP/6-31G(d,p) level, results in the enthalpy of formation of the most stable isomers of CHD-OH -OO• in a range of − 0.47 to −3.62 kcal mol –1 at 298 K (for (Z)-o-II, −0.47 kcal mol –1 and (E)-o-II, −3.62 kcal mol –1 ). The reaction energies (Δ r H o 298) of hydroxycyclohexadienyl radical ( C•HD-OH ) + O2 ↔ CHD-OH -OO• isomers were determined to be − 10.63 ∼ − 13.67 (− 10.56 ∼ − 13.34) kcal mol –1. Entropy (S o 298) of CHD-OH -OO• was in range of 93.17 ∼ 96.18 (90.82 ∼ 95.55) cal mol –1 K –1. Data in parentheses are based on RRHO/torsion frequency approximation with correction for the mix of conformers. The literature values of reaction energy (Δ r H o 298 K) and entropy (Δ r S o 298 K) were reported −10.5 ± 1.3 kcal mol –1 and −33.9 ± 1.4 cal mol –1 K –1, respectively, based on second-law analysis at temperature between 265 and 345 K.

Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

The tetradentate ligand system L1 combines two N-heterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL1(MeCN)2](PF6)2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH⧧ = 15.1 kcal mol–1, and ΔS⧧ = −4.7 cal mol–1 K–1). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL1(CO)(MeCN)](PF6)2 (2) or neutral [FeL1(N3)2] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1–3. Mössbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe–CNHC bonds in the basal plane constituted by the macrocyclic ligand L1. Oxidation of 1 has been monitored by UV–vis spectro-electro chemistry, and the resulting iron(III) complex [FeL1(MeCN)2](PF6)3 (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = 1/2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL1} core are basically invariant with respect to changes in the metal ion’s oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C2N2} hybrid ligand L1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C4} tetracarbene ligand.

Microsolvation, Dimerization, and sp2-Stereoinversion of Monomeric α-(2,6-Diisopropylphenyl)vinyllithium

The title compound is entirely monomeric in THF as the solvent, but its two ortho-diisopropyl substituents (in contrast to two tert-butyl groups) do not suffice to prevent its dimerization in Et2O: At −107 °C, the disolvated dimer (43% of the total material in monomeric formula units) was accompanied by the disolvated (15%) and the trisolvated monomers (42%). Increasing temperatures reduced trisolvation by Et2O to disolvation and diminished also the combined monomer portions (10% at +25 °C); hence, dimerization in Et2O is endothermic (+3.1 kcal mol–1) but favored by a strongly positive entropy. Microsolvation by only one tert-butyl methyl ether (t-BuOMe) ligand per Li cation of the dimer in t-BuOMe solution was discovered through NMR integration; this disolvated dimer was also observed in cyclopentane as the solvent from −85 to at least +25 °C (hence, no monomer, no higher aggregate). Aryl rotations about the (partial) single bond to the carbanionic center C-α were assessed from the temperatures of NMR signal coalescences of the diastereotopic methyls in an isopropyl group: they are slowest in THF as the solvent because the monomer is then always trisolvated, and delocalization of the negative charge from C-α into the aromatic π-system is therefore most efficient (hence, more rotation-resistant). Microsolvation controls also the cis/trans sp2-stereoinversion, which is faster in THF than in the other three solvents because only one additional THF ligand must temporarily be immobilized (performing THF catalysis) on the way to the tetrasolvated, ionic transition state that involves rehybridization (close to sp) of C-α (pseudoactivation entropy ΔSψ⧧ = −23.6 cal mol–1 K–1). Most of the pertaining microsolvation numbers became available through their empirical relationship with the measured one-bond 13C–6Li NMR coupling constants.

Inferring the microscopic surface energy of protein-protein interfaces from mutation data.

Mutations at protein-protein recognition sites alter binding strength by altering the chemical nature of the interacting surfaces. We present a simple surface energy model, parameterized with empirical ΔΔG values, yielding mean energies of -48 cal mol(-1) Å(-2) for interactions between hydrophobic surfaces, -51 to -80 cal mol(-1) Å(-2) for surfaces of complementary charge, and 66-83 cal mol(-1) Å(-2) for electrostatically repelling surfaces, relative to the aqueous phase. This places the mean energy of hydrophobic surface burial at -24 cal mol(-1) Å(-2) . Despite neglecting configurational entropy and intramolecular changes, the model correlates with empirical binding free energies of a functionally diverse set of rigid-body interactions (r = 0.66). When used to rerank docking poses, it can place near-native solutions in the top 10 for 37% of the complexes evaluated, and 82% in the top 100. The method shows that hydrophobic burial is the driving force for protein association, accounting for 50-95% of the cohesive energy. The model is available open-source from http://life.bsc.es/pid/web/surface_energy/ and via the CCharpPPI web server http://life.bsc.es/pid/ccharppi/.

On the molecular basis of the high affinity binding of basic amino acids to LAOBP, a periplasmic binding protein from Salmonella typhimurium.

The rational designing of binding abilities in proteins requires an understanding of the relationship between structure and thermodynamics. However, our knowledge of the molecular origin of high-affinity binding of ligands to proteins is still limited; such is the case for l-lysine-l-arginine-l-ornithine periplasmic binding protein (LAOBP), a periplasmic binding protein from Salmonella typhimurium that binds to l-arginine, l-lysine, and l-ornithine with nanomolar affinity and to l-histidine with micromolar affinity. Structural studies indicate that ligand binding induces a large conformational change in LAOBP. In this work, we studied the thermodynamics of l-histidine and l-arginine binding to LAOBP by isothermal titration calorimetry. For both ligands, the affinity is enthalpically driven, with a binding ΔCp of ~-300 cal mol(-1) K(-1) , most of which arises from the burial of protein nonpolar surfaces that accompanies the conformational change. Osmotic stress measurements revealed that several water molecules become sequestered upon complex formation. In addition, LAOBP prefers positively charged ligands in their side chain. An energetic analysis shows that the protein acquires a thermodynamically equivalent state with both ligands. The 1000-fold higher affinity of LAOBP for l-arginine as compared with l-histidine is mainly of enthalpic origin and can be ascribed to the formation of an extra pair of hydrogen bonds. Periplasmic binding proteins have evolved diverse energetic strategies for ligand recognition. STM4351, another arginine binding protein from Salmonella, shows an entropy-driven micromolar affinity toward l-arginine. In contrast, our data show that LAOBP achieves nanomolar affinity for the same ligand through enthalpy optimization.

One-Pot Synthesis of Poly(vinyl alcohol) (PVA) Copolymers via Ruthenium Catalyzed Equilibrium Ring-Opening Metathesis Polymerization of Hydroxyl Functionalized Cyclopentene

Well-defined poly(vinyl alcohol-alt-propenylene) (4) was synthesized in one-pot reaction via equilibrium ring-opening metathesis polymerization (ROMP) or acyclic diene metathesis (ADMET) of nonprotected 3-cyclopentene-1-ol (2) and 1,6-heptadiene-4-ol using (1,3-bis(2,4,6-trimethylphenyl)-2-midazolidinylidene)dichloro(phenylmet hylene)(tricyclohexylphosphine)ruthenium (6) and (1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene)dichloro(o-isoprop oxyphenylmethylene)ruthenium (7) metathesis catalysts. The activation enthalpy and entropy of the equilibrium ROMP were determined as ΔH = -6.2 kcal mol^(-1) and ΔS = -18.9 cal mol^(-1) K^(-1). The observed thermodynamic parameters were supported by computational studies. The calculated ring strain energy for 2 (-6.8 kcal mol^(-1) is comparable with the observed activation enthalpy for its equilibrium ROMP reaction catalyzed by 6 or 7. The cis:trans olefinic bond ratio analysis indicated a 20:80 cis:trans selectivity. The hydrogenation of 4 resulted in poly(vinyl alcohol-alt-propylene) (11) in high yield. Because of the similar ring strain energies of cyclopentene (1) and 2, the equilibrium copolymerization results in a polymer having randomly distributed dyads. In general, it means that the polymer formed contains approximately 50% alternating polymer, 25% 3, and 25% 4 homopolymer dyads as expected for a random polymerization.

Biophysical characterization of the interaction of O-acylcholines with the major bovine seminal plasma protein, PDC-109.

Thermodynamic parameters characterizing the interaction of O-lauroylcholine (OLC) and O-myristoylcholine (OMC) with PDC-109 have been determined by isothermal titration calorimetric studies. Titration of OLC or OMC with PDC-109 resulted in exothermic peaks, the magnitude of which decreased with subsequent injections, showing saturation of the binding sites at high protein concentration and indicating that the binding process is associated with negative enthalpy change (ΔH° < 0). Association constant (Ka), Stoichiometry (n), and thermodynamic parameters, ΔH°, ΔS° and ΔG° for the binding of OLC to PDC-109 have been obtained at different temperatures, with the values obtained at 25 °C being 5.08 × 105 M−1, 0.47, −4.46 kcal mol−1, 11.13 cal mol−1 K−1 and −7.78 kcal mol−1, respectively. Values of thermodynamic parameters obtained at different temperatures indicate that the binding of PDC-109 to O-lauroylcholine is governed by a negative enthalpy contribution. The interaction of O-lauroylcholine with PDC-109 was also studied by monitoring changes in the protein intrinsic fluorescence emission intensity. The association constant, Ka, for the interaction of PDC-109 with O-lauroylcholine from fluorescence studies at 25 °C was estimated to be 5.46 × 104 M−1, which is somewhat lower than the Ka value determined by ITC. Circular dichroism spectroscopic studies indicate that interaction of O-lauroylcholine with PDC-109 induced changes in the secondary and tertiary structure of PDC-109. Differential scanning calorimetric studies revealed that the thermal unfolding temperature of PDC-109 increases in the presence of OMC, indicating that binding of O-acylcholines stabilizes the protein structure.

Dihydrogen bonding formed by (hydrido)[hydrotris(pyrazolyl)borato]ruthenium. The effect of ligands on the proton-accepting ability of ruthenium complexes

Reactions of [1,2-bis(diphenylphosphino)ethane](hydrido)[hydrotris(pyrazolyl)borato]-ruthenium with proton donors were studied by IR and 1H NMR spectroscopy in CH2Cl2 over a wide temperature range. Dihydrogen bonding to CF3CH2OH was detected; its spectral and thermodynamic parameters (ΔH° =–3.3±0.2 kcal mol–1, ΔS° =–6.7±0.6 cal mol–1 K–1) were determined. The basicity factor E j of the hydride ligand is 0.81. The H⋯H distance in the dihydrogen-bonded complex (2.05 A) was estimated from the changes in the spin-lattice relaxation time T 1min of the hydride resonance. Comparison of the results obtained with the literature data revealed a correlation between the length and enthalpy of formation of the dihydrogen bond as well as a correlation between the basicity of the hydride ligand and the tendency of the fragment [ML n ] toward stabilization and oxidative addition of H2.

Spin equilibria and thermodynamic constants for (C5H4R)2Mn, R = H or Me, in solid solutions of diamagnetic diluents

The solid state structure of (C5H4Me)2Mn is determined and its solid state magnetism investigated. (C5H4Me)2Mn is a chain polymer, as is the parent (C5H5)2Mn (Bünder W.; Weiss, E. Z. Naturforsch.1978, 33b, 1237), and they exhibit very similar solid state magnetic susceptibilities that are modeled as an antiferromagnetically-coupled linear Heisenberg-chain of high-spin Mn(II) centers (S = 5/2) (König, E.; Desai, V. P.; Kanellakopulos, B.; Klenze, R. Chem. Phys.1980, 54, 109). Doping of these manganocenes into diamagnetic hosts such as (C5H4Me)2Fe and (C5H5)2Fe, respectively, stabilizes the low-spin configuration (S = 1/2) relative to the high-spin configuration (S = 5/2). At higher temperatures the solid solutions of these manganocenes show spin-crossover behavior modeled by a Boltzmann distribution of spins to give ΔH0 ≈ 1.7 kcal mol−1 and ΔS0 ≈ 5 cal (mol K)−1 for the LS ⇌ HS equilibria.

Experimental and Modeling Study of Methyl trans-3-Hexenoate Autoignition

This work presents the results of an experimental and computational study of methyl trans-3-hexenoate autoignition. Experimental autoignition studies were conducted using the University of Michigan rapid compression facility. Pressure time histories were used to determine ignition delay times as a function of test gas composition and experimental conditions. The fuel/oxygen equivalence ratio and dilution level were ϕ = 0.3 and inert/O2 = 3.76 (mole basis). End of compression conditions targeted an average pressure of 10.5 atm and temperatures ranging from 884 to 1085 K. A correlation in Arrhenius form was developed by regression analysis of the experimental data, where the ignition delay time is τign (ms) = 1.4 × 10–6 exp[30 100/(R(cal mol–1 K–1)T)] with a R2 value of 0.99. Gas-sampling experiments were also conducted to measure stable intermediates formed during autoignition. A detailed reaction mechanism was developed and model predictions were compared to the experimental data. While ignition delay ti...

Kinetics and Mechanism of the Thermal Decomposition Reaction of 3,6-diphenyl- 1,2,4,5-tetroxane in 2-methylcellosolve Solution

3,6-diphenyl-1,2,4,5-tetraoxane(DFT) has been prepared in order to study the effect of temperature on the reaction kinetics in methylcellosolve solution. The thermolysis reaction fulfills a kinetic law of the first order up to conversions approximately 60 % of the DFT. The values of ΔH#=20.2±1.0 kcal mol-1 and ΔS#= -25.3±1.4 cal mol-1 K-1 of the rupture of one O-O bond in the diperoxide molecule were obtained by measuring the remanent diperoxide at different reaction times by GC technique. Benzaldehyde and benzoic acid were detected by GC as the major organics products of the reaction.

Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mn(V) -oxo complex.

Addition of an anionic donor to an MnV(O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [MnV(O)(TBP8Cz)(CN)]− was generated from addition of Bu4N+CN− to the 5-coordinate MnV(O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives Mn–O=1.53 Å, Mn–CN=2.21 Å. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN− complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e−-reduced MnIII(CN)− complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ΔH≠=14 kcal mol−1, ΔS≠=−10 cal mol−1 K−1. Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate MnV(O) complexes.

Microsolvation, aggregation, and pseudomonomolecular, ionic sp2-stereoinversion mechanism of two exocyclic β,β-di-tert-alkyl-α-arylvinyllithiums

A THF-solvated, crystalline, exocyclic alkenyllithium, (tert-alkyl)2CC(Li)–Ph, was synthesized and shown to be a disolvated dimer in the solid state and in toluene as the solvent; increasing amounts of the monomeric species emerged on cooling the toluene solution. In THF as the solvent, only the trisolvated monomer was present and identified as a contact ion pair (CIP) through its scalar 13C,6Li NMR coupling. This ground-state needs only one further THF ligand as a catalyst for breaking the C–Li bond with formation of a tetrasolvated, solvent-separated ion pair (SSIP) on the way to the transition state of cis/trans sp2-stereoinversion. The ensuing pseudomonomolecular, ionic mechanism is confirmed by low pseudoactivation parameters: enthalpy ΔHψ‡ = 6.9(3) kcal mol−1; entropy ΔSψ‡ = −23.3(9) cal mol−1 K−1. Similar parameters were found with 2,6-dimethylphenyl in place of Ph.

Enolisation Kinetics of m-Nitro Acetophenone

m-Nitroacetophenone was chosen for the study of kinetics of enolisation. The rate of the reaction was studied by iodination. The kinetics of the reaction was monitored under several conditions of variation of ketone concentration, dielectric constant of the medium , temperature, effect of catalyst etc. In addition to this four different amino acids viz. β-alanin, DL-alanin, L-alanin and Glycine were tested as catalyst for the enolisation process. The rate of enolisation was found to increase with the increase in then ketone concentration , percentage composition of the solvent mixture and also with the increase in the dipole moments of the amino acids. Pseudofirst order rate kinetics was operational and the rate constants were found to increase with the increase in the amino acid molarity. Linear plots obtained for log of rate constants versus reciprocal of temperature which were in good agreement with Arrhenius equation. The values of thermodynamic parameters like Entropy (∆S≠) , Enthalpy (∆H≠), energy of activation (∆Ea) and Free energy(∆F≠) were calculated and were found to be 2.6186 e.u. , 20.85 e.u. ,23.46 k cal mol-1 and 20.0 k cal mol-1 respectively. © 2014 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Hydration Changes Accompanying Helix-to-Coil DNA Transitions

We applied ultrasonic velocimetric and high-precision densimetric measurements to characterizing the helix-to-coil transition of the GGCATTACGG/CCGTAATGCC decameric DNA duplex. The transition was induced either by temperature or by mixing the two complementary single strands at isothermal conditions. The duplex dissociation causes increases in volume and expansibility while resulting in a decrease in compressibility. Our volumetric data in conjunction with computer-generated structural information are consistent with the picture in which the duplex dissociation is accompanied by an uptake of ∼180 water molecules from the bulk phase into the hydration shell of the DNA. Analysis of our compressibility and expansibility data reveals that the single-stranded conformation is likely to exist as a heterogeneous mixture of nearly isoenergetic subspecies differing in volume and enthalpy. We use our estimate of the change in hydration to evaluate the hydration and configurational contributions to the helix-to-coil transition entropy. The duplex dissociation is accompanied by an increase in configurational entropy, ΔSconf, of ∼23 cal mol(-1) K(-1) per nucleotide, which signifies liberation of manifold frozen degrees of freedom involved in maintaining the conformational stability of the duplex and the related stiffening of the heterocyclic bases and the sugar-phosphate backbone. To the best of our knowledge, this is the first experimental estimate of the change in configurational entropy associated with the helix-to-coil transition of a DNA.

Solvent Isotope Effect on the Dark Adaptation of Bacteriorhodopsin in Purple Membrane: Viewpoints of Kinetics and Thermodynamics

The thermal retinal isomerization from all-trans, 15-anti to 13-cis, 15-syn of bacteriorhodopsin in purple membrane in H2O and D2O during dark adaptation was investigated at 30-55 °C at neutral pH. In this temperature range, phase transition of purple membrane and destruction of the tertiary structure of bacteriorhodopsin did not take place. We found that the solvent isotope effect is inverted below about 45 °C; i.e., k(f)(D2O)/k(f)(H2O) > 1. Applying the transition state theory, the changes in enthalpy from the initial state to the transition state along the thermal trans-to-cis forward reaction coordinate, ΔH(f)*, were determined to be 24.7 ± 1.2 and 20.1 ± 0.4 kcal mol(-1) in H2O and D2O, respectively. The relative entropic change of the transition state in H2O and D2O, ΔΔS(f)* = ΔS(f)*(D2O) - ΔS(f)*(H2O), was -14.4 ± 3.9 cal mol(-1) K(-1). In addition, the Gibbs free energy of trans-to-cis thermal isomerization reaction in D2O is 0.4-0.7 kcal mol(-1) lower than that in H2O. It is the first time the entropy and enthalpy of the transition state have been quantified to elucidate the solvent isotope effect in the retinal thermal isomerization of bacteriorhodopsin during dark adaptation. The solvent isotope effect on the thermodynamics properties and kinetics implied that the hydrogen bonding in the transition state during the dark adaptation of bR is stronger than that in the initial state.

Radical Reactivity of the Fe(III)/(II) Tetramesitylporphyrin Couple: Hydrogen Atom Transfer, Oxyl Radical Dissociation, and Catalytic Disproportionation of a Hydroxylamine.

The chemistry of low-valent iron porphyrin complexes with oxyl radical reagents has been explored. (Meso-tetramesityl porphyrinato) iron(III) hydroxide, (TMP)FeIII(OH) reacts with the hydroxylamine TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperdine) to yield the ferrous porphyrin, (TMP)FeII, together with H2O and TEMPO. This reaction has a second order rate constant k1 = 76 ± 5 M-1 1 s-1 and likely occurs by concerted e-/H+ transfer. Hydrazines PhNHNHPh and PhNHNH2 similarly yield (TMP)FeII. A subsequent reaction between TEMPO (2,2,6,6-tetramethylpiperdinyl radical) and (TMP)FeII is observed to reversibly form the TEMPO-ligated ferric porphyrin, (TMP)FeIII(TEMPO). A combination of 1H NMR and optical spectroscopies were used to determine the thermodynamic parameters for TEMPO binding: K4 (25°C) = 535 ± 20 M-1, ΔH°4 = -7.0 ± 1.5 kcal mol-1, ΔS°4= -11 ± 5 cal mol-1 K-1, ΔG‡4(235K) = 21.3 ± 0.5 kcal mol-1, ΔG‡-4(235K) = 16.9 ± 0.5 kcal mol-1. The Fe-O bond is remarkably weak. The stable phenoxyl radical 2,4,6- t Bu3C6H2O• (ArO•) forms a stronger bond to (TMP)FeII to irreversibly make a similar FeIII(OR) complex. Both (TMP)FeII and (TMP)FeIII(OH) are catalysts for the disproportionation of excess TEMPO-H to TEMPO and TEMP-H (2,2,6,6-tetramethylpiperdine). The lack of reactivity between (TMP)FeII and the alkylated TEMPO-H analogue, TEMPO-CH3, suggests that the disproportionation involves a hydrogen atom transfer step. These results highlight the importance and versatility of the heme FeIII/II couple that is often overshadowed by its higher-valent counterparts.

Relationship between the quantity and intensity of potassium in some maize soils of central Kashmir (India)

Plant K uptake depends on its intensity, capacity and removal rate of soils which can be modeled using K exchange-equilibrium obtained from quantity-intensity (Q/I) isotherms i.e. activity ratio at equilibrium (AReK), labile K (KL), potential buffering capacity (PBCK) and free energy change (−ΔG). These relations were studied in five major maize growing soils of district Ganderbal (Kashmir, India). AReK ranged from 0.26 to 1.21 x 10−3 (Mol L−1)0.5, KL from 0.26 to 0.67 cmolc kg−1, PBCK from 23.2 to 37.6 cmolc kg−1/(Mol L−1)0.5 and -ΔG from 3.98 to 4.81 K cal mol−1oK−1. AReK was found significantly correlated with clay (r = 840**) and K0 (r = 988*). Magnitude of labile K (KL), K held on non-specific sites (K0) and specific sites (KX) were higher in Sumbal silty clay. The negative values of ΔG indicated that K- (Ca + Mg) exchange in all the soils was spontaneous.

Kinetics of Enolisation of Acetophenone and p-Bromoacetophenone: Comparative Studies

Biomolecules (Amino Acids) have been used as catalysts for the study of kinetics involved in enolosation of acetophenone and p-bromoacetophenone, and comparative study has been carried out. The process of enolisation was studied by halogenations reactions using iodine. The stoichiometery was determined in each case and the kinetic reactions were carried out to study the effect of catalyst and temperature. The enolisation process was investigated as a bimolecular reaction. The rate of enolisation was of the order p- Bromoacetophenone &gt; Acetophenone depending on the type of substituent. The process was investigated to follow Arrhenius equation. Values for various thermodynamic parameters like Entropy (∆S≠) and Enthalpy (∆H≠) were found to be -2.126 e.u. &amp; -10.88 e.u. and 19.06 K. cal mol-1 &amp; 19.01 K. cal mol-1 for acetophenone and p-bromoacetophenone respectively. © 2013 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

High-Spin S = 2 Ground State Aminyl Tetraradicals

Aminyl tetraradicals with planar tetraazanonacene backbones have quintet (S = 2) ground states and do not show any detectable thermal population of the low-spin excited states up to the highest temperature investigated (100 K) in the 2-methyltetrahydrofuran (2-MeTHF) matrix. This indicates that the nearest electronic excited state (triplet) is at least ~0.3 kcal mol(-1) higher in energy, that is, the triplet-quintet energy gap, ΔE(TQ) > 0.3 kcal mol(-1), which is consistent with the broken-symmetry-DFT-computed ΔE(TQ) of about 5 kcal mol(-1). In concentrated (ca. 1-10 mM) solutions of tetraradical 4 in 2-MeTHF at 133 K, a fraction of tetraradicals form a dimer (association constant, K(assoc) ≈ 60 M(-1)), with a weak, antiferromagnetic exchange coupling, J/k ≈ -0.1 K ~ 0.2 cal mol(-1), between the S = 2 tetraradicals. This weak intradimer exchange coupling is expected for two tetraradicals at the distance of about 6 Å. The most sterically shielded tetraradical 5 in 2-MeTHF has a half-life of 1 h at room temperature; the product of its decay is the corresponding tetraamine, suggesting that the hydrogen atom abstraction from the solvent is primarily responsible for the decomposition of the tetraradical.