Simple Protonation Research Articles

Spectroscopic and Optimization Modeling Study of Nitrous Acid in Aqueous Solution

Nitrous acid (HONO) and the nitrite ion represent a particularly important conjugate pair of trace species with regard to heterogeneous behavior within the bulk, and on the surface, of aqueous atmospheric dispersions: this role results from their chemical reactivity, photolysis pathways, solubility, and ambient concentration levels. The actual ratio of NO(2)(-): HONO in solution is determined by the pH and the nitrous acid dissociation constant (pK(a)) which is generally quoted in the literature as 3.27 at 298 K. However there is much disagreement in published works as to the exact value, which should be used in model calculations relevant to the atmosphere. Furthermore even though the nitrite ion is known to absorb solar radiation in the 300-400 nm region and represents a dominant source of *OH radicals in surface seawater, large variations in the measured molar decadic absorption coefficients, epsilon, for nitrite ions (and aqueous HONO) are evident in the literature. In the current study, using a UV-vis spectrometric approach with careful baseline subtraction, the relevant epsilon values for the nitrite ion were determined to be 8.16 +/- 0.08 M(-1) cm(-1) for the npi transitions at 290 nm and 22.1 +/- 0.22 M(-1) cm(-1) at 354 nm. For HONO, the wavelength maximum for the strongest vibronic band in solution was found at 372 nm with an epsilon value of 60.52 +/- 0.6 M(-1) cm(-1). Using the Henderson-Hasselbalch equation and the above data, a value of 2.8 +/- 0.1 is therefore reported here for the pK(a) of nitrous acid. A Newton-Gauss method was then employed to solve a set of nonlinear equations defining the chemical speciation model for HONO in solution using an algorithm written in FORTRAN 90. A model based on a simple one-step protonation worked well for intermediate pHs (6-3) but departed from the experimental observations in highly acidic media. A two-step equilibrium model involving the nitroacidium ion, H(2)ONO(+), gave a much closer fit in the very acidic region, while having little or no effect on the pH 6-3 section of the profile.

Highly active catalysts in alkene metathesis: first observed transformation of allenylidene into indenylidene via alkenylcarbyne--ruthenium species.

A cat. for all seasons: The transformation of the allenylidene-ruthenium complexes [RuCl(η6-arene)(CCCR2)(PR)][CF3SO3] into indenylidene species [RuCl(η6-arene)(indenylidene)(PR)][CF3SO3]2 by the simple protonation with CF3SO3H and formation of an alkenylcarbyne intermediate (see picture; arene=p-cymene) is observed by low temperature NMR experiments. The new in situ generated 18 electron ionic indenylidene species are highly active in the polymerization of cyclooctene and cyclopentene, ring-closing metathesis of dienes and enynes, and the acyclic diene metathesis of decadiene, thus making it a catalyst for all seasons.

Chemistry of photogenerated α-hydroxy-p-nitrobenzyl carbanions in aqueous solution: protonation vs. disproportionation

The photochemistry of p-nitrobenzyl derivatives 6–10 has been studied in aqueous solution as a function of pH, using product analysis, UV–vis spectrophotometry, and laser flash photolysis (LFP). The compounds were chosen with the aim of further exploring the propensity of these systems to give rise to α-hydroxy-p-nitrobenzyl carbanions on photolysis, and to study their mechanisms of subsequent reaction. α-Hydroxy-substituted carbanions are anions that cannot be readily formed using thermal routes but which are believed to have some interesting chemistry. Three methods were employed for photogenerating these carbanions: (i) decarboxylation; (ii) retro-Aldol reaction; and (iii) carbon acid deprotonation. All three methods proved to be successful using the p-nitrobenzyl chromophore. Photogenerated α-hydroxy-p-nitrobenzyl carbanions react via disproportionation, giving rise to oxidized and reduced products; simple protonation of the anion was undetectable.Key words: photodecarboxylation, nitrobenzyl carbanions, photoredox, nitroaromatic compounds, excited-state carbon acids.

Ligand dependent structural changes in the acid–base chemistry of electron deficient benzoheterocycle triosmium clusters

The reactions of the electron deficient benzoheterocycle clusters Os 3(CO) 9(μ 3-η 2-LH)(μ-H) (L=phenanthridine, 2; 5,6-benzoquinoline, 3; quinoxaline, 4; 2-methyl benzimidazole, 5; 2-methyl benzotriazole, 6; 2-methyl benzothiazole, 7; benzothiazole, 8; 2-methyl benzoxazole, 9; benzoxazole, 10) with n-butylamine and with the protic acids HBF 4 and CF 3CO 2H are reported. Complexes 2 and 3 behave very similarly to the analogous quinoline complex (L=quinoline, 1) forming adducts with n-butylamine with similar spectral properties to 1 and undergoing simple protonation at the metal core with both HBF 4 and CF 3CO 2H. Complexes 4– 10 on the other hand form an additional amine adduct with different spectral characteristics than 1– 3 and in the case of 5 this is the only detectable product. In addition, these compounds also form adducts with acetonitrile while 1– 3 do not. Compounds 4– 9 also exhibit much more complex behavior in the presence of CF 3CO 2H than 1– 3, giving trifluoroacetate adducts subsequent to initial protonation. Although 4, 5 and 10 behave differently than 1 with n-butylamine and protic acids they form exactly the same triphenyl phosphine adduct as 1 and 3 based on multinuclear NMR data. The solid-state structure for the amine adduct of 2 is reported and suggested structures for the other amine and acid adducts, based on NMR data, are presented. The reasons for the differences in the behavior of the three distinct groups of benzoheterocycles ( 1– 3, 4 and 5– 10) are discussed.

Formation of Hypercoordinated Carbon inside a (μ-Hydrocarbyl)bis(group 4 metallocene) Framework by a Simple Protonation Route

(Alkenyl)zirconocene chlorides Cp2Zr(Cl)(CHCHR) (4, R = H, phenyl, n-butyl, or cyclohexyl) react with “Cp2Zr” generated from various precursors to yield the neutral dinuclear alkenyl-bridged bis(metallocene) complexes 6 [Cp2Zr(μ-Cl)(μ-η1:η2-CHCHR)ZrCp2]. The complexes 6 are regioselectively protonated by treatment with (HNMe2Ph+)(BPh4-) (8a) to yield the unusually structured products 9 [Cp2Zr(μ-Cl)(μ-η1:η2-C1H2C2HR)ZrCp2+], which exhibit an unsymmetrically bridged hydrocarbyl ligand containing a novel type of a hypercoordinated carbon center (C1) inside the rigid organometallic framework. From the NMR analysis and an X-ray crystal structure determination of the example 10c [(MeCp)2Zr(μ-Cl)(μ-C1H2C2H-n-butyl)Zr(MeCp)2+], it is evident that the hypercarbon atom C1 is coordinated to four close neighboring atoms (Zr1, Zr2, C2, and H1a) in a distorted square-planar arrangement with the remaining C1−H1b bond being oriented perpendicular to it. H1b thus marks the apex of a distorted square pyramid; the hypercarbon atom C1 is located in the center of the basal plane. This unusual structural coordination geometry around C1 is determined by the stereoelectronic features of the two adjacent group 4 bent metallocene units. From the dynamic features of complex 9a (R = H), a stabilization energy of ca. 10 kcal mol-1 is estimated for the uncommon coordination mode of C1, which is favored here, relative to a “normal” sp3-hybridized structure in a C2v-symmetric metallacyclic framework. Part of the pronounced thermodynamic stabilization of this unusual pentacoordinate carbon geometry originates from the strong α-agostic Zr···H1a−C1 interaction in the basal plane. From the monodeuterated derivative 9a-C1HD, an energy difference of 220 cal mol-1 between D and H favoring the bridging position was determined.

Green Fluorescent Protein as a Noninvasive Intracellular pH Indicator

It was found that the absorbance and fluorescence of green fluorescent protein (GFP) mutants are strongly pH dependent in aqueous solutions and intracellular compartments in living cells. pH titrations of purified recombinant GFP mutants indicated >10-fold reversible changes in absorbance and fluorescence with pK a values of 6.0 (GFP-F64L/S65T), 5.9 (S65T), 6.1 (Y66H), and 4.8 (T203I) with apparent Hill coefficients of 0.7 for Y66H and ∼1 for the other proteins. For GFP-S65T in aqueous solution in the pH range 5–8, the fluorescence spectral shape, lifetime (2.8 ns), and circular dichroic spectra were pH independent, and fluorescence responded reversibly to a pH change in <1 ms. At lower pH, the fluorescence response was slowed and not completely reversed. These findings suggest that GFP pH sensitivity involves simple protonation events at a pH of >5, but both protonation and conformational changes at lower pH. To evaluate GFP as an intracellular pH indicator, CHO and LLC-PK1 cells were transfected with cDNAs that targeted GFP-F64L/S65T to cytoplasm, mitochondria, Golgi, and endoplasmic reticulum. Calibration procedures were developed to determine the pH dependence of intracellular GFP fluorescence utilizing ionophore combinations (nigericin and CCCP) or digitonin. The pH sensitivity of GFP-F64L/S65T in cytoplasm and organelles was similar to that of purified GFP-F64L/S65T in saline. NH 4Cl pulse experiments indicated that intracellular GFP fluorescence responds very rapidly to a pH change. Applications of intracellular GFP were demonstrated, including cytoplasmic and organellar pH measurement, pH regulation, and response of mitochondrial pH to protonophores. The results establish the application of GFP as a targetable, noninvasive indicator of intracellular pH.

Cationic Trinuclear 48-Electron Ruthenium Carbonyl Cluster Complexes Containing No Hydride Ligands

In previous papers, we reported that the carbonyl cluster compound [Ru3@-H)@3-ampy)(CO)g] (1; Hampy = 2-amino6-methylpyridine) reacts with diphenylacetylene to give the alkenyl-bridged derivative [Ru3@3-ampy)@,q1:q2-PhC=CHPh)(co)S] ( 2 ) ' ~ ~ and that complex 2 can be protonated with HBF4.OEt2 to give [Ru3@-H)@3-ampy)@,q1 :q2-PhC=CHPh)(CO)s][BF4] (3) (Scheme 1).2a The presence of the alkenyl and hydride ligands in complex 3 prompted us to study reactions which might induce the reductive elimination of cis-stilbene and that would lead to cationic cluster compounds containing no hydride ligands. Simple protonation of neutral carbonyl cluster complexes frequently leads to hydridic cationic derivative^.^,^ However, in contrast with the geat number of neutral and anionic carbonyl cluster compounds reported to date: very few nonhydridic cationic carbonyl clusters have been d e s ~ r i b e d . ~ ~ As far as we are aware, and with the exception of the 50-electron compound [ R U ~ @ P P ~ ~ ) ~ ( C O ) ~ ] + , ~ all of the other nonhydridic cationic carbonyl cluster complexes are either 47-electron

Thermodynamic Studies on the Complexation of o-Phenylenediamine-N,N,N′,N′-tetraacetate with Divalent Cadmium, Mercury, and Lead Ions in Aqueous Solutions

Abstract Potentiometric studies have been carried out on the complexation of o-phenylenediamine-N,N,N′,N′-tetraacetate (phdta, L4−) with the cadmium and lead ions (M2+) in aqueous solutions at 25 ± 0.1 °C and I = 1.00 M (NaClO4). The ligand buffer method was successfully applied to the determination of the formation constant (M2+ + L4− = ML2− : KML) by use of Cd and Pb ion-selective electrodes. This method gave log KML = 13.37 for CdL and 13.89 for PbL. Calorimetry and potentiometry gave the values of enthalpy and entropy of complexation (−ΔH°/ kJ mol−1 = 18.9, ΔS°/J K−1 mol−1 = 193 (CdL); 34.8, 149 (PbL); and 50.6, 165 (HgL), respectively). Thermodynamic parameters of the protonation to ML were also examined for the PhDTA complexes of these and some divalent transition metal ions. The metal complexes are classified into two categories according to the thermodynamic parameters: A) Mn2+, Cd2+, and Pb2+ (−ΔH°MHL = −1.1–1.1 kJ mol−1; ΔS°MHL = 43–49 J K−1 mol−1). B) Co2+, Ni2+, Cu2+, and Zn2+ (−ΔH°MHL = 8.0–11.3 kJ mol−1; ΔS°MHL = 18–37 J K−1 mol−1). The smaller value of −ΔH°MHL for A has been interpreted in terms of simple protonation to a carboxylate oxygen in the complex ML, with any acetate group not being dissociated. The entire process of protonation for B, however, may be composed of two steps: i) Detachment of one carboxylate group from the metal ion, followed by the hydration resulting in M(L′) (H2O)2−, where L′ represents a ligand anion with a free carboxylate group, ii) Protonation to an oxygen atom of the detached carboxylate group.

Protonation of 18-crown-6 by dichloropicric acid in anhydrous and in water saturated 1,2-dichloroethane. Homoconjugation of dichloropicric acid

From conductometric and UV-VIS spectrophotometric studies of the reaction between 18-crown-6 (L) and dichloropicric acid (HA) in dry and water saturated 1,2-dichloroethane, it has been concluded that formation of a 1:1 homoconjugate HA 2 − accompanies the simple protonation of L, viz, L+HA⇆LH+A− and L+2HA⇆LH+HA 2 − . The electrolytes LH+A− and LH+HA 2 − are extensively, or practically completely dissociated in both solvents under the experimental conditions. The specie LH+A− appears to be a contact ion pair in DCE. The stability constant of HA 2 − in the dry solvent, 5.7×103 mol−1-cm3, is some 102.4 times that in propylene carbonate reflecting the difference in H-bond accepting capacity of the two solvents. Hydration of HA, A− and HA 2 − in wet dichloroethane is negligible or slight. As expected, LH+ is rather strongly hydrated, the ratio of the hydration constants of LH+ and L being about 1×101.

Hydride transfer and dihydrogen elimination from osmium and ruthenium metalloporphyrin hydrides: model processes for hydrogenase enzymes and the hydrogen electrode reaction

A series of metalloporphyrin hydride complexes of the type K[M(Por)(L)(H)] (M - Ru, Os; Por - OEP, TMP; L = THF, *Im, PPh{sub 3}, pyridine) has been synthesized by stoichiometric protonation of the corresponding K{sub 2}[M(Por)], followed by addition of L. The addition of excess acids to these hydrides resulted in the elimination of dihydrogen. The kinetics showed no evidence for a bimolecular mechanism for this process and suggest simple protonation of the metal-hydride bond followed by dihydrogen loss. One-electron oxidation of the metal hydrides also resulted in dihydrogen formation. The kinetics of the oxidatively induced hydrogen evolution step from K[Ru(OEP)(THF)(H)] were examined and indicate a biomolecular mechanism in which two metal hydrides reductively eliminate one dihydrogen molecule. The rate constant was determined to be 88 {+-} 14 M{sup -1} s{sup -1}. These reaction mechanisms are discussed in the context of designing bimetallic proton reduction catalysts. The metal hydride K[Ru(OEP)(THF)(H)], was also synthesized by heterolytic activation of H{sub 2}. This hydride is a good one-electron reductant (-1.15 V vs FeCp{sub 2}) and is capable of reducing, by hydride transfer, the NAD{sup +} analogue, 1-benzyl-N,N-diethyl-nicotinamide. This nicotinamide reduction by a hydride formed from heterolytic dihydrogen activation is suggested as the mechanismmore » for hydrogenase enzymes. 38 refs., 4 figs., 3 tabs.« less

Reactions of methylpropene-2- 13C on H-mordenite : II. Influence of aging and reaction temperature

Catalyzed reactions of methylpropene-2- 13C on H-mordenite have been carried out at 50, 120, 170 (before and after catalyst aging) and 400 °C. At low temperature (50 °C) no hydrocarbon residues are formed, and simple protonation takes place leading to trimethylpentenes. At 120 °C during aging and at 170 °C the carbon-13 distribution shows that the gaseous C 5C 7 products are deeply rearranged. The distributions cannot be accounted for by the same mechanism of partial scrambling—i.e., addition of scrambled monolabeled C 1, C 2, and C 3 units to an isobutene molecule (mechanism D)—observed on aged mordenite at 120 °C (Part I) and are best explained by combining linearly a distribution associated with mechanism D and a distribution involving complete redistribution of all carbon atoms (statistical). At 400 °C isobutene yields saturated hydrocarbons and aromatics, all of which have a statistical distribution of the carbon-13. These results and the mass spectrometric analysis of the hydrocarbon residues allowed us to propose the following sequence of reactions. At 120 °C, during aging, and at 170 °C polymeric cations attached to the zeolite framework are deactivated by β-fission producing carbenium ions and adsorbed olefln radicals. These radicals may then act as hydride and proton donors leading to saturated hydrocarbons and adsorbed alkadiene radicals. Such adsorbed olefinic and dienic radicals, which represent the strongly adsorbed residues left on the zeolite at 120 and 170 °C, yield at 400 °C paraffins by hydride transfer and triene precursors of the aromatics by β-fission. In this reaction scheme, the formation of deeply rearranged hydrocarbons is explained by successive adsorption-desorption steps of the primary olefins and cations.

THE FORMATION OF LONG WAVELENGTH ABSORBING SPECIES FROM SHORT WAVELENGTH ABSORBING LINEAR CONJUGATED POLYENES*

Abstract— Even linear conjugated polyenes absorb at a wavelength much shorter than their odd polyene analogues. One way even polyenes can be converted into odd polyenes is by simple protonation. The odd polyene, the retinyliccation, absorbs at a wavelength long enough, 600 nm, to accommodate known visual pigments. Protonated unsymmetrical π‐systems terminating in nitrogen have the ability to absorb at intermediate and long wavelengths. The actual excitation energy is a function of the anionic radius. Consequently, the wavelength in a visual pigment can be regulated by the distance between the centers of positive and negative charges for the N‐retinylidenealkylammonium cation.

The mechanism of the fixation of tissue components by osmium tetroxide via hydrogen bonding

Considerable electron density, observable in membranous elements in thin sections of buffered OsO4-fixed tissue with electron microscopy, was removed by the action of K4Fe(CN)6 in acetic acid. This reagent reacted only with Os(VIII) and not with lower oxidation states of osmium to give an insoluble crystalline precipitate, K2FeOs(CN)6. Supporting this finding was the fact that the osmate esters, Os(VI), originally synthesized by Criegee (8, 9), were unreactive with acetic ferrocyanide. Acetic acid alone decreased membrane density of fixed sections by simple protonation but did not remove osmium from osmate esters. Further, fresh tissue incubated in a saturated urea solution, then fixed in buffered OsO4 not only failed to blacken, as routinely observed after OsO4 fixation, but also tissue structure was poorly preserved and membranous structures showed less electron density. Tissue fixed in a 1:1 solution of urea and OsO4 also demonstrated competition between the two compounds for binding sites in membranes. Thus, osmium in the Os(VIII) state appears responsible for some, if not much, of the membrane density, and is probably hydrogen bound in its native state as OsO4, to protein and to aliphatic side chains of lipid in membranes. Theoretical calculations pertaining to molecular interactions of OsO4 urea, and model proteins and lipids indicate that the above mechanistic hypothesis of the interaction of OsO4 with tissue components is favored over the mechanism of osmate ester formation with the double bonds in lipids as has been believed to date.

Semi-empirical calculations on carbinol—carbonium ion equilibria

Extended Hückel MO (EHMO) calculations have been carried out on the reactants and products in the ionization reaction of meta- and para-substituted aryl carbinols to the corresponding carbonium ions. The calculated energy differences between the carbinols and the corresponding carbonium ions have been compared with σ+ values and the charge on the exocyclic C atom (c7) for the benzyl ions, and with σ+, σ and the experimental ΔG values for the triarylmethyl carbonium ions. The calculated charge on the exocyclic C atom has been found to give a good correlation with the experimental pKR+ values. All simple protonation reactions are predicted to be exothermic, as in the gas phase, and are found to be almost independent of the substituents.

SCF wavefunction for the ground state of CN − and the change of the correlation energy in some simple protonation processes

The SCF MO LCAO wavefunction of CN − has been calculated with a double-ζ basis plus 3d polarization functions (Slater type orbitals). A comparison is made between the SCF and the experimental values of the heat of protonation, supplemented with data pertinent to other systems. A discussion on the changes in the correlation energy in these processes is also made.

On sulfenium ions in 100% sulfuric acid

Cryoscopic and conductimetric measurements on solutions of 2:4 dinitrobenzene sulfenyl chloride in 100% H2SO4 and dilute oleum are found to be more consistent with simple protonation of the sulfenyl chloride rather than with sulfenium ion formation as previously reported by Kharasch and co-workers (1) in99%H2SO4.

Die Raschneutralisation von Wolframaten und Wolframatophosphaten in der Strömungsapparatur

AbstractWith a rapid flow apparatus the investigation of the simple protonation of WO42− was achieved (pK1 + pK2 = 8,1, both H+ being added almost simultaneously). The rapid condensation which follows the protonation leeds during the first decisecond to a product of Z = 1.25, probably H5W4O163−. A few minutes later, a new preequilibrium is established with (according to Sasaki) HW6O215− (Z = 1.167) as practically the only polynuclear product present. The final equilibrium is reached only very slowly.