RESUMO
Due to the pronounced effect of the confined environment on the photochemical properties of 4-hydroxybenzylidene imidazolinone (HBI), a GFP-related chromophore, imidazolidinone and imidazothiazolone analogues have been studied as fluorescent probes. Their photoisomerization and their thermal reversion were studied under 365-nm-irradiation, resulting in observation of an enthalpy-entropy compensation effect. Theoretical studies were carried out to shed light on the thermal reversion mechanism. Moreover, photophysical studies of benzylidene imidazothiazolone in the presence of dsDNA revealed fluorescence enhancement. The prepared compounds could be considered as a valuable tool for the detailed investigation of physicochemical, biochemical, or biological systems.
Assuntos
Corantes Fluorescentes , Proteínas de Fluorescência Verde/química , Fluorescência , Termodinâmica , EntropiaRESUMO
Collisional quenching of NO A (2)Σ(+) (v = 0) by N(2)O and CO(2) has been studied through measurements of vibrationally excited products by time resolved Fourier transform infrared emission. In both cases vibrationally excited NO X (2)Π (v) is seen and quantified in levels v≥ 2 with distributions which are close to statistical. However the quantum yields to produce these levels are markedly different for the two quenchers. For CO(2) such quenching accounts for only ca. 26% of the total: for N(2)O it is ca. 85%. Far more energy is seen in the internal modes of the CO(2) product than those of N(2)O. The results are rationalised in terms of cleavage of the N(2)-O bond being dominant in the latter case, with either a similar O atom production or a specific channel producing almost exclusively NO in low vibrational levels (v = 0,1) for quenching by CO(2). Minor reactive channels yielding NO(2) are seen in both cases, and O((1)D) is observed with low quantum yield in the reaction with N(2)O. The results are discussed in terms of previous models of the quenching processes, and are consistent with the very high yield of NO X (2)Π (v = 0) previously observed by laser induced fluorescence for quenching of NO A (2)Σ(+) (v = 0) by CO(2).
RESUMO
This paper reports for the first time the synthesis and characterization of trifluoromethyl fluoroformyl trioxicarbonate, CF(3)OC(O)OOOC(O)F. The new trioxide is obtained from the gas-phase photolytic reaction of CF(3)C(O)OC(O)CF(3) and FC(O)C(O)F at 223-228 K. It is a very thermally labile molecule that decomposes at room temperature by rupture of either of the CF(3)OC(O)O-O-OC(O)F bonds. These bonds are nonequivalent, and a branching ratio of 0.8 for fragmentation through the CF(3)OC(O)OO-OC(O)F bond was obtained. Unambiguous identification was possible through reaction of the trioxide with an excess of NO(2). Potential-energy surfaces (PES) of the different rotamers were studied by the B3LYP/6-311+G* method, and analysis of the IR frequency of the possible mixture of rotamers agrees excellently with the experimental IR spectrum. This molecule is the first nonsymmetric acyl trioxide reported in the literature.
Assuntos
Carbonatos/química , Hidrocarbonetos Fluorados/química , Óxidos/química , Carbonatos/síntese química , Simulação por Computador , Hidrocarbonetos Fluorados/síntese química , Modelos Químicos , Óxidos/síntese química , Espectrofotometria Infravermelho , TemperaturaRESUMO
Gas phase thermal decomposition of CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3) was studied at temperatures between 64 and 98 degrees C (CF(3)OC(O)OOC(O)F) and 130-165 degrees C (CF(3)OC(O)OOCF(3)) using FTIR spectroscopy to follow the course of the reaction. For both substances, the decompositions were studied with N(2) and CO as bath gases. The rate constants for the decomposition of CF(3)OC(O)OOC(O)F in nitrogen and carbon monoxide fit the Arrhenius equations k(N)2 = (3.1 +/- 0.1) x 10(15) exp[-(29.0 +/- 0.5 kcal mol(-1)/RT)] and k(CO) = (5.8 +/- 1.3) x 10(15) exp[-(29.4 +/- 0.5 kcal mol(-1)/RT)], and that for CF(3)OC(O)OOCF(3) fits the equation k = (9.0 +/- 0.9) x 10(13) exp[-(34.0 +/- 0.7 kcal mol(-1)/RT)] (all in units of inverted seconds). Rupture of the O-O bond was shown to be the rate-determining step for both peroxides, and bond energies of 29 +/- 1 and 34.0 +/- 0.7 kcal mol(-1) were obtained for CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3). The heat of formation of the CF(3)OCO(2)(*) radical, which is a common product formed in both decompositions, was calculated by ab initio methods as -229 +/- 4 kcal mol(-1). With this value, the heat of formation of the title species and of CF(3)OC(O)OOC(O)OCF(3) could in turn be obtained as Delta(f) degrees (CF(3)OC(O)OOC(O)F) = -286 +/- 6 kcal mol(-1), Delta(f) degrees (CF(3)OC(O)OOCF(3)) = -341 +/- 6 kcal mol(-1), and Delta(f) degrees (CF(3)OC(O)OOC(O)OCF(3)) = -430 +/- 6 kcal mol(-1).
RESUMO
A complete study of the reaction of CF(3) radicals in the presence of CO and O(2) was carried out by using isotopically labeled reagents to form, selectively, all the possible isotopomers of the intermediate trioxide, CF(3)OC(O)OOOC(O)OCF(3), and of the stable peroxide, CF(3)OC(O)OOC(O)OCF(3). Analyses were carried out by means of FTIR spectroscopy in combination with ab initio calculations. At temperatures close to 0 degrees C, the acyloxy radicals formed were shown to exist long enough to yield a statistical mixture of isotopomers. In previous reports their lifetime was considered to be too short.
RESUMO
Irradiation of the equilibrated gas mixture CF(3)O(2)NO(2)<==>CF(3)O(2) + NO(2) at room temperature using the output from UV fluorescent "blacklamps" provides a rapid and simple method for the production of pure samples of CF(3)ONO(2) in high yield (ca. 80%). This synthetic procedure is superior to that described in the literature in two aspects: (i) the yield of CF(3)ONO(2) is approximately a factor of 5 greater, and (ii) the present method avoids the need for a high pressure (70 bar) reactor.
RESUMO
The open-chain trioxide CF(3)OC(O)OOOC(O)OCF(3) is synthesised by a photochemical reaction of CF(3)C(O)OC(O)CF(3), CO and O(2) under a low-pressure mercury lamp at -40 degrees C. The isolated trioxide is a colourless solid at -40 degrees C and is characterised by IR, Raman, UV and NMR spectroscopy. The compound is thermally stable up to -30 degrees C and decomposes with a half-life of 1 min at room temperature. Between -15 and +14 degrees C the activation energy for the dissociation is 86.5 kJ mol(-1) (20.7 kcal mol(-1)). Quantum chemical calculations have been performed to support the vibrational assignment and to discuss the existence of rotamers.
RESUMO
The synthesis of CF(3)OC(O)OOC(O)F is accomplished by the photolysis of a mixture of (CF(3)CO)(2)O, FC(O)C(O)F, CO, and O(2) at -15 degrees C using a low-pressure mercury lamp. The new peroxide is obtained in pure form in low yield after repeated trap-to-trap condensation and is characterized by NMR, IR, Raman, and UV spectroscopy. Geometrical parameters were studied by ab initio methods [B3LYP/6-311+G(d)]. At room temperature, CF(3)OC(O)OOC(O)F is stable for many days in the liquid or gaseous state. The melting point is -87 degrees C, and the boiling point is extrapolated to 45 degrees C from the vapor pressure curve log p = 8.384 - 1715/T (p/mbar, T/K). A possible mechanism for the formation of CF(3)OC(O)OOC(O)F is discussed, and its properties are compared with those of related compounds.