Vibration-vibration energy transfer in gaseous HBr

The vibration-vibration energy transfer of HBr gas initially excited to the first vibrational state has been observed. Collisional pumping to the V = 2 level is measured by monitoring the fluorescence of the 2−1 transition. The rate constant for the process: HBr(V=0) + HBr(V=2) → 2HBr(V=1) is found to be 1.4×10⁵ sec⁻¹ torr⁻¹.     

A time-resolved lif study of the kinetics of OH(ν = 0) and OH(ν = 1) with HCl and HBr

The kinetics of OH(ν = 0) and OH(ν = 1) have been followed using pulsed photolysis of H₂O or HNO₃ to generate hydroxyl radicals, and time-resolved, laser-induced fluorescence to observe the rates of their subsequent removal in the presence of HCl or HBr. The experiments yield the following rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K: OH(ν = 0) + HCl: k_o = (6.8 ± 0.25) × 10^?13; OH(ν = 0) + HBr: k_o = (11.2 ± 0.45) × 10^?12; OH(ν = 1) + HCl: k₁ = (9.7 ± 1.0) × 10^?13; OH(gn = 1) + HBr; k₁ = (8.1 ± 1.05) × 10^?12 For OH(ν = 1), the measurements do not distinguish between loss by reaction and relaxation, and the fact that k₁ > k_o for HCl is tentatively attributed to relaxation, probably by near-resonant vibrational—vibrational energy transfer. Clearly, neither of these exothermic, low-activation-energy reactions is enhanced to any great extent, if at all, by vibrational excitation of the OH radical.ft]*|Present address: Battelle/Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352, USA.     

Infrared chemiluminescence from the reactions F + HBr,F + H2Se,and Cl + H2Se

Infrared chemiluminescence from HF and HCl has been observed and yielded vibrational and rotational population distributions for the reactions F + HBr, F + H₂Se, and Cl + H₂Se. Evaluation of the spectra recorded by a commercial Fourier-transform spectrometer under low-flow conditions gave the following relative vibrational populations: for F ? HBr. N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} = 0.45 : 0.31 : 0.13 : 0.11: for F + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} : N_{υ = 4} : N_{υ = 5} = 0.29 : 0.35 : 0.24 : 0.09 : 0.03: for Cl + H₂Se, N_{υ = 1} : N_{υ = 2} : N_{υ = 3} = 0.40 : 0.51 : 0.09. All three vibrational surprisal plots show a significant deviation from linearity. Neglecting the contributions from N_{υ = 0}, the total energy is partitioned into vibration and rotation as follows: 〈f_V〉 = 0.49 and 〈f_R〉 = 0.09 for F + HBr, 〈f_V〉 = 0.41 and 〈f_R〉 = 0.07 for F + H₂Se, 〈f_V〉 = 0.53 and 〈f_R〉 = 0.10 for Cl + H₂Se. Inclusion of estimates for N_{υ = 0} gives the more realistic values 〈f_V〉 = 0.24, 0.34, and 0.49 respectively. Whereas 9 ± 3% of the collisions between F + HBr yield Br in the excited ²P_{$\text{1}\text{2}$} state, no rovibrationally excited HSe fragments were detected in the two other systems. Consistent values for the bond dissociation energy D₀⁰(HSeH) = 329 ± 5 kJ/mol and the enthalpy of formation ΔH₁₀⁰ (HSe) = 137 ± 5 kJ/mol are derived from the highest observed HCl and HF levels.     

Production of electronically excited atoms. H + Br2 → HBr + Hb(*), H + HBr → H2 + Br(*)

By measurement of infrared chemiluminescence we have obtained for the branching ratio of the room temperature reaction H + Br₂ (1), k*₁/k₁ = 0.015 ± 0.004 and for H + HBr (2), k*₂/k₂ ? 0.013. For H + Br₂ → HBr(υ^· ? 6) + Br (1), the detailed rate constant k(υ^* = 6) = 0.014 ± 0.003 relative to k(υ^· = 4) = 100.     

Penning ionization and photoionization electron spectrometry of hydrogen bromide

An electron spectrometric study has been performed on HBr using metastable helium and neon atoms as well as helium resonance photons. High resolution electron spectra were obtained for a mixed He(2¹ S, 2³ S) beam, a pure He(2³ S) beam, a mixed Ne(3s ³ P ₂,³ P ₀) beam, and for HeI VUV light. From the comparison of vibrational populations of HBr⁺ (X, v′) and HBr⁺ (A, v′), formed by either He^* and Ne^* Penning ionization (PI) or HeI photoionization, we conclude that HBr⁺ (X) formation by PI exhibits only little perturbation of HBr potentials, whereas HBr⁺ (A) formation by PI exhibits substantial bond stretching of HBr due to metastable atom attack preferably from the H end. For He(2¹ S) + HBr theX- andA-state vibrational peak shapes are substantially broader than for the He(2³ S) + HBr case pointing to an additional, charge exchanged interaction (He⁺ + HBr^?) in the entrance channel of the former system which is also responsible for a broad feature found at lower electron energies in the He(2¹ S, 2³ S) induced PI electron spectra. For the first time, we have detected the low energy electrons in both the He(2¹ S) + HBr and He(2³ S) + HBr spectra, associated with the major mechanism for the formation of Br⁺ ions: energy transfer to repulsive HBr** Rydberg states, dissociating to H(1s) and autoionizing Br** atoms. The HBr⁺ (X)² II _3/2:² II _1/2 fine structure branching ratios vary significantly with the ionizing agent in a similar way as for the isoelectronic, atomic target case krypton.     

Kinetics of the reaction of H(2S) with HBr

The rate constants for the reaction H + HBr → H₂ + Br were measured between 217 and 383 K using pulsed laser photolysis of HBr and cw resonance fluorescence detection of H(²S). The temporal profiles of the product Br atoms were also monitored to obtain the rate constant at 298 K. The yield of Br from the reaction was determined to be unity. The rate coefficient as a function of temperature is given by the Arrhenius expression, k ₁ = (2.96 ± 0.44) × 10^?11 exp(?(460 ± 40)/T) cm³ molecule^?1 s^?1. The quoted errors are at the 95% confidence level and include estimated systematic errors. Our results are compared with those from previous direct measurements. © John Wiley & Sons, Inc.     

Hydrogen abstraction by fluorine atoms: F + HX and F + DX (X = I,Br, Cl)

Absolute reaction rates for F + HX and F + DX (X = I, Br, Cl) have been obtained by monitoring the rise time of HF (DF) vibrational fluorescence following multiphoton dissociation of SF₆ in mixtures of HX (DX) and argon. The cross sections for reaction are, in units of 10^?16 cm², 4.37, 5.26, and 1.16 for HI, HBr, and HCl, respectively. The isotope effects k_HX/k_DX, are 1.29 ± 0.14, 1.29 ± 0.18, and 1.38 ± 0.29, respectively.     

Interpretation of the vibrational spectra of methylglyoxal and biacetyl in their first singlet excited electronic states

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm^?1) of methylglyoxal (CH₃COCHO) and its perdeutero analog (CD₃COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH₃(CD₃) rotation is changed during the transition. In the excited state the potential function is ternary (V₁ = 95 (1 + cos 3θ)cm^?1) as in the fundamental state (V₀ = 134.5 (1 - cos 3θ)cm^?1), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH₃COCH₃CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V₁ = 117.5 (1 + cos 3θ) cm^?1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.     

Hydrolytic precipitation of calcium polyvanadates from vanadium(IV) and vanadium(V) solutions

The effect of VO²⁺ ions on the composition and kinetics of calcium polyvanadate precipitation from solutions with 1.5 ≤ pH ≤ 9 at 80–90°C has been studied. For 1,5 ≤ pH < 3 and V⁴⁺/V⁵⁺ = 0.11–9, the precipitated compounds have the general formula Ca _x V _y ⁴⁺ V _12?y ⁵⁺ O_31?δ · nH₂O (0.8 ≤ x ≤ 1.06, 2 ≤ y ≤ 3, 0.94 ≤ δ ≤ 1.5). The maximum vanadium(IV) proportion (y = 3) in the precipitates is achieved when V⁴⁺/V⁵⁺ = 0.5?1.0 in the solution and pH is 3. Polyvanadate precipitation at pH 1.7 has a long induction period (up to 30 min), which is not observed for V⁴⁺/V⁵⁺ > 0.1. Precipitation in solutions with pH 3 occurs only when VO²⁺ ions are added, with a maximum rate near V⁴⁺/V⁵⁺ = 0.2 and in presence of chloride ions. The processes are controlled by a secondorder reaction on the polyvanadate surface.     

EPR kinetic study of the reactions of F and Br atoms with H2CO

The absolute rate constants of the reactions F + H₂CO → HF + HCO (1) and Br + H₂CO → HBr + HCO (2) have been measured using the discharge flow reactor-EPR method. Under pseudo-first-order conditions (¦H₂CO¦?¦F¦or¦Br¦), the following values were obtained at 298 K: k₁ = (6.6 ± 1.1) × 10^?11 and k₂ = (1.6± 0.3) × 10^?12, Units are cm³ molecule^?1s^?1. The stratospheric implication of these data is discussed and the value obtained for k makes reaction (2) a possible sink for Br atoms in the stratosphere.     

Structure of two new compounds of fluoroquinolone antibiotics with mineral acids

New compounds of sparfloxacin (C₁₉H₂₂F₂N₄O₃, SfH) and levofloxacin (C₁₈H₂₀FN₃O₄, LevoH) with mineral acids, namely, sparfloxacinium bromide (SfH · HBr, I) and levofloxacindium diperchlorate (LevoH · 2HClO₄, II), have been synthesized and characterized by X-ray diffraction. Crystallographic data are a = 7.7151(7) Å, b = 26.109(3) Å, с = 10.008(1) Å, β = 103.556(1)°, V = 1959.7(3) ų, space group P2₁/n, Z = 4 for I and a = 9.727(6) Å, b = 20.440(12) Å, с = 12.286(7) Å, β = 104.327(8)°, V = 2367(2)ų, space group P2₁, Z = 4 for II. The structures of these compounds are stabilized by intra- and intermolecular hydrogen bonds and π–π interaction between SfH₂⁺ or LevoH₃²⁺ ions.     

Spectroscopic detection of excited-state electron transfer in porphyrin viologen systems

Pulsed laser excitation (354.7 nm, 10 ns pulse) of a pyridyltritolylporphyrin chromophore covalently linked to a dibenzylviologen, Bz₂V²⁺, electron acceptor (porphyrin—viologen, PV²⁺) in CH₃CN leads to intramolecular electron transfer quenching of the porphyrin singlet excited state within the laser pulsewidth to reduce the linked Bz₂V²⁺ to Bz₂V^+·. Transient Bz₂V^+· can be detected directly by resonance Raman spectroscopy. The same transient features are obtained from pulsed laser excitation of a mixture of porphyrin (P) and dibenzylviologen in CH₃CN where Bz₂V²⁺ quenches the porphyrin fluorescence, establishing bimolecular excited state electron transfer quenching to yield Bz₂V^+·. Confirmation of our assignment of the transient Bz₂V^+· comes from comparison of the spectra with the resonance Raman spectrum of an authentic sample of Bz₂V^+·, and of electrochemically reduced PV²⁺ which has been spectroscopically confirmed to form PV^+·. Fluorescence lifetime determinations for PV²⁺ and P yield a rate constant for intramolecular electron transfer, k_et = 8 × 10⁷ s⁻¹, consistent with the ability to observe electron transfer within the laser pulsewidth     

Kinetics of CN(v=0) and CN(v=1) with H2 HCl and HBr

CN radicals have been generated in their X ²Σ⁺ (v=0) and (v= 1 ) levels by pulsed laser photolysis of NCNO at 532 nm, and time-resolved laser-induced fluorescence has been used to measure the rates of their removal by H₂, HC1 and HBr. The rate constants for removal of CN(v= 1 ) by these three species are 1.2 ± 0.3, 1.1 ± 0.2 and 1.3 ± 0.1 times the rate constants for reaction of CN(v=0). The results can be interpreted in terms of vibrationally adiabatic theory and a CN vibrational frequency which is almost the same in the transition state as in the isolated radical.     

Structural aspects of the metal-insulator transitions in V0.985Al0.015O2

The crystal structure of V_0.985Al_0.015O₂ has been refined from single-crystal X-ray data at four temperatures. At 373°K it has the tetragonal rutile structure. At 323°K, which is below the first metal-insulator transition, it has the monoclinic M₂ structure, where half of the vanadium atoms are paired with alternating short (2.540 Å) and long (3.261 Å) V-V separations. The other half of the vanadium atoms form equally spaced (2.935 Å) zigzag V chains. At 298°K, which is below the second electric and magnetic transition, V_0.985Al_0.015O₂ has the triclinic T structure where both vanadium chains contain V-V bonds, V(1)-V(1) = 2.547 Å and V(2)-V(2) = 2.819 Å. At 173°K the pairing of the V(1) chain remains constant: V(1)-V(1) = 2.545 Å, whereas that of the V(2) chain decreases: V(2)-V(2) = 2.747 Å. From the variation of the lattice parameters as a function of temperature it seems that these two short V-V distances will not become equal at lower temperatures. The effective charges as calculated from the bond strengths at 298 and 173°K show that a cation disproportionation has taken place between these two temperatures. About 20% of the V⁴⁺ cations of the V(1) chains have become V³⁺ and correspondingly 20% of the V⁴⁺ cations of the V(2) chains have become V⁵⁺. This disproportionation process would explain the difference between the two short V-V distances. Also it would explain why the T → M₁ transition does not take at lower temperatures.     

New defect vanadium dioxide phases

Two new monoclinic V₂O₄ phases were prepared at high pressure from the regular monoclinic (M₁) form of V₂O₄. The unit cell dimensions for the unmodified monoclinic (M₂) phase are: a = 9.083, b = 5.763, c = 4.532 Å, and β = 91.30°. The space group $\text{C 2}\text{m}$ is consistent with the crystallographic data. The new vanadium dioxide exhibited a structural transition and an abrupt, reversible change in resistivity (approx. 4 orders of magnitude) at 66°C similar to that observed in M₁-type V₂O₄. This new form of V₂O₄ is believed to be stabilized by chemical and structural defects. Controlled substitution of V⁵⁺ for V⁴⁺ in the structure led to yet another monoclinic (M₃) phase. This phase is closely related to the M₂ phase. The M₃ unit cell dimensions are: a = 4.506, b = 2.899, c = 4.617 Å, and β = 91.79°, having the space group $\text{P 2}\text{m}$. The substitution of V³⁺ yielded only monoclinic (M₁) derivatives. The modified products have varied semiconductor to metal transition temperatures which depend on the type and amount of substitution and defect structure.     

Phase diagram and some physical properties of V2O3+x1 (0 ? x ? 0.080)

The magnetic and electric properties of V₂O_3+x were investigated by measurements of magnetic susceptibility, electrical resistivity, magnetotorque, Mössbauer of doped ⁵⁷Fe, and NMR of ⁵¹V, and the results were compared with those of the (V_1?xTi_x)₂O₃ system or highly pressured V₂O₃. The results obtained are as follows: (1) The metallic state shows an antiferromagnetic ordering at T_N (x). The value of T_N for metallic V₂O₃, obtained by interpolation to x = 0, shows the coincidence between V₂O_3+x and the (V_1?xTi_x)₂O₃ system. (2) Magnetic susceptibility of V₂O_3+x is expressed as χ_M(V₂O_3+x) = (1?x)χ_M(V³⁺) + xχ_M(V⁴⁺). χ_M(V⁴⁺) obeys the Curie-Weiss law $\text{(χ}{}_{\mathrm{<mtext>M</mtext>}}\text{(}\text{V}{}^{\mathrm{4+}}\text{) =}\text{0.77}\text{T}\text{+ 17)}$. (3) In the insulating phase, the electrical resistivity ? is expressed as a common equation: $\text{? = 10}{}^{\mathrm{?1.8}}\text{exp}\text{(}\text{E}\text{kT}\text{)}$. This implies that the substitution of Ti or nonstoichiometry (V⁺⁴ + metal vacancies) has little influence on the carrier mobility (or bandwidth). (4) There is a critical length in the c-axis (? 14.01 Å) where the metal-insulator transition takes place. This suggests that the length of the c-axis plays an important role in the metal-insulator transition of V₂O₃-related compounds.     

Molecular structure and conformation of gaseous chloroacetyl chloride as determined by electron diffraction

Chloroacetyl chloride is studied by gas-phase electron diffraction at nozzle-tip tempera- tures of 18, 110 and 215°C. The molecules exist as a mixture of anti and gauche confor- mers with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 2σ is found to be 0.770 (0.070), 0.673 (0.086) and 0.572 (0.086) at 18, 110 and 215°C, respectively. These values correspond to an energy difference with estimated standard deviation ΔE^o = E^o_g -E^o_a = 1.3 ± 0.4 kcal mol^?1 and an entropy difference ΔS^o = S^o_g -S^o_a = 0.7 ± 1.1 cal mol^?1 K^?1. Certain of the diffraction results permit the evaluation of an approximate torsional potential function of the form 2V = V₁(1 - cos φ) + V₂(1 - cos 2φ) + V₃(1 - cos 3φ); the results are V₁ = 1.19 ± 0.33, V₂ = 0.56 ± 0.20 and V₃ = 0.94 ± 0.12, all in kcal mol^?1. The results for the distance (r_a), angle (_∠α) and r.m.s. amplitude parameters obtained at the three temperatures are entirely consistent. At 18°C the more important parameters are, with estimated uncertainties of 2σ, r(C-H) = 1.062(0.030) Å, r(CO) = 1.182(0.004) Å, r(C-C) = 1.521(0.009) Å. r(CO-Cl) = 1.772(0.016) Å, r(CH₂-Cl) = 1.782(0.018) Å, ∠C-C-0 = 126.9(0.9)°, ∠CH₂-CO-C1 = 110.0(0.7)°,∠CO-CH₂-C1 = 112.9(1–7)°, ∠H-C-H = 109.5° (assumed), ∠φ (gauche torsion angle relative to 0° for the anti form) = 116.4(7.7)°, δ (r.m.s. amplitude of torsional vibration in the anti conformer) == 17.5(4.2)°.     

The electronic structure of FeV-cofactor in vanadium-dependent nitrogenase

The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V³⁺, 3Fe³⁺, 4Fe²⁺]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined ⁵¹V (I = 7/2) hyperfine splitting, a_iso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V³⁺, 4Fe³⁺, 3Fe²⁺]. Our findings suggest that the V³⁺ does not change valency throughout the catalytic cycle.

Active site FeV-cofactor of the V-nitrogenase and the EPR spectrum of the reduced cofactor showing ⁵¹V-hyperfine coupling.     

Electronic State of Vanadium Ions in Li1+xV3O8 According to EPR Spectroscopy

EPR spectroscopy is used to study the electronic state of vanadium ions in HT- and LT-Li_1+xV₃O₈. It is shown that in both cases the EPR spectra observed are attributed to vanadyl VO²⁺ ions (localized electron centers) with weak exchange interaction. The other type of registered electrons is characterized by larger mobility through a few V⁵⁺ ions, i.e., by a higher degree of delocalization (electron gas). Based on the analysis of the temperature dependence of the EPR line width, it is stated that the exchange interaction between localized electron centers proceeds through electron gas (C-S-C relaxation). It is found that HT-Li_1+xV₃O₈ differs from LT-Li_1+xV₃O₈ by the sloping form of its spectrum at g_∥ range connected with two types of VO²⁺ ions different in the direction of the crystal field axis corresponding to a short V=O²⁺ bond.