Single Protonation Research Articles

Calorimetric analysis of the protonation process for 3-arylimino-3H-indole N-oxides and N, N '-dioxides in acetonitrile

Calorimetric analysis on two series of indole derivatives (both previously studied from the calorimetric and electrochemical points of view) has been carried out. Both series show a single protonation process, but only for one series does the electron charge distribution enable the site of protonation to be recognized.

ChemInform Abstract: Electrochemical Investigation of Potassium Heptacyanorhenate(III) in Aqueous Solution.

AbstractOnly two processes are observed: a one‐electron oxidation and the protonation of (II).

Electrochemical Investigation of Potassium Heptacyanorhenate(III) in Aqueous Solution

AbstractThe electrochemistry of potassium heptacyanorhenate(III) in aqueous solution was studied by cyclic and by rotating disk voltammetry at planar microelectrodes. The results are consistent with a single, reversible electron transfer: Re(CN)3−7 + e⇄Re(CN)4−7 with E′0 = 643 mV vs. NHE. A single protonation equilibrium is observed: Re(CN)4−7 + H+⇄ Re(CN)7H3− with pK = 1.31 determined from combined voltammetric and pH data. The Re–CN bond appears to be kinetically inert, and none of the cyano complexes in other oxidation states of Re claimed in the literature was found in the potential range − 2 V to + 1 V.

Substrate specificity and protonation state of ornithine transcarbamoylase as determined by pH studies.

The ornithine transcarbamoylase catalyzed reaction and its inhibition by L-norvaline have been investigated between pH 5.5 and 10.5. The steady-state turnover rate (kcat) of the enzyme from Escherichia coli increases with pH and plateaus above pH 9. Its change with pH conforms to a single protonation process with an apparent pKa of 7.3. The effect of pH on the apparent Michaelis constant (KMapp) of L-ornithine suggests that this diamino acid in its cationic form is not the substrate. Treating only the zwitterions of ornithine as substrate, the pH profile of the pseudo-first-order rate constant (kcat/KMz) of the reaction is a bell-shaped curve characterized by pKa's of 6.2 and 9.1 and asymptotic slopes of +/- 1. Similar pKa's (6.3 and 9.3) are obtained for the pKi profile of zwitterionic L-norvaline, a competitive inhibitor. The pKi profile further indicates that the alpha-amino group of the inhibitor must be charged for binding. Together, these pH profiles provide sufficient information to suggest that only the minor zwitterionic species of ornithine, H2N(CH2)3CH(NH3+)COO-, binds the enzyme productively. The selection of this substrate form by the enzyme leads to a Michaelis complex in which ornithine is poised for nucleophilic attack. Following such binding, the need for deprotonation of the delta-NH3+ group is avoided, and transcarbamoylation becomes energetically more feasible. Reaction schemes accounting for the effects of pH are proposed for the enzymic reaction.

Quantum-chemical investigation of the mechanism of aldol condensation. Communication 3. Cationic mechanism of condensation and elimination

1. The MINDO/3 and CNDO/BW quantum-chemical methods have been used to investigate fragments of the potential energy surface for the reaction of protonated formaldehyde with vinyl alcohol. 2. Two monomolecular mechanisms have been proposed for the isomerization of the protonated aldol, and an analysis of the corresponding transition states has been made. 3. It has been shown that a single protonation of the aldehyde molecule is sufficient for catalysis of the entire sequence of its conversions to the product of the crotonaldehyde condensation (α,β-unsaturated carbonyl compound) and H2O.

Spectral study of the structure of protonated forms of phthalocyanines

The problem of the structure of protonated forms of phthalocyanines is controversial: according to thermodynamic data [i], in the reaction of a phthalocyanine molecule with acids, only one single protonation is possible, but in the electronic spectra the formation of several products was observed, depending on the strength of the acid, or change in the pH of the solution. We could thus assume the existence of several protonated forms, differing in the number of protons added [2, 3]. We carried out a spectral investigation of the reaction between layers of phthalocyanines (nonmetallic phthalocyanine H=PhC and its metal complexes MPhC, where M = Cu, Zn, Fe, CO) and gaseous HCI and HBr. By comparing IR absorption spectra we concluded that there is a qualitative difference in the structure of the products formed at different stages of this reaction: at the first stage a protonated form is formed, with a proton localized on one of the bridge peripheral nitrogen atoms, and at subsequent stages, complexes with different composition of the protonated form are formed with hydrogen halides~ The nonmetallic H2PhC reacts at the first stage with hydrogen halides in the same way as ~hC, and at the seeond stage, apparently, forms a double protonated form with one of the protons localized on one of the central nitrogen atoms.

Studies of an acid-induced species of purple membrane from Halobacterium halobium.

A new spectral species of the purple membrane of Halobacterium halobium has been observed below pH 3.2. The formation of this new species is temperature-dependent and is favoured by increasing temperature up to the physiological range of the organism. The rate of formation at pH 3.0 and 22 degrees C is 7.9 x 10-3s-1. The spectral distribution and temperature-dependence of the new species suggest that it may be phototransiet O, stabilized by low pH. Flash-photolytic experiments in the pH range 7.2-2.7 show a pH-dependence corresponding to the static events and are consistent with a single protonation of bacteriorhodopsin below pH 3.22. These results can also be interpreted in terms of the stabilization of phototransient O at low pH. The temperature-dependence of the formation of the acid-induced species may reflect a relationship with the phase transition of the membrane.

Theory of lone pairs. Successive protonations of multi-lone pair bases

While single protonation is always exothermic in the gas phase multiple protonations are shown to be endothermic. The relevance of these results to acid catalysed reactions is discussed.

Protonation du N,N-diméthylformamide: étude par spectrométries vibrationnelle et électronique

Infrared and Raman spectra of acidic (hydrochloric or sulfuric) aqueous solutions of dimethylformamide indicate the existence of a single protonation equilibrium.[Formula: see text]The pK for this reaction was evaluated as −1.35 ± 0.05 from u.v. spectra; a less precise value (−1.1 ± 0.5) was obtained from Raman spectra. This result disagrees with earlier conclusions which favored the existence of two ionization equilibria for dimethylformamide. [Journal translation]

Zero field splittings in the lowest excited triplet states of aza- and diazaphenanthrenes and their protonated species

ESR spectra of the lowest excited ππP * triplet states of a series of azaphenanthrenes (AP) and diazaphenanthrenes (DAP) are measured in rigid glasses. The zfs parameters are evaluated for 4-AP, 1,7-DAP, 4,7-DAP, 1,10-DAP and for their mono- and diprotonated species, BH + and BH 2+ 2. The zfs parametesr of the bases are a litte higher than those of phenanthrene. On single protonation they fall below the latter values. Second protonation causes again an increase in zfs, with the exception of the 1,10-DAP divalent cation which behaves peculiarly. Spin-orbi contributions to the zfs seem not to be responsible for the observed differences.

Spectroscopic studies on the protonation of s-triaminobenzene

The protonation of s-triaminobenzene (TAB) has been studied by measuring IR, UV and NMR spectra, special attention being paid to the structure of the single protonation product (TABH +) of TAB. The IR spectrum at room temperature shows that the NH stretching vibrations due to the neutral amino groups of TAB remain unaltered in TABH +. Furthermore, in the NMR spectrum of TABH +, the proton signal of the methylene group appears at τ = 6·7 ppm. These facts and other supporting evidence, demonstrate that the first protonation at room temperature mainly occurs on the ring C atom and produces the triaminobenzenium ion. It was further concluded, from the detailed analysis of the temperature dependence of UV spectra, that TAB in aqueous solution of pH 4·3 consists of two components, the triaminobenzenium ion and the amino ▪ overwhelmingly predominant at room temperature, the amount of the latter increases with the decreasing temperature. The peaks at 27,400, 36,800 and 45,500 cm −1 observed for TABH + at room temperature have been ascribed to the triaminobenzenium ion, since these peak positions coincide with the transition energies calculated for this ion by the Pariser-Parr-Pople + CI method.

THE WALLACH REARRANGEMENT: PART II. KINETICS AND MECHANISM OF THE ACID-CATALYZED REARRANGEMENT OF AZOXYBENZENE

The rate of rearrangement of azoxybenzene to p-hydroxyazobenzene has been measured in 75.3–96.4% sulfuric acid at 25° and in 65.0–90.4% sulfuric acid at 75.5° by spectrophotometric methods. The pKa of azoxybenzene in aqueous sulfuric acid has also been determined. It is found that although azoxybesssnzene is almost completely protonated over the entire range of acid concentration studied, the rate increases by more than 1 000-fold. A two-proton process is therefore indicated and mechanisms are proposed involving a dication (II) as the key intermediate. The rate data do not allow differentiation between two proposed mechanisms, one involving two equilibrium protonations, and the other a single equilibrium protonation followed by rate-determining proton transfer. Past mechanisms of the Wallach rearrangement are discussed.