Kinetics following addition of sulfur fluorides to a weakly ionized plasma from 300 to 500 K: rate constants and product determinations for ion-ion mutual neutralization and thermal electron attachment to SF5, SF3, and SF2

Rate constants for several processes including electron attachment to SF(2), SF(3), and SF(5) and individual product channels of ion-ion mutual neutralization between SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) were determined by variable electron and neutral density attachment mass spectrometry. The experiments were conducted with a series of related neutral precursors (SF(6), SF(4), SF(5)Cl, SF(5)C(6)H(5), and SF(3)C(6)F(5)) over a temperature range of 300-500 K. Mutual neutralization rate constants for SF(6)(-), SF(5)(-), and SF(4)(-) with Ar(+) are reported with uncertainties of 10-25% and show temperature dependencies in agreement with the theoretical value of T(-0.5). Product branching in the mutual neutralizations is temperature independent and dependent on the electron binding energy of the anion. A larger fraction of product neutrals from the SF(6)(-) mutual neutralization (0.9 ± 0.1) are dissociated than in the SF(5)(-) mutual neutralization (0.65 ± 0.2), with the SF(4)(-) (0.7 ± 0.3) likely lying in between. Electron attachment to SF(5) (k = 2.0 × 10(-8) ±(1)(2) cm(3) s(-1) at 300 K) and SF(3) (4 ± 3 × 10(-9) cm(3) s(-1) at 300 K) show little temperature dependence. Rate constants of electron attachment to closed-shell SF(n) species decrease as the complexity of the neutral decreases.     

Electron attachment to SF5X compounds: SF5C6H5, SF5C2H3, S2F10, and SF5Br, 300-550 K

Rate constants and ion product channels have been measured for electron attachment to four SF5 compounds, SF5C6H5, SF5C2H3, S2F10, and SF5Br, and these data are compared to earlier results for SF6, SF5Cl, and SF5CF3. The present rate constants range over a factor of 600 in magnitude. Rate constants measured in this work at 300 K are 9.9+/-3.0x10(-8) (SF5C6H5), 7.3+/-1.8x10(-9) (SF5C2H3), 6.5+/-2.5x10(-10) (S2F10), and 3.8+/-2.0x10(-10) (SF5Br), all in cm3 s-1 units. SF5- was the sole ionic product observed for 300-550 K, though in the case of S2F10 it cannot be ascertained whether the minor SF4- and SF6- products observed in the mass spectra are due to attachment to S2F10 or to impurities. G3(MP2) electronic structure calculations (G2 for SF5Br) have been carried out for the neutrals and anions of these species, primarily to determine electron affinities and the energetics of possible attachment reaction channels. Electron affinities were calculated to be 0.88 (SF5C6H5), 0.70 (SF5C2H3), 2.95 (S2F10), and 2.73 eV (SF5Br). An anticorrelation is found for the Arrhenius A-factor with exothermicity for SF5- production for the seven molecules listed above. The Arrhenius activation energy was found to be anticorrelated with the bond strength of the parent ion.     

Determination of the rate constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination reactions with collision partners CH4, C2H6, CO2, CF4, and SF6 at low pressures and 296 K. Part 2

The recombination rate constants for the reactions NH2(X2B1) + NH2(X2B1) + M → N2H4 + M and NH2(X2B1) + H + M → NH3 + M, where M was CH4, C2H6, CO2, CF4, or SF6, were measured in the same experiment over presseure ranges of 1-20 and 7-20 Torr, respectively, at 296 ± 2 K. The NH2 radical was produced by the 193 nm laser photolysis of NH3. Both NH2 and NH3 were monitored simultaneously following the photolysis laser pulse. High-resolution time-resolved absorption spectroscopy was used to monitor the temporal dependence of both species: NH2 on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A2A1 ← (0,0,0)X2B1 electronic transition near 675 nm and NH3 in the IR on either of the inversion doublets of the qQ3(3) rotational transition of the ν1 fundamental near 2999 nm. The NH2 self-recombination clearly exhibited falloff behavior for the third-body collision partners used in this work. The pressure dependences of the NH2 self-recombination rate constants were fit using Troe’s parametrization scheme, k(inf), k(0), and F(cent), with k(inf) = 7.9 × 10(-11) cm3 molecule(-1) s(-1), the theoretical value calculated by Klippenstein et al. (J. Phys. Chem. A113, 113, 10241). The individual Troe parameters were CH4, k(0)(CH4) = 9.4 × 10(-29) and F(cent)(CH4) = 0.61; C2H6, k(0)(C2H6) = 1.5 × 10(-28) and F(cent)(C2H6) = 0.80; CO2, k(0)(CO2) = 8.6 × 10(-29) and F(cent)(CO2) = 0.66; CF4, k(0)(CF4) = 1.1 × 10(-28) and F(cent)(CF4) = 0.55; and SF6, k(0)(SF6) = 1.9 × 10(-28) and F(cent)(SF6) = 0.52, where the units of k0 are cm6 molecule(-2) s(-1). The NH2 + H + M reaction rate constant was assumed to be in the three-body pressure regime, and the association rate constants were CH4, (6.0 ± 1.8) × 10(-30); C2H6, (1.1 ± 0.41) × 10(-29); CO2, (6.5 ± 1.8) × 10(-30); CF4, (8.3 ± 1.7) × 10(-30); and SF6, (1.4 ± 0.30) × 10(-29), with units cm6 molecule(-1) s,(-1) and the systematic and experimental errors are given at the 2σ confidence level.     

Kinetics of electron attachment to OH and HNO3 and mutual neutralization of Ar+ with NO2(-) and NO3(-) at 300 and 500 K

The electron attachment rate constant to nitric acid (HNO(3)) has been measured in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 and 500 K using three independent methods: the traditional FALP technique of monitoring electron depletion, "one-gas" VENDAMS (variable electron and neutral density attachment mass spectrometry), and "two-gas" VENDAMS. The three measurements are in agreement with a 300 K weighted average of 1.4 ± 0.3 × 10(-7) cm(3) s(-1), 2 to 10 times higher than previously reported values. Attachment is primarily dissociative yielding NO(2)(-) as previously reported, but for the first time a small endothermic channel to produce OH(-) was also observed at 500 K. From the one-gas VENDAMS data, associative attachment to the OH produced in the primary attachment was found to occur with an effective two body rate constant of 1.2±(0.7) (3)×10(-11) cm(3) s(-1) at 300 K, the first reported rate constant for this radical species. Finally, ion-ion neutralization rate constants of NO(2)(-) and NO(3)(-) with Ar(+) were determined to be 5.2±(2.5) (1.5) × 10(-8) and 4.5 ± 2.5 × 10(-8) cm(3) s(-1) at 300 K, respectively.     

Rate constants for the reactions between OH and perfluorinated alkenes

The rate constants for the reactions of OH radicals with fully fluorinated alkenes containing different numbers of -CF(3) groups next to olefinic carbon, CF(2)═CF(2), CF(2)═CFCF(3), CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), were measured between 230 and 480 K using the flash photolysis resonance fluorescence technique to give the following expressions: k(C(2)F(4))(250-480 K) = 1.32 × 10(-12) × (T/298 K)(0.9) × exp(+600 K/T) cm(3) molecule(-1) s(-1), k(C(3)F(6))(230-480 K) = 9.75 ×10(-14) × (T/298 K)(1.94) × exp(+922 K/T) cm(3) molecule(-1) s(-1), k(trans-C(4)F(8))(230-370 K) = 7.50 × 10(-14) × (T/298 K)(1.68) × exp(+612 K/T) cm(3) molecule(-1) s(-1), k(cis-C(4)F(8))(230-370 K) = 2.99 × 10(-14) × (T/298 K)(2.61) × exp(+760 K/T) cm(3) molecule(-1) s(-1), and k(C(6)F(12))(250-480 K) = 2.17 × 10(-15) × (T/298 K)(3.90) × exp(+1044 K/T) cm(3) molecule(-1) s(-1). The kinetics of the OH reaction in an industrial sample of octofluoro-2-propene (a mixture of the cis- and trans-isomers of CF(3)CF═CFCF(3)) was studied to determine the "effective" reaction rate constant for the typically industrial mixture: k()(230-480 K) = 7.89 × 10(-14) × (T/298 K)(1.71) × exp(+557 K/T) cm(3) molecule(-1) s(-1). On the basis of these results, the atmospheric lifetimes were estimated to be 1.2, 5.3, 21, 34, and 182 days for CF(2)═CF(2), CF(3)CF═CF(2), trans-CF(3)CF═CFCF(3), cis-CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), respectively. The general pattern of halolalkene reactivity toward OH is discussed.     

Studies of the kinetics and thermochemistry of the forward and reverse reaction Cl + C6H6 = HCl + C6H5

The laser flash photolysis resonance fluorescence technique was used to monitor atomic Cl kinetics. Loss of Cl following photolysis of CCl4 and NaCl was used to determine k(Cl + C6H6) = 6.4 x 10(-12) exp(-18.1 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 578-922 K and k(Cl + C6D6) = 6.2 x 10(-12) exp(-22.8 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 635-922 K. Inclusion of literature data at room temperature leads to a recommendation of k(Cl + C6H6) = 6.1 x 10(-11) exp(-31.6 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) for 296-922 K. Monitoring growth of Cl during the reaction of phenyl with HCl led to k(C6H5 + HCl) = 1.14 x 10(-12) exp(+5.2 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 294-748 K, k(C6H5 + DCl) = 7.7 x 10(-13) exp(+4.9 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 292-546 K, an approximate k(C6H5 + C6H5I) = 2 x 10(-11) cm(3) molecule(-1) s(-1) over 300-750 K, and an upper limit k(Cl + C6H5I) < or = 5.3 x 10(-12) exp(+2.8 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 300-750 K. Confidence limits are discussed in the text. Third-law analysis of the equilibrium constant yields the bond dissociation enthalpy D(298)(C6H5-H) = 472.1 +/- 2.5 kJ mol(-1) and thus the enthalpy of formation Delta(f)H(298)(C6H5) = 337.0 +/- 2.5 kJ mol(-1).     

Dissociative electron attachment to C(2)F(5) radicals

Dissociative electron attachment to the reactive C(2)F(5) molecular radical has been investigated with two complimentary experimental methods; a single collision beam experiment and a new flowing afterglow Langmuir probe technique. The beam results show that F(-) is formed close to zero electron energy in dissociative electron attachment to C(2)F(5). The afterglow measurements also show that F(-) is formed in collisions between electrons and C(2)F(5) molecules with rate constants of 3.7 × 10(-9) cm(3) s(-1) to 4.7 × 10(-9) cm(3) s(-1) at temperatures of 300-600 K. The rate constant increases slowly with increasing temperature, but the rise observed is smaller than the experimental uncertainty of 35%.     

Electron attachment and detachment, and the electron affinities of C5F5N and C5HF4N

Rate constants have been measured for electron attachment to C5F5N (297-433 K) and to 2, 3, 5, 6-C5HF4N (303 K) using a flowing-afterglow Langmuir-probe apparatus (at a He gas pressure of 133 Pa). In both cases only the parent anion was formed in the attachment process. The attachment rate constants measured at room temperature are 1.8 +/- 0.5 X 10(-7) and 7 +/- 3 X 10(-10) cm(-3) s(-1), respectively. Rate constants were also measured for thermal electron detachment from the parent anions of these molecules. For C5F5N- detachment is negligible at room temperature, but increases to 2530 +/- 890 s(-1) at 433 K. For 2, 3, 5, 6-C5HF4N-, the detachment rate at 303 K was 520 +/- 180 s(-1). The attachment/detachment equilibrium yielded experimental electron affinities EA(C5F5N)=0.70 +/- 0.05 eV and EA(2, 3, 5, 6-C5HF4N)=0.40 +/- 0.08 eV. Electronic structure calculations were carried out for these molecules and related C5HxF5-xN using density-functional theory and the G3(MP2)//B3LYP compound method. The EAs are found to decrease by 0.25 eV, on average, with each F substitution by H. The calculated EAs are in good agreement with the present experimental results.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

Atmospheric chemistry of C2F5CH2OCH3 (HFE-365mcf)

FTIR smog chamber techniques were used to measure k(Cl + C(2)F(5)CH(2)OCH(3)) = (2.52 ± 0.37) × 10(-11) and k(OH + C(2)F(5)CH(2)OCH(3)) = (5.78 ± 1.02) × 10(-13) cm(3) molecule(-1) s(-1) in 700 Torr of air diluent at 296 ± 1 K. The atmospheric lifetime of C(2)F(5)CH(2)OCH(3) is estimated to be 20 days. Reaction of chlorine atoms with C(2)F(5)CH(2)OCH(3) proceeds 18 ± 2% at the -CH(2)- group and 82 ± 2% at the -CH(3) group. Reaction of OH radicals with C(2)F(5)CH(2)OCH(3) proceeds 44 ± 5% at the -CH(2)- group and 56 ± 5% at the -CH(3) group. The atmospheric fate of C(2)F(5)CH(2)OCH(2)O radicals is reaction with O(2) to give C(2)F(5)CH(2)OCHO. The atmospheric fate of C(2)F(5)CH(O)OCH(3) radicals is C-C bond-cleavage to give C(2)F(5) radicals and CH(3)OCHO (methyl formate). The infrared spectrum was recorded and used to estimate a global warming potential of 6 (100 year time horizon) for C(2)F(5)CH(2)OCH(3).     

Vacuum-UV negative photoion spectroscopy of SF5CF3

Ion pair formation, generically described as AB-->A(+)+B(-), from vacuum-UV photoexcitation of trifluoromethyl sulfur pentafluoride, SF(5)CF(3), has been studied by anion mass spectrometry using synchrotron radiation in the photon energy range of 10-35 eV. The anions F(-), F(2)(-), and SF(x)(-) (x=1-5) are observed. With the exception of SF(5)(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from ion pair formation. SF(5)(-) arises from dissociative electron attachment, following photoionization of SF(5)CF(3) as the source of low-energy electrons. Cross sections for anion production are put on to an absolute scale by calibration of the signal strengths with those of F(-) from both SF(6) and CF(4). Quantum yields for anion production from SF(5)CF(3), spanning the range of 10(-7)-10(-4), are obtained using vacuum-UV absorption cross sections. Unlike SF(6) and CF(4), the quantum yield for F(-) production from SF(5)CF(3) increases above the onset of photoionization.     

Communication: Revised electron affinity of SF6 from kinetic data

Previously determined experimental data for thermal attachment of electrons to SF(6) and thermal detachment from SF(6)(-) over the range 590-670 K are reevaluated by a third-law analysis. Recent high precision calculations of SF(6)(-) harmonic frequences and anharmonicities (for several of the modes) lead to considerable changes in modeled vibrational partition functions which then have to be accommodated for by a smaller value of the derived adiabatic electron affinity EA of SF(6). The previously estimated value of EA = 1.20 (±0.05) eV in this way is reduced to a value of EA = 1.03 (±0.05) eV. In addition, the bond dissociation energy E(0,dis) for SF(6)(-) → SF(5)(-) + F is reduced to E(0,dis) = 1.44 (±0.05) eV. Finally, the consequences for modeled specific rate constants k(det)(E,J) of electron detachment from SF(6)(-) are discussed.     

C6F5Xe]+ and [C6F5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C6F5)

New examples of [C6F5Xe]+ salts of the weakly coordinating [BY4]- (Y = CN, CF3, or C6F5) anions were synthesized by metathesis of [C6F5Xe][BF4] with MI[BY4] (MI = K or Cs; Y = CN, CF3, or C6F5) in CH3CN at -40 degrees C, and were crystallized from CH2Cl2 or from a CH2Cl2/CH3CN solvent mixture. The low-temperature (-173 degrees C) X-ray crystal structures of the [C6F5Xe]+ cation and of the [C6F5XeNCCH3]+ adduct-cation are reported for [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4]. The [C6F5Xe]+ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF3)4]- anion. The solid-state Raman spectra of the [C6F5Xe]+ and [C6F5XeNCCH3]+ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C6F5XeNCCH3]+ is approximately half that of [FXeNCCH3]+. Coordination of CH3CN to [C6F5Xe]+ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.     

B(C6F5)3- vs Al(C6F5)3-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.     

Reaction of Cl atoms with C6F13CH2OH, C6F13CHO, and C3F7CHO

The Cl atom initiated oxidation of C(6)F(13)CH(2)OH, C(6)F(13)CHO, and C(3)F(7)CHO was investigated at 298 K and 1000 mbar pressure of air in a photoreactor using in situ Fourier transform infrared (FTIR) analysis. The rate coefficient for the reaction Cl + C(6)F(13)CH(2)OH (reaction 2) was measured using a relative method: k(2) = (6.5 +/- 0.8) x 10(-13) cm(3) molecule(-1) s(-1). C(6)F(13)CHO was detected as the major primary product, while CO and CF(2)O were found to be the major secondary products. A fitting procedure applied to the concentration-time profiles of C(6)F(13)CHO provided a production yield of (1.0 +/- 0.2) for this aldehyde in reaction 2, and the rate coefficient for the reaction Cl + C(6)F(13)CHO (reaction 4) was k(4) = (2.8 +/- 0.7) x 10(-12) cm(3) molecule(-1) s(-1). A high CO yield observed in the oxidation of C(6)F(13)CH(2)OH, (52 +/- 1)%, is attributed to the Cl atom initiated oxidation of C(6)F(13)CHO. High CO yields, (61 +/- 2)% and (85 +/- 5)%, were also measured in the Cl atom initiated oxidation of C(3)F(7)CHO in air and nitrogen, respectively. These high CO yields suggest the occurrence of a decomposition reaction of the perfluoroacyl, C(6)F(13)CO, and C(3)F(7)CO radicals to form CO which will compete with the combination reaction of these radicals with oxygen to form perfluoroacyl peroxy radicals in the presence of air. The latter radicals C(n)F(2)(n)(+1)CO(O)(2) (n = 6-12), through their reaction with HO(2) radicals, are currently considered as a possible source of persistent perfluorocarboxylic acids which have been detected in the environment. The consequences of the present results would be a reduction of the strength of this potential source of carboxylic acids in the atmosphere.     

Spectroscopic characterization of the isolated SF6- and C4F8- anions: observation of very long harmonic progressions in symmetric deformation modes upon photodetachment

Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.     

Low-energy electron attachment to SF6. III. From thermal detachment to the electron affinity of SF6

The thermal attachment of electrons to SF(6) is measured in a flowing-afterglow Langmuir-probe apparatus monitoring electron concentrations versus axial position in the flow tube. Temperatures between 300 and 670 K and pressures of the bath gas He in the range of 0.3-9 Torr are employed. Monitoring the concentrations of SF(6)(-) and SF(5)(-), the latter of which does not detach electrons under the applied conditions, an onset of thermal detachment and dissociation of SF(6) at temperatures above about 530 K is observed. Analysis of the mechanism allows one to deduce thermal detachment rate coefficients. Thermal dissociation rate coefficients for the reaction SF(6)(-)-->SF(5)(-)+F can only be estimated by unimolecular rate theory based on the results from Part I and II of this series. Under the applied conditions they are found to be smaller than detachment rate coefficients. Combining thermal attachment and detachment rates in a third-law analysis, employing calculated vibrational frequencies of SF(6) and SF(6)(-), leads to the electron affinity (EA) of SF(6)(-). The new value of EA=1.20(+/-0.05) eV is significantly higher than previous recommendations which were based on less direct methods.     

Model studies of DNA C5' radicals. Selective generation and reactivity of 2'-deoxyadenosin-5'-yl radical

The reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations. Pulse radiolysis revealed that this reaction was complete in approximately 0.3 mus, and, at this time, no significant absorption was detected. The spectrum of a transient developed in 20 mus has an absorbance in the range 300-500 nm (epsilon(max) congruent with 9600 M(-1) cm(-1) at 360 nm), and it was assigned to aromatic aminyl radical 3. Computed vertical transitions (TD-UB3LYP/6-311+G) are in good agreement with the experimental observations. Radical 3 is obtained by the following reaction sequence: one-electron reductive cleavage of the C-Br bond that gives the C8 radical, a fast radical translocation from the C8 to C5' position, and an intramolecular attack of the C5' radical at the C8,N7 double bond of the adenine moiety. The rate constant for the cyclization is 1.6 x 10(5) s(-1). On the basis of the theoretical findings, the cyclization step is highly stereospecific. The rate constants for the reactions of C5' and aminyl 3 radicals with different oxidants were determined by pulse radiolysis methods. The respective rate constants for the reaction of 2'-deoxyadenosin-5'-yl radical with dioxygen, Fe(CN)(6)(3)(-), and MV(2+) in water at ambient temperature are 1.9 x 10(9), 4.2 x 10(9), and 2.2 x 10(8) M(-1) s(-1). The value for the reaction of aminyl radical 3 with Fe(CN)(6)(3-) is 8.3 x 10(8) M(-1) s(-1), whereas the reaction with dioxygen is reversible. Tailored experiments allowed the reaction mechanism to be defined in some detail. A synthetically useful radical cascade process has also been developed that allows in a one-pot procedure the conversion of 8-bromo-2'-deoxyadenosine to 5',8-cyclo-2'-deoxyadenosine in a diastereoisomeric ratio (5'R):(5'S) = 6:1 and in high yield, by reaction with hydrated electrons in the presence of K(4)Fe(CN)(6).     

Atmospheric chemistry of CF3CF═CH2 and (Z)-CF3CF═CHF: Cl and NO3 rate coefficients, Cl reaction product yields, and thermochemical calculations

Rate coefficients, k, for the gas-phase reactions of Cl atoms and NO(3) radicals with 2,3,3,3-tetrafluoropropene, CF(3)CF═CH(2) (HFO-1234yf), and 1,2,3,3,3-pentafluoropropene, (Z)-CF(3)CF═CHF (HFO-1225ye), are reported. Cl-atom rate coefficients were measured in the fall-off region as a function of temperature (220-380 K) and pressure (50-630 Torr; N(2), O(2), and synthetic air) using a relative rate method. The measured rate coefficients are well represented by the fall-off parameters k(0)(T) = 6.5 × 10(-28) (T/300)(-6.9) cm(6) molecule(-2) s(-1) and k(∞)(T) = 7.7 × 10(-11) (T/300)(-0.65) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and k(0)(T) = 3 × 10(-27) (T/300)(-6.5) cm(6) molecule(-2) s(-1) and k(∞)(T) = 4.15 × 10(-11) (T/300)(-0.5) cm(3) molecule(-1) s(-1) for (Z)-CF(3)C═CHF with F(c) = 0.6. Reaction product yields were measured in the presence of O(2) to be (98 ± 7)% for CF(3)C(O)F and (61 ± 4)% for HC(O)Cl in the CF(3)CF═CH(2) reaction and (108 ± 8)% for CF(3)C(O)F and (112 ± 8)% for HC(O)F in the (Z)-CF(3)CF═CHF reaction, where the quoted uncertainties are 2σ (95% confidence level) and include estimated systematic errors. NO(3) reaction rate coefficients were determined using absolute and relative rate methods. Absolute measurements yielded upper limits for both reactions between 233 and 353 K, while the relative rate measurements yielded k(3)(295 K) = (2.6 ± 0.25) × 10(-17) cm(3) molecule(-1) s(-1) and k(4)(295 K) = (4.2 ± 0.5) × 10(-18) cm(3) molecule(-1) s(-1) for CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF, respectively. The Cl-atom reaction with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF leads to decreases in their atmospheric lifetimes and global warming potentials and formation of a chlorine-containing product, HC(O)Cl, for CF(3)CF═CH(2). The NO(3) reaction has been shown to have a negligible impact on the atmospheric lifetimes of CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF. The energetics for the reaction of Cl, NO(3), and OH with CF(3)CF═CH(2) and (Z)-CF(3)CF═CHF in the presence of O(2) were investigated using density functional theory (DFT).     

Toward a more general synthetic route to paramagnetic solids containing RCNSSS*+ radical cations. A structure-property correlation for RCNSSS*+ (R = F5C2, Cl3C)

Reaction of Cl3CN and F5C2CN with a 1:1 mixture of S4(AsF6)2 and S8(AsF6)2 affords the paramagnetic solids Cl3CNSSSAsF6 (1CCl3AsF6) and F5C2CNSSSAsF6 (1C2F5AsF6). Isotropic electron paramagnetic resonance spectra of 1CCl3AsF6 and 1C2F5AsF6 in SO2 consist of a single line with g = 2.01675 and 2.01580, respectively. The structure of 1CCl3AsF6 contains chains of radical cations with relatively close interchain interactions. In contrast, chains are isolated in 1C2F5AsF6. The magnetic behavior of both compounds was interpreted as that of 1D Heisenberg antiferromagnetic chains (1CCl3AsF6, J = -34 cm(-1), theta = -9 cm(-1), TIP = 0.00082, rho = 0.012; 1C2F5AsF6, J = -21 cm(-1), theta = -4.2 cm(-1), TIP = 0.00092, rho = 0.065). Density functional theory calculated and experimental magnetic coupling constants were in good agreement. The correlation between intermolecular S...S contacts and the strength of magnetic couplings was established.