Reaktionsmechanismen des Oxoniumions von neuen Gesichtspunkten der Säure—Basen-Katalyse

Hydroxide ion is shown to react with α-glucose in two ways. Firstly, it catalyses mutarotation of α-glucose in conjunction with the hydrate sheath, and secondly, it reacts extremely rapidly as partially dehydrated hydroxide ion with formation of α-glucosate ion without mutarotation of α-glucose. This knowledge, combined with the author's determination of the hydrogen bond entropy of the hydrated oxonium ion leads to new views on the mechanism of very rapid oxonium ion reactions. The hydrogen bond entropy of the hydrated oxonium ion ΔS was determined by the author from the coefficients of the water catalysisk _W and oxonium ion catalysiskH₃O⁺ of the mutarotation of α-glucose $$\Delta S = R ln {{k_w } \mathord{\left/ {\vphantom {{k_w } {k_{H_3 O^ + } }}} \right. \kern-\nulldelimiterspace} {k_{H_3 O^ + } }},$$ As the proton of the hydrated oxonium ion is attracted by both oxygen atoms with the same intensity, (ΔS)/2 is the hydrogen bond entropy of the single hydrated proton, which is to be considered as the activated oxonium ion of the reactions between the oxonium ion and the anions. While only the activation of the oxonium ion is necessary for the reaction of the oxonium ion with hydroxide ion, the reaction of the oxonium ion with acetate ion needs furthermore the activation of this anion, which consists of rupture of the internal hydrogen bond of this anion. The enthalpy of activation of the acetate ion is determined by the author from the acetate ion catalysis and the water catalysis of the α-glucose mutarotation. The activation enthalpy of the reaction of the oxonium ion with acetate is therefore the sum of the activation enthalpies of the oxonium ion and of the anion. Furthermore it is shown that the high migration velocity of the hydrogen ion in aqueous solution is due to the proton exchange between the water molecules, initiated by the single hydrated proton.     

Infrared diode laser spectroscopy of the ν2(2+ ← 1−) band of H3O+

Rotational lines in the ν₂ = 2⁺ ← 1⁻ “hot” band of the inversion mode of the oxonium (H₃O⁺) ion have been recorded by diode laser absorption spectroscopy. The ion was generated in low pressure gas discharges and detected using both velocity modulation and modulated hollow cathode techniques. Analysis of the spectra using a simple oblate symmetric top model has allowed the rotational parameters describing the 2⁺ inversion state to be determined for the first time. The band origin lies at 521.4383(52) cm⁻¹. These data will be useful in refining the oxonium ion inversion potential function and should aid in the analysis of other bands involving or perturbed by the 2⁺ level.     

Kristallstrukturen von Säurehydraten und Oxoniumsalzen. XX. Die Oxoniumtetrafluoroborate H3OBF4, [H5O2]BF4 und [H(CH3OH)2]BF4

Crystal Structures of Acid Hydrates and Oxonium Salts. XX. Oxonium Tetrafluoroborates H₃OBF₄, [H₅O₂]BF₄, and [H(CH₃OH)₂]BF₄ The crystal structures of three oxonium tetrafluoroborates were determined. H₃OBF₄, oxonium tetrafluoroborate proper, is triclinic with space group P1 , Z = 2 and the unit cell dimensions a = 4.758, b = 6.047, c = 6.352 Å and α = 80.40, β = 79.48, γ = 88.25° at ?26°C. Cations H₃O⁺ and anions BF₄^? are linked by hydrogen bonds O? H…?F into ribbons of condensed rings. In [H₅O₂]BF₄ (diaquohydrogen tetrafluoroborate, monoclinic, P2₁/c, Z = 4, a = 6.584, b = 9.725, c = 7.084 Å, β = 95.15° at ?100°C) the hydrogen bond in the cation H₅O₂⁺ is 2.412 Å short, asymmetric and approximately centered and the linking of cations and anions three-dimensional. In [H(CH₃OH)₂]BF₄ (Bis(methanol)hydrogen tetrafluoroborate, monoclinic, P2₁/c, Z = 4, a = 5.197, b = 14.458, c = 9.318 Å, β = 94.61° at ?50°C) the cation [H(CH₃OH)₂]⁺ is characterized for the first time in a crystal structure with an again very short (2.394 Å), asymmetric and effectively centered hydrogen bond. By further hydrogen bonds cations and anions form only dimers of the formula unit of centrosymmetric cyclic structure.     

The Structure and Properties of Phenol-2,4-disulfonic Acid Dihydrate

The crystal and molecular structure of phenol-2,4-disulfonic acid dihydrate was determined by X-ray structure analysis. All hydrogen positions in the crystal structure were found using difference Fourier syntheses. Oxonium cations and acid anions were linked in the crystal structure by short H-bonds, and the phenol OH group participated in two weak H-bonds with sulfo group oxygens simultaneously. The IR frequency corresponding to ν_{s, as} (H₃O⁺) vibrations decreased to 2700 cm^?1 under the influence of short H-bonds between oxonium cations and anions. The contour of the corresponding absorption band became anomalously broad. A discrete maximum was observed at 3412 cm^?1 on the high-frequency wing of this band; this maximum was assigned to OH stretching vibrations of the phenol group. The protonic conductivity of the compound measured by impedance spectroscopy was 2.5 × 10^?6 Ω^?1 cm^?1 at 298 K in a vacuum, E _a = 0.37 ± 0.01 eV. An increase in the humidity of the environment to 15% at room temperature increased conductivity from 10^?6 to 10^?5 Ω^?1 cm^?1, E _a = 0.27 ± 0.02 eV.     

Acidolysis of poly(4-methyl-1,3-dioxolane)

The onset of decomposition of poly(4-methyl-1,3-dioxolane) was lowered to 70°C by 0.1 wt% p -toluene sulfonic acid from 170°C in the absence of acid to produce more than 81% yield of monomer. Protonation forms cyclic oxonium ion followed by depolymerization. Minor products are isomers of hydroxymethyl-2-hydroxyl-2-methyl ethyl ether and bis(2-hydroxyl-2-methyl ethoxyl)methane from rearrangements of the oxonium ions. The first order rate constant of acidolysis of poly(4-methyl-1-1,3-dioxolane) is about 8.5 kcal mol^?1, which is much smaller than about 17 kcal mol^?1 for the acidolysis of poly(1,3-dioxolane).     

Gas-phase oxirane addition to acylium ions on reaction with 1,3-dioxolanes elucidated by tandem and triple stage mass spectrometric experiments

Acylium ions containing a variety of substituents all undergo an unprecedented reaction with 1,3-dioxolanes which gives rise to a cyclic, resonance-stabilized oxonium ion, formally the product of oxirane (C₂H₄O) addition to the reagent ion. The structure for the ion–molecule product is supported by multiple-stage mass spectrometric experiments, performed in a pentaquadrupole mass spectrometer, which show the expected fragmentation by C₂H₄O loss to yield the original reactant acylium ions. The oxonium ions are formed in relatively high abundance in many cases and are observed even when proton-transfer reactions would be expected to occur competitively owing to the acidity of some of the acylium ions studied. This ion–molecule reaction is proposed to serve as a general method for identification and/or trapping of ions of the whole acylium ion class and also for the gas-phase generation of the oxonium ions. The reaction with 1,3-dioxolane is also useful in distinguishing the most stable C₂H₃O⁺ ion, the acetyl cation, from its two stable isomers, O-protonated ketene and the oxiranyl cation. The thioacetyl cation, the only sulfur analog investigated, also reacts with dioxolane to form the corresponding oxirane addition product, indicating that the C₂H₄O addition reaction occurs and that it may be useful for identification of the thioacylium class and for the gas-phase generation of sulfur analogs of oxonium ions.     

Formation of β-distonic oxonium ions: Study of ROCH2CH2OR′ radical cations

Radical cations derived from the ethers ROCH₂CH₂OR′ (R, R′ = H, CH₃, C₂₅) were studied, since β-distonic oxonium ions are often prepared from ionized ethers of glycol. The first step in the fragmentation is a 1,5-transfer of an α-hydrogen to oxygen of a terminal alkoxy group leading to a δ-distonic oxonium ion. This step is thermo-neutral and reversible in the ROCH₂CH₂OH radical cations and exothermic and irreversible in the dialkyl ether radical cations. Depending on R and R,′ these δ-distonic oxonium ions fragment by three reactions: the loss of an alcohol or a water molecule, the formation of a β-distonic oxonium ion ˙CH₂CH₂O(H)⁺R and a 1,4-H migration between carbon atoms. Competition between these processes is discussed.     

Heterotricyclodecane XVI. 2-Oxa-7-aza-isotwistane und 2-Oxa-7-aza-twistane

The synthesis of several derivatives of 2-oxa-7-aza-isotwistane ( 20–29 and 35 ) and 2-oxa-7-twistane ( 30–34 ) is described starting from cis,cis-cycloocta-1, 5-diene ( 1 ). The bicyclic acetoxy-olefin 10 was used as a key intermediate. The 10^N(7)-isotwistane 22 was treated under reaction conditions suitable for molecular rearrangements involving an oxonium ion g , by neighbouring group participation, leading to 2-oxa-7-aza-twistanes.     

The structure of H3O+TiF5−

Oxonium pentafluorotitanate was prepared by the reaction of H₂O with TiF₄ in HF. Single crystal x-ray diffraction studies on H₃O⁺TiF₅- show that the compound crystallizes in monoclinic form. The space group is C2/c and the unit cell dimensions are a = 14.528(5), b = 4.839(1), c = 13.798(5)A^o α = 115.59(5)^o with 8 formula units per unit cell.     

Distinctive MS/MS Fragmentation Pathways of Glycopeptide‐Generated Oxonium Ions Provide Evidence of the Glycan Structure

Post‐translational glycosylation of proteins play key roles in cellular processes and the site‐specific characterisation of glycan structures is critical to understanding these events. Given the challenges regarding identification of glycan isomers, glycoproteomic studies generally rely on the assumption of conserved biosynthetic pathways. However, in a recent study, we found characteristically different HexNAc oxonium ion fragmentation patterns that depend on glycan structure. Such patterns could be used to distinguish between glycopeptide structural isomers. To acquire a mechanistic insight, deuterium‐labelled glycopeptides were prepared and analysed. We found that the HexNAc‐derived m/z 126 and 144 oxonium ions, differing in mass by H₂O, had completely different structures and that high‐mannose N‐glycopeptides generated abundant Hex‐derived oxonium ions. We describe the oxonium ion decomposition mechanisms and the relative abundance of oxonium ions as a function of collision energy for a number of well‐defined glycan structures, which provide important information for future glycoproteomic studies.     

Kristallstrukturen von Säurehydraten und Oxoniumsalzen. XVIII. Schmelzdiagramm H2O?HF und Strukturen der 1 : 1- und einer 1 : 2-Phase

Crystal Structures of Acid Hydrates and Oxonium Salts. XVIII. Melting Diagram H₂O? HF and Structures of the 1:1 and a 1:2 Phase The quasi binary system H₂O-HF contains three intermediate phases of compositions 1:1, 1:2, and 1:4, which melt at ?36, ?78 (decomposition), and ?100°C. With a composition of 1:2 there is also a low-temperature phase (transition at ?103°C) and a metastable phase. The 1:1 phase is orthorhombic, space group Pnma, z = 4, a = 621.6, b = 418.4, c = 623.3 pm at ?62°C. One of the 1:2 phases (assignment still to be done) is monoclinic, space group P2₁/c, z = 4, a = 347.8, b = 603.9, c = 1141.5 pm, β = 96.57° at ?95°C. Both crystal structures are those of oxonium salts, H₃OF and H₃OFHF, respectively, and built up by strong hydrogen bonds.     

Neue Oxonium-Bromochalkogenate(IV) — Darstellung,Struktur und Eigenschaften von [H3O][TeBr5] · 3C4H8O2 und [H3O]2[SeBr6]

New Oxonium Bromochalcogenates(IV) — Synthesis, Structure, and Properties of [H₃O][TeBr₅] · 3 C₄H₈O₂ and [H₃O]₂[SeBr₆] Dark red crystals of the composition [H₃O][TeBr₅] · 3 C₄H₈O₂ ( 1 ) were isolated from a saturated solution of TeBr₄ in 1,4-dioxane containing a small amount of water. In this compound (space group P2₁/m, a = 8.922(4) Å, b = 13.204(7) Å, c = 9.853(5) Å, β = 91.82(4)° at 150 K) a square pyramidal [TeBr₅]^? anion has been isolated for the first time. The coordination sphere of the anion is completed to a distorted octahedron by weak interaction with a dioxane molecule of the cationic system. The [H₃O]⁺ cations are connected to chains by dioxane molecules. At room temperature the compound is stable only in its mother liquor. Crystalline [H₃O]₂[SeBr₆] ( 2 ) (space group Fm3m, a = 10.421(1) Å at 170 K) is a bromoselenous acid of high symmetry. The [H₃O]⁺ ion is only weakly coordinated by Br atoms of the anion. The anions are isolated octahedral [SeBr₆]^2? units. The structure is isotypic to the K₂[PtCl₆] structure. Despite being a halogenochalcogen(IV) acid, 2 exhibits a remarkable thermal stability. Both oxonium compounds were characterized by single-crystal X-ray structure analyses. Vibrational spectra of 2 are reported.     

Fragmentation of substituted oxonium ions: The role of ion-neutral complexes

Disubstituted trialkyloxonium ions R₁OCH₂O⁺(R₁)CH₂OR₂ (R₁, R₂ = CH₃ or C₂H₅) have been prepared by chemical ionization of dimethoxy- and diethoxymethane individually or as a mixture, and their fragmentation has been studied by means of metastable ion and collision-induced dissociations. It is found that when a methoxymethyl group is attached to the charged oxygen atom, the oxonium ions can fragment by C-O bond cleavage to generate a methoxymethyl cation/dialkoxymethane ion-neutral complex, in which methyl cation transfer occurs to expel neutral formaldehyde. When an ethoxymethyl group is connected with the central oxygen atom, a reaction channel involving loss of C₂H₄O is observed and found to be insensitive to collisions. This process is proposed to involve isomerization prior to fragmentation leading to methylated dialkoxymethanes coordinated with neutral acetaldehyde in ion-neutral complexes; these ion-neutral complexes are estimated to be 35 kJ mol⁻¹ more stable than the original oxonium ions.     

Comparative studies of electron-impact (EI) and field ionization (FI) mass spectra of pentaerythritol derivatives

The EI Mass spectra of the polyfunctional pentaerythritol derivatives show that the molecular ions [M]⁺· exhibit extensive ion fragmentation. No Information about [M]⁺·. can be obtained. In Contrast to this, the FI Mass spectra of these compounds show intense [M]⁺· and/or [M + 1]⁺, and a characteristic ion at m/e 31, which is assumed to be the oxonium ion \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {\rm O}\limits^ + {\rm H} $\end{document}. Because of surface adsorption and field attraction, FI mass spectrometry presents a serious problem in quantitative analysis of a mixture containing compounds with quite different degrees of polarization.     

Reactions of α-chloronitrones with unsaturated ethers

The Ag⁺ induced reaction of α-chloronitrones with unsaturated ethers goes in two parallel competitive directions. On the one hand cycloadducts like 3 are formed and on the other hand oxonium ions like 4. Formation of analogous oxonium compounds occurs also when α-alkoxynitrones are treated with Et₃O⁺BF₄^? in 1,2-dichloroethane. Formation of oxonium compounds with the enol-ethers is practically irreversible and leads to enol-ether fragmentation. With the saturated ethers compounds like 25 were formed and were used as a potential source of N-alkyl-N-vinyl-nitrosonium ions.     

Norbornanes. Part 21. Bridging strain in norbornyl and oxanorbornyl cations

Further evidence is presented that the 2-norbornyl cation is stabilized primarily by C(2)–C(6) bridging, and that C(2)–C(7) bridging leads to prohibitive strain. Thus, a comparison of the heats of hydrogenation of nortricyclene 17 and bicyclo[3. 2. 0. 0.^2,7]heptane 18 indicates that the strain energy of the latter is ca. 21.5 kcal/mol higher that that of 17 . Furthermore, 6-exo-2-oxabicyclo[2. 2. 1]heptyl sulfonates 8 ionize with strong O(2) participation to the bridged oxonium ion 12. In contrast, 2-endo-7-oxabicyclo[2.2.1]heptyl sulfonates 11 ionize without O(7) participation to form the unbridged carbenium ion 15 .     

Formaldehyde in Super Acids: A Succession of Products from Carbenium through Oxonium Ion to Hydroxymethyl(methylidene)oxonium Salts

An oxonium ion with hydroxymethyl and methylidene substituents is found in the salts 1 -MF₆ (M=As, Sb), which can be isolated from solutions of formaldehyde in superacids HF/MF₅. Their formation was at first surprising, since NMR spectroscopy indicates that protonated monofluoromethanol is present in solution.     

Heterotricyclodecane X. 2-Oxa-7-thia-isotwistan und 2-Oxa-7-thia-twistan sowie Derivate

The synthesis of 2-oxa-7-thia-isotwistane ( 11 ) and 2-oxa-7-thia-twistane ( 22 ) as well as of several of their derivatives is described starting from endo-2-hydroxy-9-thiabi cyclo[3.3.1]non-6-ene ( 4 ). The 10^O(2)-isotwistane bromide 13 , iodide 14 and tosylate 30 as well as the 10^S(7)-isotwistane iodide 15 , tosylate 20 , and the 10^S(7)-twistane tosylate 21 were treated under reaction conditions suitable for molecular rearrangements involving the epi-sulfonium ion i and the oxonium ion g . respectively, by neighbouring group participation.     

Kristallstrukturen von Säurehydraten und Oxoniumsalzen. 36 [1]. Selensäure-Tetrahydrat. Ionisch gemäß (H5O2)2SeO4 in einer orthorhombischen sowie einer tetragonalen Form

Crystal Structures of Acid Hydrates and Oxonium Salts. 36 [1]. Selenic Acid Tetrahydrate. Ionic as (H₅O₂)₂SeO₄ in an Orthorhombic as well as a Tetragonal Form Low-melting selenic acid tetrahydrate has been studied for the first time by crystal structure analysis. Two forms have been observed, an orthorhombic one with space group Pnma and Z = 4 and a tetragonal one with space group P4 2₁c and Z = 2. The lattice parameters at ?150°C are a = 6.130(3), b = 12.776(6) and c = 9.299(5) Å for the orthorhombic and a = 7.676(4) and c = 6.378(3) Å for the tetragonal form. Both forms are oxonium salts, corresponding to (H₅O₂)₂SeO₄. The ions are hydrogen-bonded into a three-dimensional array. The bonds within the diaquahydrogen cations have O ?O distances of 2.422(5) and 2.425(4) Å. The tetragonal form is isotypic to the tetrahydrate of sulfuric acid.     

Beitrag zur Chemie der Phosphor-Stickstoff-Verbindungen. Reaktion von Cl2(O)P?NH?P(O)Cl2 mit Tetrahydrofuran

Contribution to the Chemistry of Phosphorus-Nitrogen Compounds. Reaction of Cl₂(O)P? NH? P(O)Cl₂ with Tetrahydrofuran As well as many phosphorus compounds, the imidodiphosphoryl tetrachloride HN(P(O)Cl₂)₂ reacts with a large excess of tetrahydrofuran to give the polytetrahydrofuran. Otherwise, if we use smaller molecular ratios THF/HN(P(O)Cl₂)₂ (1/2 to 3) a polytetrahydrofuran with short chains and N(ω-hydroxypolytetramethylenoxy)imidodiphosphoryl tetrachloride R? N(P(O)Cl₂)₂; R = H(O(CH₂)₄)_n- are obtained at 22° or 30°C. The ¹H and ³¹P n.m.r. spectra show that oxonium ions are formed with progressive additions of THF to HN(POCl₂)₂/CCl₄ solution. Then two mechanisms have been considered by nucleophilic attack on carbon α of oxonium ion coming from: the free electronic dublett on oxygen of THF to give polytetrahydrofuran or (and) from the nitrogen of imido diphosphoryl tetra chloride anion ((Cl₂OP)₂N)^? to obtain N(ω-hydroxypolytetramethylenoxy)imidodiphosphoryl tetrachloride.