Dye‐sensitised photo‐oxidation of 2‐dimethylamino‐5,6‐ dimethylpyrimidin‐4‐01 in aqueous solution

Abstract

The sensitisation by Rose Bengal of the photo‐oxidation of 2‐dimethylamino‐5,6‐ dimethylpyrimidin‐4‐01 (I) has been studied in aqueous solution by monitoring the rate of oxygen consumption in the system. The results are in accord with a mechanism whereby I is oxidised via a reaction with singlet oxygen. Values for the rate constant for the reaction of I with singlet oxygen have been determined over the temperature range 279‐296 K, and the energy of activation and entropy of activation for the reaction have been evaluated as 71.9 kJ mol−1 and 150.5 J K−1 mol−1, respectively. The positive value for the entropy of activation suggests that the reaction intermediate could have a zwitterionic form. The effect of pH on the reaction rate has been investigated, and the increased rate at high pH has been attributed to an increase in singlet oxygen production as the pH is raised.

Environmental photodegradation of pyrimidine fungicides — Kinetics of the visible-light-promoted interactions between riboflavin and 2-amino-4-hydroxy-6-methylpyrimidine

“…In spite of the high value of the rate constant for the interaction 1 Rf*-AHMPD ( 1 k q , process [7]) -close to the diffusion limit -relatively high concentrations of AHMPD are necessary to prevent triplet state population (75 mM, to quench 50% 1 Rf*, process [7]). The competitive pathways represented by processes [3] (and, subsequently, [5] and [6]) and [8] are generated from 3 Rf* (37,38). While processes [5] and [8] can be merely physical events, process [6] can involve the consumption of both Rf and AHMPD.…”

“…Results indicate the operation of a complex mechanism that possibly includes specific interactions of AHMPD with singlet ( 1 Rf*) and triplet ( 3 Rf*) excited states of Rf, represented by processes [7] and [8]. , which is equal to the ratio 1 k q / 1 k d , where 1 k d is the rate constant for thermal deactivation of 1 Rf* (process [9]).…”

“…The triplet decay was measured at low Rf concentration (typical 0.02 mM) and at low enough laser energy, to avoid self-quenching and triplet-triplet annihilation effects, respectively. Employing the expression 1/ 3 τ = (1/ 3 τ o ) + 3 k q [AHMPD] (where 3 τ o and 3 τ are the experimentally determined lifetimes of 3 Rf* in the absence and in the presence of AHMPD, respectively), a value for the bimolecular rate constant 3 k q of 2.7 × 10 7 M -1 s -1 was obtained (process [8], Fig. 3, inset).…”

“…Following this line, in recent papers (5,6) we have reported the kinetics of the dye-sensitized photooxygenation of several hydroxypyridines, hydroxypyrimidines, and hydroxyquinolines with the basic molecular structures of known N-heteroaromatic pesticides (1). The kinetics of the photosensitized oxidation, in aqueous or chloroform solution, of the pyrimidine fungicides ethirimol (5-nbutyl-2-diethylamino-4-hydroxy-6-methylpyrimidine), dimethirimol (5-n-butyl-2-dimethylamino-4-hydroxy-6-methylpyrimidine), and four analogous pyrimidines, all with structures closely related to that of the model compound 2-amino-4-hydroxy-6-methylpyrimidine (AHMPD), has been previously studied (7,8).…”

Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

“…For example, these molecules can be used as intermediates [1], fine products for drugs [2][3][4][5] and pesticides [6,7], dyes [8] and laser dyes [9], redox systems for solar energy [10], organized assemblies [11] and complex-forming agents [12,13]. Other areas where these molecules have been used extensively are inter- [14][15][16][17][18][19][20] and intramolecular proton transfer reactions [21] in S 0 and S 1 states.…”