Lifetime and collisional depopulation of the metastable 5D 3/2-state of Yb+

The lifetime and collisional depopulation rates of the metastable 5D _3/2 state of Yb⁺ have been determined in a radiofrequency ion trap by observation of the fluorescence count rate after ion excitation by a short laser pulse. From measurements using He, N₂ and H₂ as buffer gases between 10^?8 and 10^?6 mbar pressure and linear extrapolation to zero pressure we obtain a lifetime of τ=52.15±1.00 ms and rate constants ofR(H₂)=(1.02±0.10)×10^?9 cm³/s andR(N₂)=(1.78±0.19)×10^?10 cm³/s. The lifetime is in fair agreement with a calculated value of 74 ms.     

13C spin relaxation studies of the basic pancreatic trypsin inhibitor

The longitudinal ¹³C spin relaxation times T₁ and the ¹³C{¹H} nuclear Overhauser enhancement were measured in a concentrated aqueous solution of the basic pancreatic trypsin inhibitor. The correlation time for overall rotational motions of the basic pancreatic trypsin inhibitor molecules was found to be τ_R ≈ 2 × 10^?8 s. In connection with previous ¹H n.m.r. studies of intramolecular motions of the aromatics, we were particularly interested in the correlation times τ_G for intramolecular segmental motions of the aromatic rings. The present experiments revealed no manifestation of intramolecular motions of the aromatics, indicating that τ_G ? 2 × 10^?8 s for the aromatic ring carbon atoms. On the other hand, rapid segmental motions were evidenced for the peripheral carbon atoms of aliphatic amino acid sidechains. Comparison of the ¹H and ¹³C n.m.r. data on the basic pancreatic trypsin inhibitor indicates that the time scale of high resolution ¹H n.m.r. at high fields may in many instances be more appropriate for studies of the molecular dynamics in globular proteins than the time scale of spin relaxation measurements.     

The structure and dynamics of the copper(II)—l-proline 1:2 complex in solution

The ¹³C relaxation times (T₁ and T₂) and isotropic contact shifts (Δω) of a one molar aqueous solution of l-proline at pH = 11 (or pD = 11.4) containing ca 10^?4 M copper(II) perchlorate are measured at 62.86 MHz over a temperature range of 26–70°C. The purely dipolar longitudinal relaxation of carbon-13 nuclei contrasting with purely scalar transverse relaxation allowed us to extract carbon-to-metal distances (through T₁ measurements) and hyperfine coupling constants and dynamic parameters (from T₂ and Δω measurements). The structure of the complex in solution is found closely similar to that in the solid state. Curve-fitting procedures allowed us to derive the hyperfine electron—carbon coupling constants A_c = ?1.95, + 0.42, + 1.90 and ?1.70 MHz for carbons α, β, γ, δ, of the pyrrolidinic ring, the reorientation correlation time of the complex, τ_R (25°C) = 1.15 × 10^?10 sec, the l-proline exchange rate, k_M (25°C) = 4.0 × 10⁵ sec^?1 (and the corresponding activation parameters ΔH^≠ = 9.0 kcal mol^?1 and ΔS^≠ = ?0.7 e.u.), and the electronic relaxation time, T_1e = 1.13 × 10^?8 sec (at 25°C). The latter value was found in agreement with the one computed from ESR data and the above τ_R value, showing the predominant contributions of spin—rotation interaction and, to a lesser extent, of the effect of g-tensor anisotropy to the electronic relaxation rate.     

Conductivity of crosslinked poly(N-vinylpyrrolidone) gel film containing surfactant as electrolyte salt

New polymeric solid electrolyte films, consisting of crosslinked poly(N-vinylpyrrolidone) (PVPD) as matrix, and surfactant, sodium deoxycholate (NaDC), lithium deoxycholate (LiDC), sodium laulylsulfate (R₁₂OSO₃Na), or sodium palmitate (R₁₅COONa) as electrolyte salt, are prepared; their basic structure and conductivity dependence on temperature are reported. The structure of the electrolytes is amorphous. Their conductivity is 3.1 × 10^?5 S cm^?1 (containing NaDC), 8.42 × 10^?6 S cm^?1 (LiDC), 2.18 × 10^?4 S cm^?1 (R₁₂OSO₃Na), and 7.27 × 10^?5 S cm^?1 (R₁₅COONa) at 20°C. Their temperature dependence of the conductivity is similar to that of liquid electrolyte rather than that of usual polymeric solid electrolyte, i.e., the WLF-type dependence. The values of activation energy of conductivity (E_a) were PVPD, 25.5 kJ mol^?1; PVPD/NaDC, 21.4 kJ mol^?1; PVPD/LiDC, 25.3 kJ mol^?1; PVPD/R₁₂OSO₃Na, 17.2 kJ mol^?1; PVPD/R₁₅COONa, 18.7 kJ mol^?1. © 1993 John Wiley & Sons, Inc.     

Formation of free radicals in water under high-power laser uv irradiation

It has been shown that under high-power laser UV irradiation (I = 10⁸–10¹⁰ W/cm², λ = 266 nm, τ_p = 30 ps) water becomes ionized by a two-photon mechanism to form some free radicals including the hydrated electron e^?_aq.     

The dynamics of the NH+4 ion in ammonium chloride in disordered states by cold neutrons scattering experiments

The duration of the libration of the NH⁺₄ ion, τ, at 473 K (phase I) is found to be of the same order or magnitude as the reorientational jump time, ≈0.2 × 10^?12 s. At 453 K (phase II) τ has increased to 3 × 10^?12 s and the reorientations around the two-fold symmetry axes dominate.     

Multiple state emission from rhodium(I) and iridium(I) complexes

The effect of temperature (2–100 K) on the emission spectra and lifetimes of [M(2 = phos)₂]ClO₄ (M = Rh(I), Ir(I): 2 = phos is cis-1,2-bis-(diphenylphosphino)ethylene) is interpreted with a two-level spin-orbit-split emitting manifold. For [Ir(2 = phos)₂]ClO₄, Δ? = 143cm^?1, τ(lower) = 999μs, and τ(higher) = 1.54 μs. For the rhodium species, Δ? = 35 cm^?1, τ(lower) = 5920 μs, and τ(higher) = 20.3 μs.     

Influence of chemical exchange on the temperature dependence of Eu(fod)3-induced shifts

Temperature dependences of the paramagnetic shifts induced by Eu(fod)₃ in ¹H NMR spectra of ethylene oxide in carbon disulphide solution are obtained in the temperature range from +40 to ? 100°C at 100 MHz and from +30 to ?60°C at 60 MHz. The influence of chemical exchange leads to a decrease of the observed paramagnetic shifts with decreasing temperature. It is shown that a modified Swift and Connick equation can be used to describe the observed dependences. Upper limits of the mean lifetimes of the Eu(fod)₃-ethylene oxide adduct are τ_p < 1·7 × 10^?8 s at 14 °C and τ_p < 1 × 10^?8 s at 20 °C, respectively. The corresponding activation energy is equal to V_a = 13·7 kcal/mol.     

Experiments on solvated electron generation and deposition in hexamethylphosphoramide

Theory is developed and compared to experiment for the cyclic voltammetric ejection and redeposition of solvated electrons e_s^? in NaClO₄ and LiCl solutions in HMPA. In current reversal chronopotentiometry, e_s^? mass transfer can be observed free from uncompensated resistance effects by cathodic generation of e_s^? for time t_f followed by anodic current redeposition leading to a reverse transition time τ_b. The ratio τ_b/t_f depends on t_f, applied current, and LiCl electrolyte concentration. Comparison of τ_b/t_f to theory detects a highly reactive scavenger at micromolar concentration levels.     

Lifetime studies of the HgNH3 complex

The luminescence induced by the Hg-photosensitized reaction of NH₃ was studied by repetitive fast pulsed excitation. From observations of the decay of the luminescence, the emitter of the 290 nm fluorescence was found to be the precursor of the 340 nm emission (the stabilized complex). The first-order decay rate of the stabilized complex was found to be given by 1/τ_O + k[NH₃], where τ₀ = 2.3 μs and k = 3.5 × 10^?13 cm³ molecule^?1 s^?1.     

Intra- and inter-chain fluctuations in entangled polymer melts in bulk and confined to pore channels

It is known that topological restraints by “chain entanglements” severely affect chain dynamics in polymer melts. In this field-cycling NMR relaxometry and fringe-field NMR diffusometry study, melts of linear polymers in bulk and confined to pores in a solid matrix are compared. The diameter of the pore channels was 10 nm. It is shown that the dynamics of chains in bulk dramatically deviate from those observed under pore constraints. In the latter case, one of the most indicative signatures of the reptation model is verified 28 years after its prediction by de Gennes: The frequency and molecular mass dependencies of the spin-lattice relaxation time obey the power law T_! ∝ M⁰ v^3/4 on a time scale shorter than the longest Rouse relaxation time τ_R. The mean squared segment displacement in the pores was also found to be compatible to the reptation law < r²>∝ M^−1/2t^1/2 predicted for τ_R < t < τ_d, where τ_d is the so-called disengagement time. Contrary to these findings, bulk melts of entangled polymers show frequency and molecular mass dependencies significantly different from what one expects on the basis of the reptation model. The data can however be described with the aid of the renormalized Rouse theory.     

Investigation of relaxation processes in linear polyethylene in the 10−9 sec region by means of high-resolution neutron spectroscopy

Results are presented of neutron incoherent scattering experiments on isotropic linear polyethylene samples of high (80%) and low (48%) crystallinity in the temperature range between ?180°C and +85°C for values of the scattering vector between 0.29 Å^?1 and 1.81 Å^?1 obtained with a high resolution backscattering spectrometer (Δ?ω = 0.25 ? 1.0 μeV) and between 0.57 Å^?1 and 2.4 Å^?1 with a time-of-flight spectrometer (Δ?ω = 420 μeV). From a comparison of the results on these samples one concludes that relaxation takes place predominantly in the noncrystalline regions. This motion cannot be adequately accounted for by any of the existing models for the γ-process. Therefore, a more liquidlike motion is suggested. Diffusion of shorter chain segments has also been ruled out since it is too slow to be observed. A simplified model of protonic jumps between equidistant sites located on the periphery of a circle of radius 2.5 Å reproduces the experimental results well. For the average time between successive CH₂-group reorientations one obtains τ₁ = τ₀ exp(E_actRT) with τ₀ = (2.0 ± 1.5) × 10^?13 sec and E_act = (4.5 ± 1.0) kcal/mole. The values join up well with those for the γ-process observed by NMR. It has been concluded that 60–90% of the protons in the noncrystalline regions participate in this motion.     

Magnetic Resonance of Paramagnetic Complexes

The line width of the ESR and NMR signals of paramagnetic transition metal complexes is determined mainly by the electron spin-lattice relaxation time τ_e. Values of τ_e greater than 10^?9 lead to ESR spectra that are readily resolved, while values smaller than 10^?11 give NMR spectra having small line widths. Since fast relaxation processes are effective in nearly all transition metal complexes with several unpaired electrons and in all complexes having an orbitally degenerate ground state, the NMR method has a wider scope. The sign and magnitude of the electron-nucleus coupling can be determined with great sensitivity from the NMR spectra, whereas only the magnitude of this interaction can be determined from the ESR spectra. Free spin densities can be found very accurately from the NMR shifts, and the method can therefore be advantageously applied to kinetic measurements, e.g. on short-lived contact complexes.     

Weak collision effects: Non-steady-state thermal unimolecular reactions

Bench-mark calculations for non-steady-state thermal systems are presented. Although the equations and calculations were modeled for the isomerization of cyclopropane in the single-pulse shock tube, the results are qualitatively and semiquantitatively correct for any experimental technique in which a finite time is required to energize the reactant to the reaction temperature. A useful parameter in describing this nonequilibrium behavior is the induction time τ₉₅; the log of τ₉₅ is linearly related to the collision frequency ω. For the model cyclopropane system a steady state is reached in ～1 msec when ω ≈ 10⁸ sec^?1 and T ≈ 1500 K.     

Réactions chimiques oscillantes. II. Comportement de la «période d'induction» dans les systémes BrO/Ce4+/Cyclohexanone et BrO/Ce4+/Cyclopentanone

Oscillating Chemical Reactions. 11. Behaviour of the “induction period” in the BrO /Ce⁴⁺/Cyclohexanon and BrO /Ce⁴⁺/Cyclopentanon systems

• (1) The addition of α-monobromoketone, one of the products of reaction of the BrO₃^?/Ce⁴⁺/Cyclohexanon (S1) and BrO₃^?/Ce⁴⁺/Cyclopentanon (S2) oscillating systems, decreases and even suppresses the induction period (τ_ind.) in the case of S2. Such is not the case with S1: τ_ind. increases and the oscillations can even be completely inhibited.
• (2) The order of addition of the reagents and the time lapse (t_adj.) preceding the addition of the last of them influences τ_ind., particularly when the last reagent added is Ce⁴⁺.
• (3) In our experimental conditions, the inhibition of the oscillatory phenomenon by Cl^? ions is definite only for | Cl^? | ≥ 5,0 · 10^?2M (S1) and |Cl^?| > 2,5 · 10^?3M (S2); for lower concentrations τ_ind. increases with | Cl^?|.

     

Two-step picosecond UV excitation of polynucleotides and energy transfer

We studied the dependence of the efficiency of two-step photoreactions in several polynucleotides: poly-U, C_spd-phage DNA and TMV RNA, on the chain length and spatial structure of the excited biopolymer under high-intensity picosecond UV irradiation (λ = 266nm, I = 10¹¹ ?10¹⁴ W/m², τ_p = 23 ps). It has been found that the effective energy transfer distance Λ in single-stranded polynucleotides is ≈ 1 or 2 nucleotides. In double-stranded nucleic acids Λ < 170. The lifetime τ₁ of the excited electronic state S₁ and the absorption cross section σ₂ from the level S₁ have been estimated for nucleotide pU (τ > 6.6 ps, σ₂ <1.7 × 10^?16 cm²).     

Aqueous polymerization of acrylonitrile by ascorbic acid–peroxodisulfate redox system

The polymerization of acrylonitrile initiated by an ascorbic acid–peroxodisulfate redox system was studied in an aqueous solution at 35°C in the presence of air. Molecular oxygen was found to have no effect on the polymerization reaction. An increase in ionic strength slightly increased the rate. The overall rate of polymerization, R_p, showed a square dependence on [monomer] and a half-order dependence on [peroxodisulfate]. A first-order dependence on [ascorbic acid] at low concentrations (<3.0 × 10^?3 mol L^?1) followed by a decrease in R_p at higher concentrations of ascorbic acid (>3.0 × 10^?3 mol L^?1) was also noted. R_p remained unchanged up to 40°C and showed a decline thereafter. Addition of catalytic amounts of cupric ions decreased the rate whereas ferric ions were found to increase the rate. Added sulfuric acid in the range (6.0?50.0) × 10^?5 mol L^?1 decreased the R_p.     

Excited state decay of tetrahalomanganese(ii) complexes

The excitation and emission spectra and decay times of several MnX^2-₄ (X = Cl^?, Br^?, 1^?) complexes of various tetraalkylammonium, -phosphonium, and -arsonium salts have been measured for the pure solids at 298°K and 77°K. High luminescence quantum yields (0.3-1.0) reveal that lifetimes fairly accurately reflect radiative decay rates. An impressive correlation exists between the lifetime, τ, of the ⁴T₁ (G) →⁶ A₁ emission and the ligand, X: for X = Cl^?, τ = 1.2 - 3,5 x 10^?3 sec; X = Br^?, τ = 0.35 - 0.43 X 10^?3 sec; X =l^?, τ = 0.036 – 0.055 X 10^?3 sec. We attribute this decreasing lifetime largely to the enhanced spin-orbital coupling associated with the heavier halide. We find that direct population of high energy charge-transfer (CT) states gives smaller emission yields than excitations in the ligand-field (LF) region.     

On the kinetics of polymer degradation in solution—VII. Laser flash photolysis of poly-α-methylstyrene in solution

Poly-α-methylstyrene (PαMS) was degraded in CHCl₃ and CCl₄ solution by flash photolysis (λ = 265 nm). The degradation, as detected by light scattering measurements, is caused by the attack on PαMS by solvent radicals, assumed to be formed mainly by energy transfer processes. The direct effect did not lead to detectable main chain cleavage as evidenced by experiments with PαMS dissolved in dioxane or methylene chloride. The time dependence of the decrease of the light scattering intensity (LSI) after the flash was measured. The observed first order lifetime τ(LSI) corresponds to the lifetime of lateral macroradicals P′ that decompose by main chain cleavage (k = 3.5 × 10² sec^?1). Molecular oxygen reacts with the lateral macroradicals with k = (5.5 ± 0.5) 10⁵ M^?1 sec^?1. Only a minor portion of the product of this reaction (PO₂′) decays by main chain scission. Thus O₂ inhibits main chain scission significantly. By addition of cyclohexane and ethyl mercaptan, the main chain cleavage is inhibited. τ(LSI) was not affected by these compounds in the concentration range investigated ([C₆H₁₂]: up to 8.4 M; [C₂H₅SH]: up to 3 × 10^?3 M), indicating that they reacted with solvent radicals which otherwise attack the polymer forming lateral macroradicals.     

Picosecond Fluorescence of Merocyanines Derived from Photochromic Spironaphthoxazines

The emission of the merocyanines derived from the photochromic 1, 3, 3-trimethylspiro[indoline-2,3′[3H]naphth[2,1-b][1, 4]oxazine] 1, the 4, 5-dimethyl-substituted compound 2 and the 6′-piperidino-substituted derivative 3, in solution with acetonitrile or ethanol, was studied by means of a time-correlated single photon counting system with ca 10 ps time resolution. The signal decays were described by a sum of two or three exponential components with τ₁=10–60 ps, τ₂= 0.5–1ns, τ₃=1–2 ns, corresponding to the contribution of different fluorescing merocyanine isomers. The most intense emission, the shortest lived one, is characterized by extremely low quantum yields (<10^?6). The effect of the addition of LiClO₄ to acetonitrile solutions of 2 is in agreement with the formation of ground-state complexes with particular open isomers.