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Mechanisms of decomposition reaction of thymine cation radicals (T${}^{+•}$), 1-methyl substituted T${}^{+•}$ (1-MeT${}^{+•}$), and 1,3-dimethyl substituted T${}^{+•}$ (1,3-Me₂T${}^{+•}$) generated by ionizing radiation or one-electron oxidation to thymine, 1-methylthymine and 1,3-dimethylthymine in aqueous solution have been investigated by theoretical calculations. Seven reaction paths including H₂O or OH${}^{-}$ addition, and deprotonation were considered. The most likely reaction for T${}^{+•}$ decomposition is N1 deprotonation forming T(-N₁H)${}^{•}$. When N1 hydrogen is replaced by methyl, T${}^{+•}$ would deprotonate from C5 methyl group generating 1-methyl substituted UCH${}_2^{•}$ in neutral solution, and reaction with OH${}^{-}$ producing 1-methyl substituted T6OH${}^{•}$ in basic solution. When N1 and N3 hydrogens are both replaced by methyl, T${}^{+•}$ decomposition reaction is similar to 1-MeT${}^{+•}$, indicating that N3 hydrogen barely influences the subsequent reaction of 1-MeT${}^{+•}$. These results can provide theoretical support for experimental observations.

A series of free-base corroles with different electron-donating or electron-withdrawing substituents were reacted with piperidine, 4-aminopyridine, 2-methylimidazole, 2-aminopyridine or pyridine in PhCN and the UV-visible spectral changes monitored during conversion of (Cor)H₃ to [(Cor)H₂]^- as a function of the concentration and strength of the added organic base. Analysis of the UV-visible spectral changes as a function of the added base concentration enabled calculation of equilibrium constants (logK) for deprotonation of each corrole under the given experimental conditions. Relationships are examined between the experimentally measured logK values and previously published spectroscopic and structural properties of the compounds.

Acid-base properties of zinc phthalocyanine (Pc) and zinc and metal-free tetrapyrazinoporphyrazine (TPPz) macrocycles with eight peripheral tert-butylsulfanyl or diethylamino substituents are investigated in this work by means of UV-vis spectroscopy. The Pc and TPPz are protonated on the azomethine nitrogens in acidic media. The monoprotonated form was found using trifluoroacetic acid and lower amounts of H₂SO₄. The higher concentrations of sulfuric acid lead to the appearance of the diprotonated form, and possibly also the triprotonated form in the case of Pc. It was found that the pyrazine nitrogens in the TPPz macrocycle undergo protonation at approx. 20-30% of sulfuric acid in THF solution. TPPz with diethylamino peripheral substituents are protonated preferably on these peripheral tertiary amines. Metal-free TPPz can readily lose the hydrogens on central nitrogens in strong basic media (e.g. tetrabutylammonium hydroxide) to directly form a dianion, with no presence of important or detectable amounts of the monoanion species. A different form of deprotonated macrocycle was observed in pyridine suggesting the formation of only a proton transfer complex, not a true ionic form. The acidity of the central nitrogens is dependent on the strength of the electron-donating effect of peripheral substituents. TPPz with tert-butylsulfanyl substituents is a stronger acid than the one with diethylamino substituents.

The UV-visible spectrum of free-base octaethylcorrole, (OECor)H₃, was recorded in thirteen different nonaqueous solvents as well as in a mixed acetonitrile/acid solvent containing one of seven different acids. Spectra were also measured in seven different solutions of neat concentrated acid and in CH₃CN containing piperidine or tetrabutylammonium hydroxide as an added base. The overall data was analyzed as a function of solvent acidity or basicity parameters and the number of protons on the central nitrogens of the macrocycle, the predominant form of the corrole in these solutions being respresented as (OECor)H₃, [(OECor)H₄]⁺, [(OECor)H₂]⁻ or [(OECor)H]²⁻ where OECor = trianion of octaethylcorrole. The mono-protonated corrole, [(OECor)H₄]⁺, is formed in concentrated acetic acid or in CH₃CN containing 0.10 M trifluoroacetic acid, H₂SO₄, HCl, H₃PO₄ or HClO₄. The mono-deprotonated corrole, [(OECor)H₂]⁻, is generated in piperidine while doubly deprotonated [(OECor)H]²⁻ exists in solutions of tetrabutylammonium hydroxide. An addition of protons to the macrocycle of [(OECor)H₄]⁺ also occurs in the presence of concentrated strong acids and this results in a loss of the characteristic Soret band of the corrole leading presumably to [(OECor)H₅]²⁺ where the second proton has been added to a meso-position of the macrocycle. The UV-visible spectral changes upon formation of [(OECor)H₄]⁺, [(OECor)H₂]⁻ or [(OECor)H]²⁻ in CH₃CN were monitored during a titration with the relevent acid or base and equilibrium constants for protonation or deprotonation of (OECor)H3 were determined using standard equations. The measured logK values are compared to protonation and deprotonation constants obtained for two related corroles and two related porphyrins under the same experimental conditions.

Dissociation of imino proton(s) in the cavity of the macrocycle of a highly water-soluble, metal-free phthalocyanine (H₂(H₄tsppc); where H₄tsppc denotes tetrakis{(2′,6′-dimethyl-4′-sulfonic acid)phenoxy}phthalocyaninate) in ethanolic and aqueous solutions has spectrophotometrically been investigated. The spectral changes associated with reaction with NaOH have been found to involve one-proton transfer process in aqueous media while two-protons process in ethanolic media. The acid-dissociation constant of the first imino proton in water (in the presence of Triton X-100) has been determined to be 12.5 ± 0.2 (as pKa) at 25 °C. The doubly deprotonated species in EtOH has been easily converted to its corresponding cobalt(II) derivative by thermal reaction with anhydrous CoCl₂.

The protonation and deprotonation reactions for a series N-confused meso-tetraaryl-substituted free-base porphyrins containing electron-donating or electron-withdrawing substituents was monitored in CHCl₃ and DMF by UV-visible spectroscopy during titrations with trifluoroacetic acid or tetra-n-butylammonium hydroxide. The spectroscopic data was also used to calculate equilibrium constants for these reactions. The examined compounds are represented as (XPh)₄NCPH₂, where "NCP" represents the N-confused porphyrin π-conjugated macrocycle and X is a CH₃O, CH₃, H or Clpara-substituent on the four meso-phenyl rings (Ph) of the compound. The porphyrins can exist in two tautomeric forms depending upon the solvent and each tautomer undergoes two stepwise protonation reactions leading to formation of the mono- and bis-protonated porphyrins, [(XPh)₄NCPH₃]⁺ and [(XPh)₄NCPH₄]²⁺. A single step deprotonation is observed for the same compounds in DMF and the product is assigned as [(XPh)₄NCPH]^-. Comparisons are made between UV-visible spectra of the protonated, neutral and deprotonated forms of the porphyrin and the effect of the porphyrin ring substituents and tautomeric form of the neutral porphyrin on the UV-visible spectra and protonation constants is discussed along with data from DFT calculations.

The UV-Vis spectra of the dicationic salt of 2,3,7,13,17,18,18,22,23-octamethyl-8,12-diethylsapphyrin {H₅β-Me₈Et₂Sap²⁺(Cl⁻)₂} in solutions with different polarity and acid-basic properties have been investigated. The tautomeric and acid-base equilibria of H₅β-Me₈Et₂Sap²⁺(Cl⁻)₂ in the presence of organic bases have been examined by spectrophotometric titration. It was demonstrated that an addition of small amounts of 1,8-diazabicyclo[5.4.0]undecene (DBU) and tetramethylammonium hydroxide (Me₄NOH) dissolved in methylene chloride (DCM) and MeOH, respectively results in a stepwise detachment of two protons from the basic nitrogen atoms of the macrocycle, with no anionic forms of sapphyrin being registered. The results obtained were supported using quantum-chemical calculations (DFT, B3LYP, cc-pVDZ including NBO analysis). The interactions of 2,3,7,13,17,18,18,22,23-octamethyl-8,12-diethylsapphyrin (H₃β-Me₈Et₂Sap) with a series of compounds of different acidity and basicity were studied and the formation of very weak hydrogen bond of H₃β-Me₈Et₂Sap as NH-acid with N,N-dimethylformamide (DMF) molecule with the value of the interaction energy (E_int equal -16.40 kJ/mol was revealed. The low acidity of bonds NH in the sapphyrin molecule was confirmed on the base of thermogravimetric data obtained for the solid samples of H₅β-Me₈Et₂Sap²⁺(Cl⁻)₂ evaporated from DMF and DCM concentrated solutions.