Dissociation Of Chloride Research Articles

Effects of Methanol on the Thermodynamics of Iron(III) [Tetrakis(pentafluorophenyl)]porphyrin Chloride Dissociation and the Creation of Catalytically Active Species for the Epoxidation of Cyclooctene

In a previous study, the authors showed that iron(III) [tetrakis(pentafluorophenyl)]porphyrin chloride [(F20TPP)FeCl] is catalytically inactive for cyclooctene epoxidation by hydrogen peroxide in acetonitrile but is catalytically active if the solvent contains methanol. It was suggested that the precursor to the active species is (F20TPP)Fe(OCH3) in methanol-containing solvents. The present study was aimed at evaluating this hypothesis. (F20TPP)Fe(OCH3) was synthesized and characterized by 1H NMR but was found to be inactive in both acetonitrile and methanol. Further investigation of the interactions of (F20TPP)FeCl with methanol in acetonitrile/methanol mixtures was then carried out using NMR. Two species, characterized by 1H NMR resonances at 82 and 65 ppm, were observed. The first resonance is attributed to the beta-pyrrole protons on molecularly dissolved (F20TPP)FeCl, whereas the second is attributed to beta-pyrrole protons of [(F20TPP)Fe]+ cations that are stabilized by coordination with a molecule of methanol, viz., [(F20TPP)Fe(CH3OH)]+. The relative concentration of [(F20TPP)Fe(CH3OH)]+ increases as the fraction of methanol in the solvent increases, suggesting that methanol facilitates the dissociation of (F20TPP)FeCl into cations and anions. A thermodynamic model of the dissociation is proposed and found to describe successfully the experimental observation over a range of solvent compositions, porphyrin concentrations, and temperatures. UV-visible spectroscopy was also used to validate the developed model. In addition, the observed rate constant for cyclooctene epoxidation was found to be proportional to the concentration of [(F20TPP)Fe(CH3OH)]+ calculated using the thermodynamic model, suggesting that this intermediate is a precursor to the species that catalyzes olefin epoxidation. The catalytic activity of [(F20TPP)Fe(CH3OH)]+ was further confirmed through experiments in which (F20TPP)Fe(OCH3) dissolved in methanol was reacted with HCl(aq). This reaction produced a product with an NMR peak at 65 ppm attributable to [(F20TPP)Fe(CH3OH)]+, and this mixture was found to have activity for cyclooctene epoxidation similar to that of (F20TPP)FeCl dissolved in methanol.

Thermal and photochemical behavior of trans-ruthenium(II) dichloride tetraphosphite complexes

trans-[RuCl 2{P(OR) 3} 4] complexes were prepared by reaction between RuCl 3 and P(OR) 3 ligands: R = CH 3, C 2H 5, i C 3 H 7 and n C 4H 9. The electronic spectra of freshly prepared solution show similar spectral behavior in two selected pH values: two absorptions bands at 250 nm ( ɛ = 10 3 M −1 cm −1) and at 408 nm ( ɛ = 10 2 M −1 cm −1) and a shoulder at 280 nm ( ɛ ≅ 10 2 M −1 cm −1). The complexes exhibit distinct dissociation behavior of ligands as function of the pH (3.0 and 7.0, respectively) with the increase in the size of the alkyl substituent in the phosphorus ligand. The 31P NMR data at the pH 3.0, show only one signal for complexes freshly prepared solutions with δ = 131, 129, 127 and 125 ppm assigned to R = CH 3, C 2H 5, i C 3H 7 and n C 4H 9, respectively. After 24 h reaction, the complexes exhibit 31P NMR signals attributed to new ruthenium compounds formed by chloride and phosphite dissociation, at δ = 145, 143, 140 and 137 ppm for ruthenium complexes, respectively, and signals at δ = 11.8, 9.7, 9.4 and 8.5 ppm, respectively, assigned to free dialkylphosphonate molecules in solution. At pH 7.0, only one 31P NMR signal for each complex was observed with δ = 136, 134, 131 and 130 ppm for R = CH 3, C 2H 5, i C 3H 7 and n C 4H 9, respectively. The complexes were photolyzed with light of λ = 410 nm centered on the lowest energy of ligand field transition band. Photoaquation generating Cl − species in solution was observed for all complexes with photoreaction quantum yield ϕ in the range of 0.34–0.35 mol/Einstein at pH 3.0 and in the range of 0.36–0.40 mol/Einstein at pH 7.0. In acidic medium, the irradiation at 408 nm leads to photoaquation of ligands with concomitant stereochemical change, while at neutral conditions only the chloride photolabilization was observed.

Syntheses of Acyl Rhodium Porphyrins by Aldehydic Carbon−Hydrogen Bond Activation with Rh(III) Porphyrin Chloride and Methyl

Rhodium(III) porphyrin chloride reacted with aryl aldehydes in solvent-free conditions to give acyl rhodium porphyrins. Selective aldehydic without any aromatic carbon−hydrogen bond activation (CHA) was observed. At lower temperature, reduction and side products were found. Alkanals reacted poorly. On the other hand, Rh(III) porphyrin methyl reacted more cleanly with both aryl and alkyl aldehydes. These reactions provided a facile, convenient synthesis of acyl rhodium porphyrins. These activations are unique CHA by high-valent Rh(III) species. Preliminary mechanistic experiments suggested that the rhodium(III) porphyrin chloride initially formed a cationic rhodium(III) porphyrin via chloride dissociation and then underwent oxidative addition or heterolysis to yield the product. On the other hand, rhodium(III) porphyrin methyl underwent either oxidative addition or σ bond metathesis.

Origin of Enantioselection in Chiral Alcohol Oxidation Catalyzed by Pd[(−)-sparteine]Cl2

A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl(2)-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl(2) prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl(2) to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) beta-hydride elimination of Pd-alkoxide to form ketone product and a Pd-hydride. Utilizing the addition of (-)-sparteine HCl to control the [Cl(-)] and [H(+)] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (-)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured k(rel) values when using the extracted constants. Besides a factor of 10 difference between the relative rates of beta-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (-)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd-alkoxides.

Kinetics and mechanism of the reactions of [FeCl4]– with PhS– or PhSH in MeCN, and the role of cations containing N–H groups

The kinetics of the reactions between [FeCl4]- and an excess of PhS- have been studied using stopped-flow spectophotometry. The associated absorbance-time curves can be fitted to two exponentials, and these first and second phases correspond to the formation of [FeCl3(SPh)]- and [FeCl2(SPh)2]-, respectively. It seems likely that the steps involving formation of [FeCl(SPh)3]- and [Fe(SPh)4]- are associated with much smaller changes in absorbance and so are not detected. The kinetics of the first phase exhibit a non-linear dependence on the concentration of PhS- indicating an associative mechanism in which PhS- rapidly binds to [FeCl4]- to form [FeCl4(SPh)]2- prior to rate-limiting dissociation of chloride and formation of [FeCl3(SPh)]-. The kinetics indicate that at high concentrations of PhS-, the five-coordinate intermediate attains stoichiometric concentrations. This is confirmed by the spectroscopic changes. The second phase shows analogous kinetics. The kinetics of the reactions between [FeCl4]- and an excess of PhSH have also been studied. For the first phase the reaction occurs at a rate independent of the concentration of PhSH, consistent with an associative mechanism in which the solvent (MeCN) is the nucleophile to form [FeCl3(NCMe)]. Subsequent rapid replacement of the coordinated solvent by PhSH yields [FeCl3(SHPh)]. The kinetics of the second phase of the reaction with PhSH exhibits a non-linear dependence on the concentration of PhSH, analogous to the kinetics observed with PhS- and consistent with an associative mechanism. The cations [NHEt3]+, [NH2Et2]+ and [lutH]+ (lut = 2,6-dimethylpyridine) form ion pairs with [FeCl4]- which undergo substitution more rapidly than free [FeCl4]-.

Inhibition of Catalytic Activity in Ionic Liquids: Implications for Catalyst Design and the Effect of Cosolvents

As a consequence of inherently weak interactions between the chloride anion and the 1-butyl-3-methylimidazolium cation (ca. 15 kJ/mol), chloride dissociation from a transition-metal complex can be thermodynamically disfavored in ionic liquids. Therefore, when chloride dissociation is a crucial step for the formation of the active catalytic species, the activity of the catalyst precursor might be completely inhibited in ionic liquids.

Characterization of condensed phase species produced during the thermal treatment of metal chlorides on a graphite platform using surface analysis techniques

The identification and distribution of condensed phase species produced on a graphite platform at temperatures between 200 and 2000 °C was studied using data obtained by scanning electron microscopy (SEM), energy dispersive (ED) X-ray spectrometry and Raman microanalysis. The first general conclusion is that there is no unique path in the thermal transformation of the metal chlorides tested, even for a particular metal chloride. Sodium, potassium and calcium chlorides showed similar vaporization characteristics, evolving mainly as monochlorides, while magnesium, beryllium and aluminium mainly vaporized as free atoms, following transformation caused by hydrolysis. The replacement of chlorine atoms by oxygen occurred at lower temperatures for beryllium and aluminium chlorides than for magnesium chloride. There were some particular areas on the platform surface where other mechanisms, such as hydrolysis of calcium chloride and dissociation of sodium chloride in the condensed phase, seemed occasionally to occur to a minor extent.

Complex reduction chemistry of (abpy)PtCl2, abpy = 2,2'-azobispyridine: formation of cyclic [(micro,eta2:eta1-abpy)PtCl]2(2+) with a new coordination mode for abpy and a near-infrared ligand-to-ligand intervalence charge transfer absorption of the one-electron reduced state.

The structurally characterized (abpy)PtCl2, abpy = 2,2'-azobispyridine, reveals a strong metal/ligand pi interaction as supported by DFT calculations. Unexpectedly, its chemical or electrochemical reduction occurs irreversibly to yield EPR-detectable [(micro,eta2:eta2-abpy)[PtCl2]2]*- and, as the main product of chloride dissociation, the structurally identified and DFT-calculated dinuclear [(micro,eta2:eta1-abpy)PtCl]2(2+) with a novel coordination mode for abpy and isolated as tetrachlorozincate. Stepwise reversible one-electron reduction of that dimer, separated by 0.24 V, exhibits an intense near-infrared band for the monocationic intermediate [(abpy(-I))(abpy)Pt2Cl2]*+ at 1220 nm (epsilon = 3370 M(-1) cm(-1)) which is attributed to a ligand-to-ligand intervalence charge-transfer transition.

Hydrogen-atom migration and Wolff rearrangement in photolysis of chlorohydroquinone to produce p-benzoquinone and 3-hydroxy-2,4-cyclopentadiene-1-ylidenemethanone

Photolysis of chlorohydroquinone in a low-temperature argon matrix was investigated by Fourier transform infrared spectroscopy with an aid of hybrid density-functional-theory (DFT) calculation. The new photoreaction pathways via a ketocarbene intermediate produced by dissociation of hydrogen chloride upon UV irradiation were found, where a five-membered ring ketene and p-benzoquinone were produced from the ketocarbene by Wolff rearrangement and intramolecular hydrogen-atom migration, respectively. The large H/D isotope effect in the branching ratio for the final products, ketene and p-benzoquinone, was found in the analysis of the growth behavior of the infrared bands, implying that the hydrogen-atom migration occurs by tunneling effect.

Complexes of [Re(CO) 3Cl] with different oxidation states of the polyfunctional bmtz/H 2bmtz ligand system (bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine)

Combining fac-[Re(CO) 3Cl] with components of the ligand redox system bmtz/bmtz − /H 2bmtz/H 2bmtz − (bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) has led to the isolation of the complexes (H 2bmtz)Re(CO) 3Cl, (μ-H 2bmtz)[Re(CO) 3Cl] 2 and (μ-bmtz)[Re(CO) 3Cl] 2. Other species characterized were (bmtz)Re(CO) 3Cl (UV/Vis, IR), [(H 2bmtz)Re(CO) 3Cl] − (UV/Vis, IR, EPR), {(μ-H 2bmtz)[Re(CO) 3Cl] 2} − (UV/Vis, IR, EPR) and {(μ-bmtz)[Re(CO) 3Cl] 2} − (UV/Vis, IR, X band and high-field EPR). The results confirm bmtz as very strong and H 2bmtz as moderate π acceptor ligand versus one or two chelate-bonded low-valent metal centers. Reactivity is observed in terms of oxidative proton and reductive chloride dissociation.

Synthesis, Dissociation Behavior and Crystal Structures of Palladium(II) Complexes with N,N‐Dimethylthiocarbamoyl, Me2NC=S, Containing Ligand: Structures of [Pd(PPh3)2(η1‐SCNMe2)(Cl)], [Pd(PPh3)(Cl)]2(μ,η2‐SCNMe2)2, [Pd(PPh3)2{η1‐C(SMe)(NMe2)}(η2‐S2CO)], and [Pd{η1‐SC(O)NMe2}Pd(Cl)](μ,η2‐SCNMe2)(μ,η2‐dppm)2

AbstractTreatment of Pd(PPh3)4 with N,N‐dimethylthiocarbamoyl chloride, Me2NC(=S)Cl, in dichloromethane at −20 °C produces complex [Pd(PPh3)2(η1‐SCNMe2)(Cl)] (1). In solution state, complex 1 shows the intramolecular dissociation of the chloride or intermolecular dissociation of the triphenylphosphine ligands to form η2‐thiocarbamoyl complex [Pd(PPh3)2(η2‐SCNMe2)][Cl] (2) or the bridging η2‐thiocarbamoyl dipalladium complex [Pd(PPh3)Cl]2(μ,η2‐SCNMe2)2 (3). Treatment of 1 with MeOCS2K yields [Pd(PPh3){η1‐C(SMe)(NMe2)}(η2‐S2CO)] (5a). The Fischer‐type carbene‐complex 5a is formed via methyl migration of the methyldithiocarbonate to thiocarbamoyl ligand. Complex 1 reacts with EtOCS2K resulting in the formation of η2‐dithiocarbonate complex [Pd(PPh3)(η1‐SCNMe2)(η2‐S2COEt)] (4b) and carbene‐complex [Pd(PPh3){η1‐C(SEt)(NMe2)}(η2‐S2CO)] (5b) with ratios of 10:1 from the integration of 31P{1H} NMR spectra. By continuously stirring the mixtures 4b and 5b in CH2Cl2 solution for 2 h, complex 5b was formed as the final product. Complex [Pd(η1‐SCNMe2)(η2‐Phen)(Cl)] (6) or [Pd{η1‐SC(O)NMe2}Pd(Cl)](μ,η2‐SCNMe2)‐(μ,η2‐dppm)2 (7) is accessible by the reaction of 1 with phen (1,10‐phenanthroline) or dppm {bis(diphenylphosphino)methane} in dichloromethane at room temperature. All of the complexes are identified by spectroscopic methods and complexes 1, 3, 5a, and 7 are determined by single‐crystal X‐ray diffraction. The complexes 1 and 3 crystallize in the monoclinic space group P21/n and P21/c with Z = 4, respectively, whereas 5a and 7 belong to the triclinic space group P&1bar; with Z = 2. The cell dimensions are as follows: for 1, a = 12.8248(1) Å, b = 18.6358(2) Å, c = 14.9692(1) Å, β = 105.1513(4)°; for 3, a = 9.8782(1) Å, b = 11.7238(1) Å, c = 20.7353(2) Å, α = 103.9126(5)°, β = 97.8453(3)°, γ = 104.9669(5)°; for 5a, a = 16.3638(2) Å, b = 9.3374(1) Å, c = 15.7117(2) Å, β = 98.2957(6)°; for 7, a = 12.1128(1) Å, b = 14.0133(2) Å, c = 17.7873(2) Å, α = 82.1939(5), β = 79.6190(6)°, γ = 80.9551(5)°.

Coordination of Amines to Palladium(II) Complexes of N21,N22-Bridged Porphyrins

Abstract Pyridine and ethylenediamine reacted with Pd(II) complexes of N21,N22-bridged porphyrins to give mixed-ligand Pd(II) complexes. The UV-vis and 1H NMR spectral features of the porphyrin ligand provide information regarding the nature of the coexisting ligands and Pd(II) coordination geometry in the products. That is, the splitting pattern in the Soret region of the UV-vis spectra was dependent on whether porphyrinatopalladium(II) was coordinated by neutral ligands or anionic ligands. In the 1H NMR spectra of ethylenediamine complexes of porphyrinatopalladium(II), signal broadening of porphyrin β-pyrrole protons and a chemical shift change of ethylenediamine protons were seen in the dichloride in comparison with the bis(perchlorate). These phenomena indicate the equilibrium of the coordination and dissociation of chloride and amine at the apical and basal sites in the square pyramid. The H–D exchange in the ethylenediamine ligand was promoted by the presence of chloride.

Kinetic Isotope and Collision Energy Effects in the Dissociation of Chloride and Bromide Adducts of Aliphatic Alcohols, Benzaldehyde, and 2,4-Pentanedione

A tandem-in-space triple quadrupole mass spectrometer was used to measure kinetic isotopic effects (KIEs) for the dissociation of chloride and bromide adducts of several compounds that bind halide anions via either hydrogen bonds or by nucleophilic attachment. Two isotopomers of each adduct were simultaneously mass-selected in the first quadrupole and dissociated by collision with argon in the second quadrupole. The KIEs were measured by comparing the extents of dissociation of the lighter versus the heavier isotopomeric adducts. In most cases, lower collision energies and multiple collision conditions favoured larger KIE values, an expected feature of easily dissociated cluster ions considering zero-point energies (ZPEs). The larger chloride adduct of cyclohexanol gave greater KIEs compared with the smaller alcohols, a consequence of slower dissociation due to the larger number of degrees of freedom. Dissociation of the chloride adducts gave greater KIEs than the corresponding bromide adducts, a result that is also consistent with expectations based on ZPEs. Both the chloride and bromide adducts of 2,4-pentanedione, when dissociated at 6 eV collision energy under single-collision conditions, displayed normal KIEs (1.0460 ± 0.0012 and 1.0092 ± 0.0035 respectively). These and the alcohol results were correctly predicted by the ZPEs calculated using commonly applied ab initio Hartree–Fock (HF) and B3LYP density functional theory (DFT) methods with large basis sets (6–311 containing both polarization and diffuse functions). Geometry optimization calculations for the 2,4-pentanedione chloride adduct using either the Restricted Hartree–Fock (RHF) method with a 6–31G* basis set or using the more accurate 6–31++G** method showed that, in the most stable form, the chloride is bonded at multiple sites by a molecule of 2,4-pentanedione. In this structure, chloride binds weakly to both the methylene and the methyl hydrogen atoms. Collision-induced dissociation furnishes chloride and 2,4-pentanedione anion ([M – H]–) as competitive negatively charged products, which is consistent with the proposed structure. It is interesting that the intermolecular KIEs in this study tend to be normal, while intramolecular isotope effects in halides, notably of the type M1Cl+M2 are inverse, as a consequence of the lower ZPEs associated with the heavier isotopomers. The difference in the two systems is that the stronger bonds are found in the products in the case of M1Cl+M2 dissociation but in the reactants in the case of MCl– dissociation.

Tin-capped trinuclear nickel clusters: redox isomerism between micro(3)-stannyl and micro(3)-stannylene clusters of the class [Ni(3)(dppm)(3)(micro3-I)(micro3-SnCl(x)](n+) (x = 2, n = 1; x = 3, n = 0).

The reaction of Ni(3)(dppm)(3)(micro(3)-I)(2)) with sodium trichlorostannate affords the first tin-capped nickel cluster Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3) (1). A site of coordinative unsaturation at tin can be introduced by the reaction of 1 with Tl[PF(6)] yielding the stannylene-capped cluster [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+) (2). Clusters 1 and 2 were characterized by 31P NMR, X-ray diffraction, and cyclic voltammetry (CV). Clusters 1 and 2 exhibit single electron redox chemistries, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl3](0/*-), [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)](+/0), that together comprise a redox equilibrium. Thus, electrochemical reduction of 1 produces first the 49e- cluster radical anion [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(3)](*-) which then yields the reduced form of 2, [Ni(3)(dppm)(3)(micro(3)-I)(micro(3)-SnCl(2)], upon chloride dissociation.

Theoretical and Experimental Assessment of Chloride Effects in the A-TIG Welding of Magnesium

Active fluxes that are deposited on the TIG torch path before alloy melting can significantly increase weld penetrations. Several mechanisms (arc constriction, surface-tension-driven flow) for the active-TIG or A-TIG welding process have been postulated. The A-TIG welding of magnesium is discussed here using a simple model for the surface tension over the weld pool, real-time monitoring data, and measured characteristics from the fusion zone region. The chlorides selected for this investigation incorporated simple-metal elements from different group numbers (LiiaCl, CaiiaCl2, CdiibC12, PbivbCl2 and CeCl3) so that correlations between their chemistry and their effects during A-TIG welding could be established. Video recordings showed that chlorides intensified the visible light emission from the arc and affected its profile. Measurements during arc welding at a constant current demonstrated that all chlorides increased the arc voltage (thus the heat input) and the arc temperature. A-TIG weld cross sections revealed that chlorides increased fusion zone dimensions, as could be expected from greater heat inputs. While calculations suggested that surface tension might have altered weld pool circulation, specially designed experiments with low-energy-density laser beams were inconclusive. Among all tested chlorides, cadmium chloride was the most effective during A-TIG welding due to the high first ionization potential of cadmium, which correlated to several observations such as: low chloride melting, boiling, and dissociation temperatures, high welding voltage, augmented arc temperature, increased fusion zone penetration, and greater depth-to-width ratio.

Ligand movement modulates the rate of proton transfer in reactions of [Fe4S4Cl4]2-.

Substitution of the first chloro-ligand in [Fe4S4Cl4]2- by 4-RC6H4S- (R = CF3, Cl, H, Me or MeO), in the presence of [H2N(CH2)3CH2]+, involves initial binding of thiolate, followed by protonation and finally chloride dissociation; the rate of protonation is facilitated by electron-withdrawing R-substituents indicating that Fe-thiolate bond length changes modulate proton transfer.

Absolute Heat of Formation and Singlet−Triplet Splitting for HCCN

The absolute heat of formation at 298 K for ground-state triplet cyanocarbene, HCCN, has been determined from a measurement of the chloride dissociation energy of ClCHCN-. Analysis of the energy-resolved collision-induced dissociation cross section as a function of center-of-mass collision energy in a flowing afterglow triple quadrupole instrument gives a chloride dissociation enthalpy of 43.7 ± 2.5 kcal/mol. Proton-transfer bracketing experiments were used to determine a gas-phase acidity, ΔHacid, of 357.7 ± 2.0 kcal/mol for ClCH2CN. The heat of formation at 298 K for ClCH2CN was determined from collision-induced dissociation of a series of protonated nitriles to be 25.5 ± 3.8 kcal/mol. The chloride ion dissociation enthalpy and the heat of formation and gas-phase acidity of ClCH2CN are combined in a simple thermochemical cycle to give an absolute heat of formation for HCCN of 115.6 ± 5.0 kcal/mol. High level theoretical calculations were performed in support of the experimental study at the G2 (I), CBS-Q (II), CBS-APNO (III), B3LYP/6-31G* (IV), and B3LYP/6-311++G** (V) levels of theory. The compound methods, I−III, give predictions for the acidity and heat of formation of ClCH2CN and for the heat of formation of the triplet ground state and first excited singlet state of HCCN that are in good agreement with experiment. The density functional theory predictions (IV,V) for these quantities are fair at best. The heat of formation of 3HCCN is used to derive additional thermodynamic quantities including a C−H bond dissociation enthalpy in CH2CN of 107.3 ± 5.4 kcal/mol and a singlet−triplet splitting for HCCN of 11.1 ± 5.8 kcal/mol.

Novel Irreversible Butyrylcholinesterase Inhibitors: 2-Chloro-1-(substituted-phenyl)ethylphosphonic Acids

2-Chloroethylphosphonic acid (ethephon) as the dianion phosphorylates butyrylcholinesterase (BChE) at its active site. In contrast, the classical organophosphorus esterase inhibitors include substituted-phenyl dialkylphosphates (e.g., paraoxon) with electron-withdrawing aryl substituents. The chloroethyl and substituted-phenyl moieties are combined in this study as 2-chloro-1-(substituted-phenyl)ethylphosphonic acids (1) to define the structure–activity relationships and mechanism of BChE inhibition by ethephon and its analogues. Phenyl substituents considered are 3- and 4-nitro, 3- and 4-dimethylamino, and 3- and 4-trimethylammonium. Phosphonic acids 1 were synthesized via the corresponding O,O-diethyl phosphonate precursors followed by deprotection with trimethylsilyl bromide. They decompose under basic conditions about 100-fold faster than ethephon to yield the corresponding styrene derivatives. Electron-withdrawing substituents on the phenyl ring decrease the hydrolysis rate while electron-donating substituents increase the rate. The 4-trimethylammonium analogue has the highest affinity (Ki=180 μM) and potency (IC50=19 μM) in first binding reversibly at the substrate site (possibly with stabilization in a dianion–monoanion environment) and then progressively and irreversibly inhibiting the enzyme activity. These observations suggest dissociation of chloride as the first and rate-limiting step both in the hydrolysis and by analogy in phosphorylation of BChE by 1 bound at the active site.

Photoreaction mechanisms of 2-chlorophenol and its multiple chloro-substituted derivatives studied by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculations

Photoreaction mechanisms of 2-chlorophenol, 2,3-dichlorophenol, 2,4-dichlorophenol, 2,5-dichlorophenol, 2,6-dichlorophenol and 2,4,5-trichlorophenol in low-temperature argon matrices are investigated by Fourier transform infrared spectroscopy. The observed infrared bands of photoproducts are assigned with an aid of density-functional-theory (DFT) calculations, where the 6-31++G ∗∗ basis set is used to optimize geometrical structures. It is concluded that five-member-ring ketene, cyclopentadienylidenemethanone (CPYM), is produced through keto carbene, 2-oxocyclohexa-3,5-dienylidene, by dissociation of hydrogen chloride. The geometries of HCl–CPYM complexes in a matrix cage are derived from the vibrational shifts of the hydrogen chloride.

Cyanide stimulated dissociation of chloride from the catalytic center of oxidized cytochrome c oxidase.

A comparison of bovine cytochrome c oxidase isolated in the presence and the absence of chloride salts reveals that only enzyme isolated in the presence of chloride salts is a mixture of a complex of oxidized enzyme with chloride (CcO.Cl) and chloride-free enzyme (CcO). Using a spectrophotometric method for chloride determination, it was shown that CcO.Cl contains one chloride ion that is released into the medium by a single turnover or by cyanide binding. Chloride is bound slowly within the heme a(3)-Cu(B) binuclear center of oxidized enzyme in a manner similar to the binding of azide. The pH dependence of the dissociation constant for the formation of the CcO.Cl complex reveals that chloride binding proceeds with the uptake of one proton. With both forms of the enzyme the dependence of the rate of reaction for cyanide binding upon cyanide concentration asymptotes a limiting value indicating the existence of an intermediate. With CcO.Cl this limiting rate is 10(3) higher than the rate of the spontaneous dissociation of chloride from the binuclear center and we propose that the initial step is the coordination of cyanide to Cu(B) and in this intermediate state the rate of dissociation of chloride is substantially enhanced.