Disproportionation pathways of aqueous hyponitrite radicals (HN2O2(*)/N2O2(*-))

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2(*)/N2O2(*-)) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release-consumption steps. In the first, rate-determining step, two N2O2(*-) radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) M(-1) s(-1) (at zero ionic strength) effectively in a redox reaction regenerating N2O2(2-) and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination-dissociation. Each NO so-produced rapidly adds to another N2O2(*-), yielding the N3O3(-) ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2(-) products. The N2O2(*-) radical protonates with pKa = 5.6 +/- 0.3. The neutral HN2O2(*) radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M(-1) s(-1). The N2O2(*-) radical shows surprisingly low reactivity toward O2 and O2(*-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(7) M(-1) s(-1). The previously reported rapid dissociation of N2O2(*-) into N2O and O(*-) does not occur. The thermochemistry of HN2O2(*)/N2O2(*-) is discussed in the context of these new kinetic and mechanistic results.     

The (SO2)2N3- anion

The recently proposed (SO2)2N3- anion was structurally characterized by single-crystal X-ray diffraction of the [Cs][(SO2)2N3] salt (P2(1)/c, a = 8.945(2) A, b = 10.454(2) A, c = 8.152(2) A, beta = 109.166(3) degrees, Z = 4, and R1 = 0.0329 at 130 K). In the (SO2)2N3- anion, both SO2 ligands are coordinated to one terminal nitrogen atom of the N3- anion.     

Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.     

Reactions of hydroxyl radical (·OH) and oxide radical anion (O·−) with 2-aminopurine in aqueous medium

Pulse radiolysis has been used to investigate the reaction of hydroxyl radical (^·OH) and oxide radical anion (O^·−) with 2-aminopurine (2AP), a fluorescent analogue of adenine, in aqueous medium. The second-order rate constant for the reaction of ^·OH with 2AP was determined to be 3 × 10⁹ dm³ mol⁻¹s⁻¹ and for the reaction of O^·− it was 7.1 × 10⁸ dm³ mol⁻¹s⁻¹. The transient absorption spectrum obtained in the reaction of ^·OH at pH 7 has absorption maxima at 370 and 470 nm. The spectrum undergoes a time-dependent transformation at higher time-scale. The intermediate species was found to react with N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). The yield of TMPD^·+ was calculated in terms of G(TMPD^·+) to be 3.3 × 10⁻⁷ mol J⁻¹ at pH 7. The ^·OH reactions were also carried out at pH 10 and the transient absorption spectra have λ _max at 400 and 480 nm. The transient spectra obtained in the reaction of O^·− at pH ≈14 have maxima at 400 and 480 nm. The transient intermediate species at pH 7 are assigned to the formation of 2AP-4-OH^· (54%), 2AP-5-OH^· (7%) and 2AP-8-OH^· (39%) based on the spectral evidence and TMPD^·+ build-up. Both 2AP-4-OH^· and 2AP-5-OH^· undergo OH⁻ elimination to form a radical cation. At higher pH (pH 10), the dehydration reaction of these OH-adducts leads to a N-centered radical (2AP-N(9)^·, 71%). Formation of 2AP-8-OH^· (29%) is also proposed at this pH. In the reaction of O^·− with 2AP, it is proposed that a similar nitrogen centered 2AP-N(9)^· radical is formed by an electron-transfer reaction at N(9).     

Microsolvation pattern of the hydrated radical anion of uracil: U-(H2O)n (n = 3-5)

The microsolvation patterns of the uracil radical anion in water clusters U-(H2O)n with n ranging from 3 to 5 were investigated by the density functional theory approach. The electron detachment energies (VDE) of the stable anionic complexes with different numbers of hydration water are predicted. The linear dependence of the VDE value of the most stable anionic complexes with respect to the hydration number suggests the importance of the clustered waters in the microsolvation of the radical anion of the nucleobases. The formation of the water clusters is found to be necessary in the most stable conformers of the tri-, tetra-, and pentahydrated radical anion of uracil. The microsolvation pattern with three or more well-separated hydration water molecules in the first hydration layer is less stable than the arrangement with the waters in tight clusters. The charge transfer between the anionic uracil and the hydration water is high. Good agreement between the experimental and the theoretical vertical detachment energy yield in this study further demonstrates the practicability of the B3LYP/DZP++ approach in the study of radical anions of the DNA subunits.     

An equilibrium analysis of the aqueous H+-MoO4(2-)-(HP)O(3)2- and H+-MoO4(2-)-(HP)O(3)2--HPO4(2-) systems

The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0相似文献   

Spectroscopic properties and electronic structure of pentammineruthenium(II) dinitrogen oxide and corresponding nitrosyl complexes: binding mode of N(2)O and reactivity

The spectroscopic properties and the electronic structure of the only nitrous oxide complex existing in isolated form, [Ru(NH(3))(5)(N(2)O)]X(2) (1, X = Br(-), BF(4)(-)), are investigated in detail in comparison to the nitric oxide precursor, [Ru(NH(3))(5)(NO)]X(3) (2). IR and Raman spectra of 1 and of the corresponding (15)NNO labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. This allows for the identification of the Ru-N(2)O stretch at approximately 300 cm(-)(1) and for the unambiguous definition of the binding mode of the N(2)O ligand as N-terminal. Obtained force constants are 17.3, 9.6, and 1.4 mdyn/A for N-N, N-O, and Ru-N(2)O, respectively. The Ru(II)-N(2)O bond is dominated by pi back-donation, which, however, is weak compared to the NO complex. This bond is further weakened by Coulomb repulsion between the fully occupied t(2g) shell of Ru(II) and the HOMO of N(2)O. Hence, nitrous oxide is an extremely weak ligand to Ru(II). Calculated free energies and formation constants for [Ru(NH(3))(5)(L)](2+) (L = NNO, N(2), OH(2)) are in good agreement with experiment. The observed intense absorption at 238 nm of 1 is assigned to the t(2g) --> pi(*) charge transfer transition. These data are compared in detail to the spectroscopic and electronic structural properties of NO complex 2. Finally, the transition metal centered reaction of nitrous oxide to N(2) and H(2)O is investigated. Nitrous oxide is activated by back-donation. Initial protonation leads to a weakening of the N-O bond and triggers electron transfer from the metal to the NN-OH ligand through the pi system. The implications of this mechanism for biological nitrous oxide reduction are discussed.     

Structural and electronic characterization of nitrosyl(octaethylporphinato)iron(III) perchlorate derivatives

The synthesis and crystallographic characterization of the five-coordinate iron(III) porphyrinate complex [Fe(OEP)(NO)]ClO4 are reported. This [FeNO]6 complex has a nearly linear Fe-N-O group (angle = 173.19(13) degrees) with a small off-axis tilt of the Fe-N(NO) vector from the heme normal (angle = 4.6 degrees); the Fe-N(NO) distance is 1.6528(13) A and the iron is displaced 0.32 A out-of-plane. The complex forms a tight cofacial pi-pi dimer in the solid state. M?ssbauer spectra for this derivative as well as for a related crystalline form are measured both in zero applied magnetic field and in a 7 T applied field. Fits to the measurements made in applied magnetic field demonstrate that both crystalline forms of [Fe(OEP)(NO)]ClO4 have a diamagnetic ground state at 4.2 K. The observed isomer shifts (delta = 0.22-0.24 mm/s) are smaller than those typically observed for low-spin iron(III) porphyrinates. Analogous M?ssbauer measurements are also obtained for a six-coordinate derivative, [Fe(OEP)(Iz)(NO)]ClO4 (Iz = indazole). The observed isomer shift for this species is smaller still (delta = 0.02 mm/s). All derivatives show a strong temperature dependence of the isomer shift. The data emphasize the strongly covalent nature of the FeNO group. The M?ssbauer isomer shifts suggest formal oxidation states greater than +3 for iron, but the NO stretching frequencies are not consistent with such a large charge transfer to NO. Differences in the observed nitrosyl stretching frequencies of the two crystalline forms of [Fe(OEP)(NO)]ClO4 are discussed.     

Formation of N2O from a nickel nitrosyl: isolation of the cis-[N2O2]2- intermediate

Addition of 2,2'-bipyridine (bipy) to [Ni(NO)(bipy)][PF(6)] (1) results in formation of a rare five-coordinate nickel nitrosyl [Ni(NO)(bipy)(2)][PF(6)] (2). This complex exhibits a bent NO(-) ligand in the solid state. On standing in acetonitrile, 2 furnishes the NO coupled product, [Ni(κ(2)-O(2)N(2))(bipy)] (8) in moderate yield. Subsequent addition of 2 equiv of acetylacetone (H(acac)) to 8 results in formation of [Ni(acac)(2)(bipy)], N(2)O, and H(2)O. Preliminary mechanistic studies suggest that the N-N bond is formed via a bimetallic coupling reaction of two NO(-) ligands.     

Antioxidant activity and superoxide anion radical interaction with 2-(ferrocenylmethylamino) benzonitrile and 3-(ferrocenylmethylamino) benzonitrile

Cyclic voltammetry was used to explore the interaction of (ferrocenylmethylamino) benzonitrile (FMAB) with superoxide anion radical (\({\text{O}}_{2}^{. - }\)), electrochemically generated by the reduction in commercial molecular oxygen in acetonitrile. The difference in the electrochemical behavior of \({\text{O}}_{2}^{. - }\) in the absence and presence of FMAB, including shifts in peak potential and decrease in anodic peak current, was successfully investigated for the determination of interaction parameters such as the binding constant, ratio of binding constants, binding free energy and mode of interaction. The anodic peak potential shifts and the magnitude of binding free energy ΔG suggest the electrostatic interaction of \({\text{O}}_{2}^{. - }\) with FMAB as the dominant mode, whereas the negative sign of ΔG indicates the spontaneity of the interaction. The antioxidant activity of FMAB derivatives was also evaluated using spectrophotometrical and electrochemical techniques. The spectrophotometrical assays were carried out using 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH), while the electrochemical assays were determined by measuring the oxidation peak current of \({\text{O}}_{2}^{. - }\). The activity was found to be highest for 3FMAB in both DPPH and \({\text{O}}_{2}^{. - }\) radicals scavenging methods (0.0183) and (0.0090 mg/mL), respectively, which is equal to half the antioxidant activity of standard antioxidant ascorbic acid (0.0117) and (0.0041 mg/mL).     

Infrared spectrum of the hyponitrite dianion, N(2)O(2)2-, isolated and insulated from stabilizing metal cations in solid argon.

Ultraviolet irradiation of a rigid 7 K argon matrix containing alkali or alkaline earth metal atoms and NO(2) isolated from each other by one or two layers of argon forms N(2)O(2)2-dianions insulated from two M(+) cations by argon atoms, and visible photolysis reverses this electron-transfer process likely involving the N(2)O(2)(-) anion intermediate. The isolated N(2)O(2)2- dianion is identified from isotopic substitution and isotopic mixtures, which show that the new 1028.5 cm(-1) metal independent absorption involves two equivalent NO subunits. DFT calculations predict a strong 1078.1 cm(-1) fundamental for the Li(NO)(2)Li molecule and isotopic frequency ratios in excellent agreement with the observed values, which provides a model for the matrix dianion system. The spectrum of solid Na(2)N(2)O(2) exhibits a 1030 cm(-1) infrared band, which strongly supports the present N(2)O(2)2- dianion assignment. The electrostatic stabilization of N(2)O(2)2-, which is probably unstable in the gas phase, is made possible by metal cations separated by one or two insulating layers of argon in the rigid 7 K matrix.     

Infrared spectra of seeded hydrogen clusters: (para-H2)N-N2O and (ortho-H2)N-N2O, N = 2-13

High-resolution infrared spectra of clusters containing para-H2 and/or ortho-H2 and a single nitrous oxide molecule are studied in the 2225-cm(-1) region of the upsilon1 fundamental band of N2O. The clusters are formed in pulsed supersonic jet expansions from a cooled nozzle and probed using a tunable infrared diode laser spectrometer. The simple symmetric rotor-type spectra generally show no resolved K structure, with prominent Q-branch features for ortho-H2 but not para-H2 clusters. The observed vibrational shifts and rotational constants are reported. There is no obvious indication of superfluid effects for para-H2 clusters up to N=13. Sharp transitions due to even larger clusters are observed, but no definite assignments are possible. Mixed (para-H2)N-(ortho-H2)M-N2O cluster line positions can be well predicted by linear interpolation between the corresponding transitions of the pure clusters.     

Microhydration of cytosine and its radical anion: cytosine.(H2O)n (n=1-5)

Microhydration effects on cytosine and its radical anion have been investigated theoretically, by explicitly considering various structures of cytosine complexes with up to five water molecules. Each successive water molecule (through n=5) is bound by 7-10 kcal mol(-1) to the relevant cytosine complex. The hydration energies are uniformly higher for the analogous anion systems. While the predicted vertical detachment energy (VDE) of the isolated cytosine is only 0.48 eV, it is predicted to increase to 1.27 eV for the lowest-lying pentahydrate of cytosine. The adiabatic electron affinity (AEA) of cytosine was also found to increase from 0.03 to 0.61 eV for the pentahydrate, implying that the cytosine anion, while questionable in the gas phase, is bound in aqueous solution. Both the VDE and AEA values for cytosine are smaller than those of uracil and thymine for a given hydration number. These results are in qualitative agreement with available experimental results from photodetachment-photoelectron spectroscopy studies of Schiedt et al. [Chem. Phys. 239, 511 (1998)].     

Effects of microsolvation on uracil and its radical anion: uracil(H2O)n (n = 1-5)

Microsolvation effects on the stabilities of uracil and its anion have been investigated by explicitly considering the structures of complexes of uracil with up to five water molecules at the B3LYPDZP++ level of theory. For all five systems, the global minimum of the neutral cluster has a different equilibrium geometry from that of the radical anion. Both the vertical detachment energy (VDE) and adiabatic electron affinity (AEA) of uracil are predicted to increase gradually with the number of hydrating molecules, qualitatively consistent with experimental results from a photodetachment-photoelectron spectroscopy study [J. Schiedt et al., Chem. Phys. 239, 511 (1998)]. The trend in the AEAs implies that while the conventional valence radical anion of uracil is only marginally bound in the gas phase, it will form a stable anion in aqueous solution. The gas-phase AEA of uracil (0.24 eV) was higher than that of thymine by 0.04 eV and this gap was not significantly affected by microsolvation. The largest AEA is that predicted for uracil(H2O)5, namely, 0.96 eV. The VDEs range from 0.76 to 1.78 eV.     

Infrared spectra of seeded hydrogen clusters: (paraH2)N - OCS, (orthoH2)N - OCS, and (HD)N - OCS, N = 2 - 7

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS.     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.     

OH stretch IR spectra of (H2O)3 and benzene-(H2O)3

The OH stretch IR spectrum of (H₂O)₃ is significantly altered upon complexation of the cluster with a benzene molecule. Theoretical calculations show that the differences in the OH stretch spectra of (H₂O)₃ and benzene-(H₂O)₃ are primarily due to mode localization induced by the benzene molecule. Received: 27 January 1997 / Accepted: 28 January 1997