Eduardo H. Staricco

Quenching of tris(2,2'-bipyridine) chromium(III) and tris(1,10-phenanthroline) chromium(III) excited states by phenols : temperature and pH effect

Abstract The quenching rate constant kq of the excited states *Cr(bpy)33+ and *Cr(phen)33+ by several substituted phenols was investigated as a function of temperature using the time resolved luminescence quenching technique. The activation parameters showed a pH dependence. The most important contribution to the total activation entropy in basic media was found to be the electrostatic factor. The data obtained from log kq vs. ΔG0 plot were analyzed on the basis of the current theories for electron transfer processes. The best fitting analysis in the ΔG# vs. ΔG0 plot yielded reorganization energies of 59 and 55 kcal/mol for the bipyridine and phenanthroline chromium complexes, respectively. The high values of reorganization energies found (λ) were interpreted as a hydrogen bond term contribution (λH) to the outer-sphere reorganization energy, λo′, where λo′ = λo+ λH. The effect of the ionic strength and [Cl−] on the quenching rate constant for the reaction of *Cr(phen)33+ with 2,6-dimethylphenol was investigated. It was found that neither the ionic pair formation nor the [Cl−] concentration seem to affect the quenching mechanism.

Quantum yield dependence on the wavelength in the gas phase photolysis of perfluorosuccinic anhydride

Abstract The photolysis of perfluorosuccinic anhydride vapour was studied in the pressure range 3.0 – 70.2 Torr using light with wavelengths of 254, 232 and 220 nm. The products were CO, CO 2 and C 2 F 4 . At higher pressures the quantum yield was 0.50 at all wavelengths, but at the zero pressure limit, the quantum yield at 254 and 232 nm was 0.74 and at 220 nm it was unity. The data are interpreted in terms of a variation either in the vibrational level of the lowest excited singlet or in the electronic transition.

The photolysis of CF3COCl in the presence of O2 and CO : Catalytic oxidation of CO to CO2 and the formation of polyoxygenated intermediates

Abstract Photolysis of CF3COCl in the presence of O2 yields the radical CF3O, which then reacts in the presence of CO and O2 to yield a variety of CF3-containing species initiated by the following reactions: CF 3 O+CO→CF 3 OCO CF 3 OCO+O 2 →CF 3 OC(O)O 2 The reactions of various oxygenated CF3-containing radicals give rise to two different compounds with the general composition CF3OC(O)OyCF3 (y=4 and 3) that show increased thermal stability as the number of oxygen atoms in the peroxidic bonding bridge decreases. A further species of the formula CF3OC(O)O2C(O)OCF3 is also invoked. Parallel reactions lead to the formation of CF3OC(O)O radicals that decompose to give CF3O and CO2, thus providing a way of maintaining the chain reaction that catalytically converts CO into CO2. Experimental evidence for the formation and thermal decay of these intermediates is presented.

Quantum yield in the gas phase photolysis of perfluoroacetyl chloride: a comparison with related compounds

Abstract The photolysis of perfluoroacetyl chloride vapour was studied in the pressure range 4.6–59.9 Torr and with addition of inert gas up to 535.2 Torr using light of 254 and 280 nm. The quantum yield for the decomposition of CF 3 COCl taken as Φ [CF 3 Cl+2C 2 F 6 ] = 0.98±0.13 was not affected by the total pressure, the intensity and the wavelength of the light within the studied range of conditions. In the presence of c-C 6 H 12 a clear hydrogen abstraction reaction took place indicating the presence of CF 3 radicals. An upper limit for the rate of hydrogen abstraction of log k H (cm 3 mol −1 s −1 ) = 7.9 at 298 K was obtained. Light emission was not observed over the range 330–600 nm. Mechanisms for the decomposition of perfluoroacyl halides are discussed.

The 254 nm gas phase photochemistry of perfluoroacetyl chloride in the presence of O2

The photolysis at 254 nm of perfluoroacetyl chloride (CF3COCl) in the gas phase and with O2 in excess yielded CO2, CF2O, CF3OOOCF3, CF3COF and Cl2 as products. It was observed that the ratio of CF3COCl consumed to Cl2 formed was 2:1 and that the rate of disappearance of CF3COCl was faster in the presence than in the absence of O2. The quantum yields of formation of the main products obtained in typical runs followed the relationship φCF3COCI=φCF2O+2XφCF3OOOCF3+φCF3COF=2XφφCl2>1 within experimental error, although individual experiments showed varying amounts of each product. From these results, we postulate that F atoms are released as a result of photolysis. Also some evidence is presented that is consistent with the formation of CF3OC(O)OyC(O)OCF3 (where y can be 2, 3 or 4) when the photo-oxidation is carried out in the presence of O2 and CO.

Quantum yield and total pressure effect in perfluoroglutaric anhydride photolysis in the gas phase

Abstract The quantum yield of the photolysis of perfluoroglutaric anhydride (PFGA) in the gas phase (φ CO = 0.34 ± 0.07) was determined in runs using light of wavelength 254 nm. This value is intermediate between the quantum yields of open-chain perfluorinated anhydrides and of strained cyclic perfluorinated anhydrides. The PFGA quantum yield is independent of time and total pressure. The photolysis of PFGA vapour produces CO, CO 2 , C 2 F 4 and c -C 3 F 6 . The ratio [C 2 F 4 ]/[ c -C 3 F 6 ] is significantly dependent on the total pressure as shown by runs with the addition of inert gases such as helium, argon, oxygen, nitrogen, CCl 2 F 2 and C 2 Cl 3 F 3 . Each of these gases produces a specific effect as can be seen from their collisional deactivation coefficients.

Gas phase reaction of CF3CN and other nitriles with chlorine atoms

Abstract The photolytically induced reaction of Cl 2 with some nitriles was investigated in the gas phase at room temperature. The reaction with CF 3 CClNCl, CF 3 CClNNCClCF 3 and CF 3 CCl 3 as well as minor quantities of CF 3 (Cl)CNCCl 2 CF 3 , CF 3 CCl 2 (CF 3 )CNNC(Cl)CF 3 , CF 3 CCl 2 NN CCl 2 CF 3 , CF 3 CCl 2 (CF 3 )C NCCl 2 CF 3 and CF 3 CCl 2 (CF 3 )CNNC(CF 3 )CCl 2 CF 3 . ClCN reacted in a similar way giving CCl 2 NCl, CCl 2 NNCCl 2 and CCl 4 as principal products. On the other hand, when HCN was allowed to react with chlorine atoms, only ClCN, CCl 2 NCl and CCl 4 were obtained. No compounds containing C-H bonds were found. Possible reaction schemes taking into account the products obtained are discussed.

Photolysis of perfluoroglutaric anhydride vapor at low pressures: effect of wavelength on the product ratio

Abstract The photolysis of perfluoroglutaric anhydride vapor has been investigated in the pressure range 0.03 – 2 Torr using light of wavelength 270, 265, 254, 240 and 232 nm. The products were CO, CO2, C2F4 and cC3F6. The [C2F4]/[c-C3F6] ratio is directly proportional to the energy supplied but is inversely proportional to the pressure.

Photochemical and thermal decomposition of perfluoroglutaryl dichloride in the gas phase

Abstract The photolysis of perfluoroglutaryl dichloride was investigated in the gas phase with the light of the full arc of a high pressure mercury lamp. At high intensities and conversions the main products are CO and ClCF2CF2CF2Cl, and the ratio CO Cl(CF 2 ) 3 Cl is 2. However, in runs with attenuated light and very short times, this ratio reaches higher values because less Cl(CF2)3Cl is formed. In the first step the stable intermediate compound ClCO(CF2)3Cl is formed, producing Cl(CF2)3Cl by absorption of a second photon. The thermal decomposition studied in the temperature range 611–713 K produces CO, CF2Cl2, C2F4Cl2, C2F4, COCl2 and a very small amount of c-C3F6. A mechanism is suggested to explain these results.

Quenching kinetics of I(2P½) by cyanides. Isotope effect in the quenching by CH3(D)CN

The quenching of electronically excited iodine atoms I(2P½ I*) by CH3CN, CD3CN, HCN, C2H5CN, and (CN)2 was studied. Good agreement was found for I* deactivation by acetonitrile and perdeuteroacetonitrile using two widely different techniques, namely time-resolved atomic absorption (TRAA) and chemical laser threshold gain (CLTG). The kinetic isotope effect determined, kH/kD, was 2.2 ± 0.5 for both methods. This low value would support the existence of a noticeable reactive path and a possible channel assisted by a curve-crossing mechnism. Values here reported for the second-order rate constants for I* deactivation are, respectively, (1.2 ± 0.1)× 10–13, (5.3 ± 0.5)× 10–14, (3.3 ± 0.2)× 10–14, (1.3 ± 0.1)× 10–13, (2.2 ± 0.2)× 10–15 cm3 molecule–1 s–1 for CH3CN, CD3CN, HCN, C2H5CN and (CN)2, according to the TRAA method. Results from CLTG measurements are (1.6 ± 0.1)× 10–13, (7.3 ± 0.2)× 10–14 and (8.3 ± 0.1)× 10–14 cm3 molecule–1 s–1 for CH3CN, CD3CN and HCN, respectively.