Proton Attack Research Articles

Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes †

The formation of ternary UO2(ac)pFq2–p–q (p = 1 or 2 and q = 1–3) complexes, and their equilibrium constants were investigated by potentiometric titrations and 19F NMR spectroscopy. The equilibrium constants have been determined from the emf data in a NaClO4 medium at constant sodium concentration, [Na+] = 1.00 M at 25 °C, except for the UO2(ac)F32– complex where 19F NMR at –5 °C was used. The magnitude of the equilibrium constant for the stepwise addition of fluoride indicates that prior co-ordination of acetate has only a small effect on the subsequent bonding of fluoride. The acetate exchange in the ternary UO2(ac)F32– complex was studied using 19F NMR. Through magnetisation transfer experiments, it was possible to confirm the provisional mechanism from a previous study and also the consistency of the rate constants for the five different exchange pathways required to describe the fluoride exchange. The exchange takes place via the intermediate UO2F3(H2O)2–, indicating that the acetate exchange follows an interchange mechanism with solvent participation in the transition state. The rates and mechanisms of the ligand exchange reactions in UO2(ox)2(H2O)2– and UO2(ac)2(H2O) have been studied using 13C NMR techniques at –5 °C. The rate law is v = k[complex][ligand], and the second order rate constant and the activation parameters for both systems have been determined. The reactions most likely take place through an Eigen–Wilkins type of mechanism, where the first step is a pre-equilibrium of an outer-sphere complex followed by a rate determining exchange of water. The rate constants for the water exchange reactions are very similar to that in UO2(H2O)52+. The information from the binary oxalate system rules out the formation of UO2(ox)2(H2O)2– as an intermediate in the exchange reactions in the previously studied UO2(ox)2F3–, also in this case confirming a previously suggested exchange mechanism. The H+/D+ isotope effects and a linear free energy relationship suggest that the main catalytic effect of H+ on ligand exchange rates is due to the formation of a protonated precursor. Hence, the catalytic effect depends on the basicity of the ligand and the site for the proton attack.

Toward Organic Superbases: The Electronic Structure and the Absolute Proton Affinity of Quinodiimines and Some Related Compounds

Several molecular systems that may well serve as potent organic superbases are examined by using ab initio and semiempirical theoretical models. It is found that the imino group attached to the semiquinoid fragment or to a backbone of several quinoid six-membered rings exhibits a very high proton affinity (PA). It is found that the reason behind their amplified basicity is appreciable aromatization of the quinoid building blocks upon protonation. The underlying mechanism in extended systems is that of the aromaticity spin-off effect, triggered by the proton attack at the imino N atom and spread along the quinoid ribbon in a typical domino fashion. It yields an increase in the PA as high as roughly 20 kcal/mol per the quinoid ring. Susceptibility toward the proton attack is further amplified by the alkyl substitution at the imino nitrogen atom and by additional substitution of the amino groups at specific positions within the framework of the quinoid building block(s). It is stressed that synthesis of the studied systems might provide very potent organic superbases.

Absolute proton affinities of some substituted toluenes: the additivity rule of thumb foripso attack

The problem of the ipso protonation of toluene and its predominantly disubstituted derivatives was considered by the MP2(fc)/6–31G**//HF/6–31G*+ZPE(HF/6–31G*) theoretical model. The substituents involved covered a wide range of different donor–acceptor capabilities. It is shown that the calculated MP2 ipso proton affinities of substituted toluenes follow mutatis mutandis the same additivity rule which was found earlier to be operative in polysubstituted benzenes, naphthalenes and biphenylenes. The additivity equation is both intuitively appealing and useful, being able to offer quantitative estimates of the proton affinity by very simple calculation. It is based on the concept of the increment, which in turn describes the influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the other were non-existent, thus giving rise to the independent substituent approximation (ISA). The performance of the additivity rule of thumb is very good, as evidenced by the average absolute deviation of 1 kcal mol−1. Larger deviations are possible, but they rarely occur, being indicative of a difference in interactions between substituents in the initial neutral base and in the final cationic conjugate acid. Finally, it follows as a corollary of the present analysis that protonation ipso to the CH3 group is never thermodynamically the most favourable site of proton attack in the benzene ring, provided that there is a single unsubstituted carbon atom within the aromatic moiety. The relevance of ipso protonation in persubstituted benzenes is briefly discussed. © 1998 John Wiley & Sons, Ltd.

Investigations of the Factors Affecting the Stability of Dihydrogen Adducts of Platinum(II)

The preparation and study of Pt(II) H_2 adducts and Pt(IV) dihydride complexes are described. The species of interest are generated by protonation of hydridoplatinum(II) complexes of the type trans-(PCy_3)_2Pt(H)X [X = SiH_3, H, CH_3, Ph, Cl, Br, I, CN, CF_3SO_3] and [trans-(PCy_3)_2Pt(H)L][BArf_4] [L = CO, 4-picoline; BArf_4 = B(3,5-C_6H_3(CF_3)_2)_4]. The proton attacks one of three different sites on these complexes (hydride, platinum, or the trans ligand), depending on which ligand is trans to hydride. These studies reveal several factors affecting the stability and reactivity of Pt(II) σ adducts, which thus have implications for C−H activation by Pt(II).

Effect of autotransformation on the layer charge of smectites determined by the alkylammonium method

AbstractHydrogen-forms of <2 µm fractions of six bentonites of various Fe contents were prepared by H+→OH-→H+ ion exchange using resins. Potentiometric titration curves revealed that the number of strong acid sites varied and accounted for 60-95% of the total acidity in the freshly prepared H-forms. The number of strong acid sites decreased and that of the weak acid sites increased on ageing. The process of autotransformation in aqueous dispersion at 90~ was completed within four days. Layer-charge distributions of all samples were inhomogeneous with layer charges from 0.25-0.39 Eq/unit O10(OH)2. Oxalate pretreatment of the samples resulted in changes in the layer-charge distribution due to the removal of readily soluble phases which may have blocked exchange sites. After autotransformation, the alkylammonium exchange method revealed inhomogeneous charge density distributions; the fraction of layers of the highest charge decreased. Comparison of total CEC obtained from potentiometric curves and interlamellar CEC calculated from the mean layer charge confirmed attack of protons from particle edges. However, for several samples the structural attack may also occur from the interlayer space. Autotransformation of the Hsmectites decreased the mean layer charge. Protons probably attack the Mg(O,OH)6 octahedra preferentially during the autotransformation.

Theoretical model calculations of the absolute proton affinities of benzonitrile, nitroso- and nitrobenzene

Protonation in benzonitrile ( 1), nitrosobenzene ( 2) and nitrobenzene ( 3) is examined by using the MP2(fc)/6-31G ∗∗//HF/6-31G ∗ + ZPE(HF/6-31G ∗) theoretical model. It is shown that the studied systems are protonated at heteroatoms: nitrogen in 1 and 2 and oxygen in 3. The absolute proton affinities (PAs) are in good accordance with the available measured data. However, PAs of nitrogen and oxygen atoms in 2 are relatively close implying that much more accurate calculations are necessary for an ultimate assignment. Analysis of the descriptors of covalent bonding reveals that the heteroatomic protonation increases the resonance interaction between substituent groups and the benzene moiety. The ring protonation, on the other hand, is energetically less favorable. It yields lower PAs than in the parent benzene molecule. Deactivation of the aromatic fragment toward electrophilic (proton) attack is rationalized by the antagonism between the two π-bond localization patterns: one caused by the electron withdrawing substituents CN, NO and NO 2 and the other arising from protonation and the subsequent formation of a sp 3 center. The ipso protonation is the least favorable in all three cases since the attacked C(1) atom has a depleted electron density due to the highly electronegative nearest neighbor (inductive effect). In addition, the resonance interaction between the benzene ring and the substituent are practically switched off, since protonation at the ipso position bends the substituent's covalent bond out of the molecular plane leading to substantial ring puckering of the aromatic moiety at the same time.

Highly diastereoselective electrophilic additions to the vinylcyclopropane moiety of a homotropylidene system

The acid catalyzed addition of AcOH and MeOH to the vinylcyclopropane moiety of tetracyclo[5.3.2.0 2,10 .0 3,6 ]dodeca-4,8,11-triene ( 1 ) afforded kinetic mixtures of 9-acetoxy and, respectively, 9-methoxytricyclo[4.3.3.0 2,5 ]dodeca-3,7,10-trienes in which the β derivatives are always, even when sterically more congested, highly prevalent over their α counterparts. Studies with deuterated acetic acid showed that there was not any relationship between the stereochemistry of the deuteron (proton) attack and that of the nucleophilic trapping of the intermediate carbocation. These findings can be rationalized on the basis of a two step process i) proton attack to the homotropylidene moiety of 1 with formation of a cyclopropylcarbinyl carbocation ii) opening of its cyclopropane ring under tight assistance by the nucleophilic solvent to give the final products with high β diastereoselectivity. By contrast, the reaction of dichloroketene, a strong uniparticulate electrophile, involved only the cyclobutene system of 1 . Graphic

Hydroconversion of Heptane over Pt/Al-Pillared Montmorillonites and Saponites. A Comparative Study

Montmorillonites and saponites have been pillared with oxo-hydroxy-aluminium solutions (OH/Al molar ratio of 1.6). The solids have been examined by X-ray diffraction. The textural properties, residual cation exchange capacities, and acid contents have been established. The pillared clays exhibit thermally stable spacings, specific surface areas, and micropore volumes typical of such materials. Higher acid contents were found for pillared saponites than for pillared montmorillonites. IR spectroscopy of pillared saponites showed a new OH stretching vibration at 3595 cm−1, absent in pillared montmorillonites. The acid character of these OH groups has been assessed by pyridine adsorption. The catalytic properties have been evaluated over Pt-impregnated samples in the heptane hydroisomerisation reaction. Al-pillared saponites, smectites with lattice Si for Al tetrahedral substitutions, showed superior catalytic performances (high conversions, high yields of isomers) compared with Al-pillared montmorillonites, clays with lattice octahedral substitutions. The higher activity of the saponites is attributed to the higher content and strength of the acid sites associated with Si–OH…Al groups produced upon proton attack of the tetrahedral Si–O–Al bonds. Such strong acid sites are absent in Al-pillared montmorillonites.

Mono- and diprotonation of dihydropyrene, 2,7-di-tert-butyl-dihydropyrene, and their conversion to pyrenium ions; Influence of the radical cation and its potential utility in NMR assignments of the arenium ions of readily oxidizable PAHs

Parent dihydropyrene 1 and 2,7-di-tert-butyldihydropyrene 3 are monoprotonated with FSO3H/SO2CIF to give their persistent monoarenium ions 1H + and 3H + by the attack of proton at C-3 (peri to the ethano-bridge not at C-1 as previously suggested). Dihydropyrene 3 is diprotonated in FSO3H.SbF5 1∶1 “Magic Acid”R/SO2CIF to give the symmetrical dication 3H 2 +2, similar diprotonation of 1 with “Magic Acid”/SO2CIF or with FSO3H.SbF5 (4∶1)/SO2CIF gave the diprotonated species 1H 2 +2 in a mixture. NMR characteristics of the mono- and dications are discussed.

On the reactivity of H-, Ga- and Cu-MFI zeolites towards t-butyl hydroperoxide (TBHP)

The highly acidic H-MFI zeolite effectively catalyzes the decomposition reaction of t-butyl hydroperoxide (TBHP) which most likely proceeds through an electrophilic attack of the zeolite protons on either oxygen of the peroxide. Replacement of the zeolitic protons by cationic Ga leads to a decrease of the reaction rate due to the removal of proton sites which also suggests that gallium cations are virtually inactive for TBHP decomposition. In contrast, copper ion-exchanged MFI is an active TBHP decomposition catalyst and, more importantly, radicals are involved as intermediates in the decomposition mechanism. The existence of radicals in the liquid phase was unambiguously evidenced utilizing N-phenyl-β-naphthylamine as a radical inhibitor in the batch reaction. The involvement of a homolytic pathway in the decomposition reaction is explained with oxidation-reduction cycles of the zeolite copper cations.

Stoichiometry of protonation of aromatic hydrocarbon radical anions by weak proton donors. A marked discrepancy between the number of protons used and those incorporated into the aromatic structure

The stoichiometries of the reaction between alkali metal radical anions of biphenyl, naphthalene, phenanthrene and anthracene, and methanol and/or other proton donors have been determined by the magnetic titration technique. In the case of naphthalene radical anion and, for example, methanol as the proton source, the stoichiometry was found to be cation-dependent: Li, 2:1; Na, 1.75:1; K, 1.33:1. The reaction products using the experimentally determined stoichiometric conditions were ca. 95% naphthalene and 5% dihydronaphthalene(s). Thus, a marked discrepanc is observed between the protons used and those incorporated into the naphthalene molecule. Radical anions, at concentrations comparable with those of preparative reactions, react with carbon acids or amines according to the first-order kinetic law, although the initial concentrations of the two reactants were of the same order of magnitude or even equal. Lithium anthacene radical anion reacts with phenylacetylene and diethylamine at comparable rates, although the two “acids” differ in their acidities by ca. 10 orders of magnitude. A deuterium isotope effect of 2.49±0.05 was observed in the reaction between lithium anthracene radical anion and diethylamine. A general reaction scheme is proposed that involves electron transfer to the proton and hydrogen-atom attack on the neutral hydrocarbon as the key reaction steps.

Effect of pyridine on the regio- and stereo-chemistry in the addition of bromine chloride to α,β-unsaturated aldehydes and ketones

The addition of bromine chloride (BrCl) in methylene dichloride (CH2Cl2) in the presence and absence of acid scavengers such as pyridine to the following α,β-unsaturated aldehydes, ketones and esters is described: acrylaldehyde 1, methyl vinyl ketone 2, phenyl vinyl ketone 3, (E)-crotonaldehyde 4, (E)-pent-3-en-2-one 5, (E)-4-phenylbut-3-en-2-one 6, 4-methylpent-3-en-2-one 7, methyl isopropenyl ketone 8,3-phenylbut-3-en-2-one 9, cyclohex-2-enone 10, methyl acrylate 11, (E)-methyl crotonate 12 and methyl methacrylate 13. The majority of the aldehydes and ketones gave primarily anti-Markovnikov (AM) bromo chloride regioisomer in the absence of pyridine. In most cases the Markovnikov (M) regioisomer increased significantly in the presence of pyridine. Addition of BrCl to 4, 5, 6 and 10 became stereospecific (erythro) in the presence of pyridine. The stereospecificity of addition to ester 12 was high with or without pyridine. These data were interpreted as follows: in the absence of an acid scavenger, an acid-catalysed reaction is involved, initiated by attack of proton on the carbonyl oxygen. When traces of acid are removed by an acid scavenger, the reactions proceed through a bromonium ion–chloride ion intermediate. The esters reacted only via a bromonium ion giving essentially the same mixture of regioisomers with and without acid scavenger.

Electron transfer as a possible initial step in nucleophilic addition elimination reactions between (radical) anions and carbonyl compounds in the gas phase

AbstractThe reactions of the HO−, CH3S−, CH2S− and CH2  C(CH3)CH2− ions with three ketones (CF3COR; R  CH3, CF3, C6H5) and three esters of trifluoroacetic acid (CF3CO2R; R  CH3, C2H5 and C6H5) have been studied with use of Fourier Transform Ion Cyclotron Resonance (FT‐ICR) mass spectrometry. All four negative ions react exclusively by proton transfer with CF3COCH3. With the other substrates, the HO− ion reacts by various pathways, such as proton transfer, SN2 substitution, E2 elimination and attack on the carbonyl group. The CH3S− ion is unreactive towards CF3COC6H5 but is able to react by hydride transfer, SN2, E2 and/or carbonyl attack with the remaining neutral species. The CH2S− radical anion reacts by electron transfer to afford stable molecular radical anions of CF3COCF3 and CF3COC6H5, whereas the main reaction with the two esters, CF3CO2CH3 and CF3CO2C2H5, is dissociative electron transfer leading to CF3CO2− and CF3− ions. The CH2C(CH3)CH2− anion displays a more complex reactivity pattern involving electron transfer, SN2, E2 as well as attack on the carbonyl group. Direct evidence for the occurrence of electron transfer as the initial step in an overall BAC2 type process has not been obtained for the systems studied. The reaction of the CH2S− ion with CF3CO2C6H5 was observed, however, to yield exclusively a CF3COCHS−. radical anion. Based upon the absence of a BAC2 process in the reaction of CH2S− with the methyl and ethyl esters of trifluoroacetic acid in combination with the facile occurrence of electron transfer from this radical anion, it is suggested that the CF3COCHS−. ion is formed by an initial electron transfer followed by coupling between the CH2S molecule and the CF3CO2C6H5− radical anion and subsequent loss of C6H5OH from the collision complex.

Theoretical Model Calculations of the Proton Affinities of Aminoalkanes, Aniline, and Pyridine

It is shown that the MP2(fc)/6-311+G**//HF/6-31G*+ZPE(HF/6-31G*) theoretical model reproduces very well the experimental proton affinities (PAs) of aminoalkanes and of some of their fluoro derivatives as well as the PAs of aniline and pyridine. In all molecules considered the nitrogen is most susceptible to proton attack. PA values of amino derivatives including aniline are shown to be linearly dependent on the nitrogen lone-pair s character. Increments I(X)α are derived for the amino group and the pyridine nitrogen, which extend the set of substituent increments derived previously for an additive estimation of PA values of polysubstituted benzenes.

Molecular Design of Dissolution Inhibitor in Chemical Amplification Resist System by Molecular Orbital Method: Transparency and Reactivities with “Protons”

The molecular orbital (MO) method has been applied to study the transparency of any dissolution inhibitors in the deep-UV region and the reactivities of those molecules with protons derived from a photoacid generator in a chemical amplification resist system for excimer laser lithography. The results indicate that 1,3-bis(tert-butoxycarbonyloxy) benzene is the best dissolution inhibitor with respect to both the transparency and the reactivities among the five dissolution inhibitors studied, because it has small absorption peaks in the wavelength range longer than 215 nm and easily decomposes into the oxycarbonyloxyl compound and the tert-butyl group upon proton attack. The validity of the MO approach is confirmed by the experimental results.

Theoretical calculations of proton affinities in phenol

It is shown that a relatively simple MP2(fc)/6–31G ∗∗//HF/6–31G ∗ model is capable of providing quantitative description of protonation in phenol. The use of the 6–31G ∗∗ basis set in the single-point MP2 calculation is crucial in this respect. The zero-point energy (ZPE) contribution to the proton affinity (PA) is estimated at the HF/6–31G ∗ level of approximation. It appears that the contribution of the ZPE energy to relative ΔPA proton affinities is negligible. The simple additivity rule for calculating empirical ZP energies works relatively well for the protonated species too. The energetically most favourable site of the proton attack is para to the OH substitution in accordance with the experimental finding. Performance of the MP2(fc)6–31G ∗∗+ZPE(HF/6–31G ∗) model in reproducing protonation at the oxygen atom is tested in some medium size alcohols and ethers. The calculated PA values are in good agreement with the measured data.

Reverse Water-Gas Shift Reaction Catalyzed by Ruthenium Cluster Anions

Abstract Reverse water-gas shift reaction (H2 + CO2 → CO + H2O) was homogeneously catalyzed by anionic Ru cluster in the presence of bis(triphenylphosphine)imminium chloride ([PPN]Cl). The catalytic reaction was estimated to proceed via dehydrogenation of hydride cluster, followed by coordination of CO2 and electrophilic attack of proton on its oxygen atom in the presence of chloride anion.

A quantum-chemical study of adsorbed nonclassical carbonium ions as active intermediates in catalytic transformations of paraffins. II. Protolytic dehydrogenation and hydrogen-deuterium hetero-isotope exchange of paraffins on high-silica zeolites

HF-21G quantum-chemical analysis of the protolytic attack of acid protons in zeolites at the C-H bonds in methane and ethane indicated that the resulting transition states depend on the sign of the bond polarization. If a hydride ion is split off from the paraffin, then the transition state resembles the adsorbed carbonium ion and the reaction results in molecular hydrogen and in formation of the surface alkoxy group. The case, when a proton tends to split off from the paraffin, corresponds to the hetero-isotope exchange of paraffins with surface OH groups. This is a concerted acid-base reaction with a transition state different from adsorbed carbonium ion.

A Computational Study of CIONO2H+: Structure and Vibrations

Abstract Two protonated forms of chlorine nitrate, HClONO+ 2 and ClONO2H+, are treated ab initio by the Hartree-Fock and the second order M⊘ller-Plesset perturbation approach with the standard 6–31G∗ basis set. Both minimum energy structures are planar (C 3 symmetry) and their structural, energy, and vibrational parameters are reported. The computations conclude that the proton attacks the chlorine nitrate at its central, not end, oxygen atom. The protonation causes a considerable elongation of the central ON bond which becomes most probable place of cleavage. The dissociation should yield the neutral HOCl and NO+ 2. These quantum-chemical findings well agree with the previous experimental indications. ∗Part XXII in the series “Computational Studies of Atmospheric Chemistry Species”; for Part XXI, see Ref. 1.

Adsorption of neopentane on HNaY-FAU zeolite studied by IR spectroscopy

Adsorption of neopentane on HNaY-FAU zeolite was studied by IR spectroscopy. At beam temperature neopentane already shows a weak interaction with Brønsted acidic sites. At high temperatures, irreversible consumption of both Brønsted and Lewis sites is observed. According to the results of quantum chemical calculations a proton attack on the central carbon of neopentane seems to be favoured.