The photophysics of trans-stilbene

The fluorescence lifetime of trans-stilbene in dilute methylcyclohexane/iso-hexane solution has been measured and the mean S₁ radiative (k_F), radiationless (k_I) and cis-isomerization (k_C) rate parameters have been determined from ?90 to 60°C. S_i consists of a fluorescent trans (¹B_u^*) state (k_F⁰ = 6.0 × 10⁸ s^?1) which undergoes reversible thermal-activated rotational internal conversion (ΔH = 1.75 kcal mole^?1, ΔS = 10.6 cal deg^?1 mole^?1) to a non-fluorescent perp (¹A_g^*) state. p(¹A_g^*) lies 610 cm^?1 above t (¹B_u^*) with an intermediate S₁ potential maximum. p(¹A_g^*) undergoes internal conversion(k_I.^∞ = 5.8 × 10⁸ s^?1) to p (¹A_g) leading to cis-isomerization. This is the main isomerization channel over the whole temperature range.     

Fourier spectrometry investigation of the 1 3Σg+ →a 3Σu+ transition of the 7Li2 molecule

The 1 ³Σ_g⁺ → a ³Σ_u⁺ transition in the ⁷Li₂ molecule has been observed in the 8200–10 000 cm⁻¹ region with a high resolution Fourier spectrometer. Rotational analysis of 1 ⩽ υ′ ⩽ 7 of 1 ³Σ_g⁺ and 0 ⩽ υ″ ⩽ 7 of a ³Σ_u⁺ has been carried out. We found D_e(a ³Σ_u⁺) = 332.5 ± 1.0 cm⁻¹ that gives T_e(a ³Σ_u⁺) = 8184.3 ± 1.5 cm⁻¹ and D_e(1 ³Σ_g⁺) = 7090.4 ± 1.5 cm⁻¹ with T_e = 16330 ± 2 cm⁻¹.     

Excess molar quantities of (a halogenated n-alkane + an n-alkane) A comparative study of mixtures containing either 1-chlorobutane or 1,4-dichlorobutane

Excess molar volumes V_m^E at 298.15 K were obtained, as a function of mole fraction x, for series I: {x1-C₄H₉Cl + (1 ? x)n-C_lH_{2l + 2}}, and II: {x1,4-C₄H₈Cl₂ + (1 ? x)n-C_lH_{2l + 2}}, for l = 7, 10, and 14. 10, and 14. The instrument used was a vibrating-tube densimeter. For the same mixtures at the same temperature, a Picker flow calorimeter was used to measure excess molar heat capacities C_{p, m}^E at constant pressure. V_m^E is positive for all mixtures in series I: at x = 0.5, V_m^E/(cm³ · mol^?1) is 0.277 for l = 7, 0.388 for l = 10, and 0.411 for l = 14. For series II, V_m^E of {x1,4-C₄H₈Cl₂ + (1 ? x)n-C₇H₁₆} is small and S-shaped, the maximum being situated at x_max = 0.178 with V_m^E(x_max)/(cm³ · mvl^?1) = 0.095, and the minimum is at x_min = 0.772 with V_m^E(x_min)/(cm³ · mol^?1) = ?0.087. The excess volumes of the other mixtures are all positive and fairly large: at x = 0.5, V_m^E/(cm³ · mol^?1) is 0.458 for l = 10, and 0.771 for l = 14. The C_{p, m}^Es of series I are all negative and |C_{p, m}^E| increases with increasing l: at x = 0.5, C_{p, m}^E/(J · K^?1 · mol^?1) is ?0.56 for l = 7, ?1.39 for l = 10, and ?3.12 for l = 14. Two minima are observed for C_{p, m}^E of {x1,4-C₄H₈Cl₂ + (1 ? x)n-C₇H₁₆}. The more prominent minimum is situated at x′_min = 0.184 with C_{p, m}^E(x′_min)/(J · K^?1 · mol^?1) = ?0.62, and the less prominent at x″_min = 0.703 with C_{p, m}^E(x″_min)/(J · K^?1 · mol^?1) = ?0.29. Each of the remaining two mixtures (l = 10 and 14) has a pronounced minimum at low mole fraction (x_min = 0.222 and 0.312, respectively) and a broad shoulder around x = 0.7.     

Analysis of bound-free fluorescence and improved characterization of the electronic and spectroscopic properties of the 11Σ u + state of Cl2

Synchrotron radiation is used to selectively excite the chlorine molecule in the VUV spectral range. Stationary fluorescence spectra of the 1¹Σ _u ⁺ state are observed following primary excitation of 1¹Σ _u ⁺ and 2¹Σ _u ⁺ . The bound-free part of the spectra is analysed with the aid of quantum mechanical computer simulations. A potential energy curve is constructed which is an approximation of the adiabatic double well potential energy curve of the 1¹Σ _u ⁺ state. The inner well is characterized byT _e =(73428±50) cm^?1,r _e =(1.85 ± 0.05) Å; for the outer well holdT _e =(64631±50) cm^?1,r _e =(2.57±0.05) Å, ω _e =(261±5) cm^?1, ω _e x _e =(0.668±0.01) cm^?1 (³⁵Cl₂;v′<30). The potential energy curve is successfully checked with fluorescence excitation spectra. Within the error limits, the results of a former synchrotron radiation study are verified. It is ruled out, that the Cl₂ “γ-state” recently observed with laser spectroscopic methods, can be attributed to the outer well of 1¹Σ _u ⁺ .     

Kinetics and mechanism of the oxidation of acrylic acid and methyl methacrylate

The kinetics of the initiated oxidation of acrylic acid and methyl methacrylate in the liquid phase were studied volumetrically by measuring oxygen uptake during the reaction. Both processes proceed via the chain mechanism with quadratic-law chain termination. The oxidation rate is described by the equation w = k ₂/(2k ₆)^1/2[monomer]w _i ^1/2 , where w _i is the initiation rate and k ₂ and k ₆ are the rate constants of chain propagation and termination. The parameter k ₂/(2k ₆)^1/2 is 7.58 × 10^?4 (l mol^?1 s^?1)^1/2 for acrylic acid oxidation and 2.09 × 10^?3 (l mol^?1 s^?1)^1/2 for the oxidation of methyl methacrylate (T = 333 K). For the oxidation of acrylic acid, k ₂ = 2.84 l mol^?1 s^?1 (T = 333 K) and the activation energy is E ₂ = 54.5 kJ/mol; for methyl methacrylate oxidation, k ₂ = 2.96 l mol^?1 s^?1 (T = 333 K) and E ₂ = 54.4 kJ/mol. The enthalpies of the reactions of RO ₂ ^? with acrylic acid and methyl methacrylate were calculated, and their activation energies were determined by the intersecting parabolas method. The contribution from the polar interaction to the activation energy was determined by comparing experimental and calculated E ₂ values: ΔE _μ = 5.7 kJ/mol for the reaction of RO ₂ ^? with acrylic acid and ΔE _μ = 0.9 kJ/mol for the reaction of RO ₂ ^? with methyl methacrylate. Experiments on the spontaneous oxidation of acrylic acid provided an estimate of the rate of chain initiation via the reaction of oxygen with the monomer: w _i,0 = (3.51 ± 0.85) × 10^?11 mol l^?1 s^?1 (T = 333 K).     

Application of the modified van der waals equation for unsaturated vapour and liquid states

The Law-Lielmezs (L-L) modification of the Van der Waals equation of state: P = RT/(V-b)-a(T)/V² where: a(T) = a(T_c)·a(T_c·a(T¹) and: a(T¹) = 1 + pT^1q has been extended to include unsaturated states in terms of a correcting function C_f(such that the α(T¹) term becomes: a(T¹) = 1 + _pC_fT^1q The proposed extension has been compared with the results obtained by the use of the original Van der Waals equation of state.     

The polarized absorption spectrum of 5-methyl indole in polyethylene and polyvinylalcohol films

The stretched film technique has been used to resolve the two overlapping ¹L_a and ¹L_b transitions of indole. 5-Methyl indole was studied as a model compound in two polymer matrices, polyethylene (PE) and polyvinylalcohol (PVA). The directions of the transition moments with respect to the long molecular axis are found to be in PE − 10° and +22° for ¹L_a and ¹L_b, respectively. These angles are found to be −25° and +29° in PVA. ¹L_a has two prominent vibrations in PE at 2040 and 2895 cm⁻¹ from the origin. ¹L_b has four prominent vibrations in PE at 761, 1308, 1937 and 3196 cm⁻¹ from the origin. In PVA, ¹L_a has two prominent vibrations at 2149 and 2874 cm⁻¹ from the origin and ¹L_b has four prominent vibrations at 1049, 1352, 2229 and 3196 cm⁻¹ from the origin.     

Sub-doppler two-photon spectrum of asymmetric rotor molecules: analysis of the qqQ rotational branch of the S1(1B2u)141 ← S0(1A1g) system of benzene-d1

The Doppler-free two-photon excitation spectrum of the ^qqQ branch of the 14¹₀ vibrational band of the S₁(¹B_2u) ← S₀(₁A_1g) transition of benzene-d₁ has been recorded using a cw single-mode dye laser coupled to an external concentric resonator. The spectrum has been analysed using a non-rigid Watson Hamiltonian. More than 200 lines with J up to 20 have been assigned and the rotational constants which best reproduce the spectrum are A¹_v = 0.181435, B¹_v = 0.169990, C¹_v = 0.089055 cm⁻¹. The K_a = odd lines of the ^qqQ₅(J) subbranch show small and quite regular perturbations of 60 ± 5 MHz which are probably due to a coupling to another vibrational state of the S₁ manifold.     

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VI. Vibrational density and statistics of matrix elements versus energy

By laser excitation of the rotationless level (J = 0) of ten vibrational levels of the S₁ (A_u) state (0⁰, 7², 5¹, 8¹, 6¹7¹, 4¹, 8¹7², 2¹, 8¹4¹ and 2¹7²) of supersonic jet cooled glyoxal, we have obtained S₁-T₁ anticrossing spectra using the homogeneous, high magnetic field (0–8 T) of a Bitter coil. As explained previously, V_st is readily obtained from the width of an anticrossing. As triplet vibrational energy increases from 2776 (0⁰ of S₁) to 4636 cm⁻¹ (2¹7² of S₁), the number of anticrossings increases from 38 (0⁰) to 871 (2¹7²). The anticrossing density is related to the vibrational density of T₁. The V_st histrograms obtained for each vibrational level are very similar: p(V_st) ∝ V^−1−α_st with 0.4 ⩽ α ⩽ 0.7. The more significant and surprising result is that <V_st > is independent of vibrational energy, even though the corresponding vibrational overlaps predicted a decrease in <V_st >, of at least two orders of magnitude between 0⁰ and 2¹7². From V_st statistics we determine ϱ<V_st > and ϱ<V²_st > which are the dominant factors for ISC (intersystem crossing). We predict that strong S-T mixing should occur above 6900 ± 500 cm⁻¹.     

Observation of the electric-field-inducedE1 transition between 5s4d 1 D 2 and 5s 2 1 S 0 levels in Sr I

The angular distribution of the electric-field-inducedE1 radiation is asymmetric with respect to the external electric field direction. Measurement of this asymmetry for the 5s4d ¹ D ₂ → 5s ^{2 1} S ₀ transition in strontium permits the experimental determination of the 5s5p ¹ P ₁ → 5s4d¹ D ₂ decay rate. The result is:A(¹ P ₁ →¹ D ₂)=(4.37±0.93±0.66) × 10³ s ^?1.     

Thermally induced incomplete high-spin (5T2) ⇌ low-spin (1A1) transition in solid bis[2-(2-pyridylamino)-4-(2-pyridyl) thiazolato]iron (II)

The ⁵⁷Fe Mössbauer effect in two samples (A and B) of [Fe(papt)₂] and in its solvates with CHCl₃ and C₆H₆ has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (⁵T₂) ? low-spin (¹A₁) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔE_Q = 2.03 mm s^?1 and δIS = +0.87 mm s^?1, characteristic of a ⁵T₂ ground state. At 257 K, a second doublet, typical for a ¹A₁ ground state, is observed and its intensity increases as the transition progresses but levels off below ～ 100 K. At 4.2 K, 83% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.56 mm s^?1 and δ^IS(¹A₁ = +0.32 mm s^?1. In an applied magnetic field, V_zz(¹A₁) < 0 and η ≈ 0.7 have been determined, whereas for the ^sT₂ ground state, V_zz(^sT₂) > 0, η ≈ 0.75, and an internal hyperfine field H_n ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5_T2 and f1_A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the ⁵T₂ and constant υ_cff for the ¹A₁ ground state. The temperature dependence of ?In f1_A1 closely follows the Debye model with Θ1_A1 = 165 K, whereas the same applies to ?ln f5_T2 only above ～ 210 K and Θ5_T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states.     

The production of H(2S) atoms in the energy-transfer reaction of O2(1Δg) with HO2(X̃2A″)

The reaction of O₂(¹Δg) with HO₂(X?) was studied in an isothermal flow reactor in the pressure range 7?p? 10.7 mbar at temperatures between 299?T? 423 K. H-atom production was observed in the reaction O₂(¹Δg) + HO₂(òA′) - H(²S)+ 2O₂ (³Σ_g^?). The rate of this reaction (k₁) is estimated to be k₁ = (1 ± 0.5) × 10¹⁴ CM³ Mol^?1 s^?1. The implications of this reaction to recent determinations of the rate of the reaction H + O₂(¹Δg) are discussed.     

Multi-reference configuration interaction calculations on the systems Xe 2 + and Xe 3 +

Potential curves for the ground (²Σ _u ⁺ ) and the three lowest excited states of the Xe ₂ ⁺ dimer ion (²Π _g ,²Π _u ,²Σ _g ⁺ ) have been calculated using pseudopotentials in MRD-CI (multi-reference single anddouble excitationconfigurationinteraction) calculations. Spin-orbit interaction — leading to the six states 1.(1/2) _u , 1.(3/2) _g , 1.(3/2) _u , 1.(1/2) _g , 2.(1/2) _u , 2.(1/2) _g — has been taken into account using a semiempirical technique [1]. Subsequently, starting with a relaxed Xe ₂ ⁺ ion in its ground state, the potential energy surface for the system Xe-Xe ₂ ⁺ was studied. We found that the collinear approach of the Xe atom leads to the most stable geometry. This is a linear symmetric molecule with bond lengths of 6.38 bohr. In the bestT-shaped structure, the Xe atom is 7.83 bohr away from the midpoint of the Xe ₂ ⁺ (r=6.1 bohr) dimer. The calculated binding energy of 0.25 eV for the equilibrium structure of the Xe ₃ ⁺ molecule (i.e. the linear symmetric geometry), is in very good agreement with experimental results of 0.27 ± 0.02 eV [2].     

Pulsed amperometric detection of sulfur compounds: Part II. Dependence of response on adsorption time

A mathematical model is developed for the response of PAD and is applied to data from the study of I_p as a function of t_ads for evaluation of the adsorption rate constants, and the maximum molar surface coverage for thiourea at a Pt electrode. The results are, respectively: k₁ = 4.1 × 10⁴ M⁻¹ s⁻¹, k₋₁ = 1.9 s⁻¹, and Γ₀ = 1.3 × 10⁻¹⁰ mol cm⁻². The calculated adsorption equilibrium constant (k₁/k₋₁) is 2.1 × 10⁴ M⁻¹, compared to 4.9 × 10⁴ M⁻¹ calculated from the plot of 1/I_p vs. 1/c^b for c^b > 1.0 × 10⁻⁴ M and t_ads = 8500 ms. Analytical calibration procedures are examined; linear plots of 1/I_p vs 1/c^b cannot be expected for cases of mixed transport-isotherm control of detector response.     

Study of singlet-triplet transitions in the ozone molecule using the multiconfigurational self-consistent field theory

The intensities and dipole moments of the lower singlet-triplet transitions ³ A ₂ ← X ¹ A ₁ and ³ B ₁ ← X ¹ A ₁ in the ozone molecule were calculated by the multiconfiguration self-consistent field theory with the quadratic response function. The results of calculations of the intensities of singlet-triplet transitions using different basis sets and complete active spaces were compared. The assignment of the ³ A ₂ ← X ¹ A ₁ transition in the ozone spectrum to the Wulf band is discussed.     

The tris(picolinate)vanadate(II) ion: Redox properties and electron transfer kinetics with cobalt(III) amines

Vanadium(II) ions form with the pyridine-2-carboxylate ligand a deep blue, tris-substituted complex absorbing at 660 nm (ε = 7.2 × 10³ M^?1) cm^?1) with a shoulder at 450 nm. Reversible spectroelectrochemistry and cyclic voltammetry were observed for this complex, with E_{$\text{1}\text{2}$} = ?0.448 V vs NHE, and ΔS_rc^θ = ?6 cal · mol^?1 · deg^?1. Electron transfer kinetics with [CO(en)₃]³⁺ led to k₁₂ = 3100 M^?1 s^?, ΔH^≠ = 12.4 kcal · mol^?1 and ΔS^≠ = ?0.9 cal · mol^?1 · deg^?1 (I = 0.10 M). For the related [Co(NH₃)₆]³⁺ complex, k₁₃ = 1.9 × 10⁴ M^?1 s^?1. The self-exchange rate constant and activation parameters were analysed in terms of relative Marcus theory.     

Resonance-enhanced multiphoton ionization of molecular hydrogen via the E,F1Σg+ state: Photoelectron energy and angular distributions

Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H₂ X¹Σ_g⁺ (ν = 0,J) + 2hv → E,F¹Σ_g⁺(ν_E,J_E = J) + hν → H₂⁺X²Σ_g⁺ (ν⁺) + e^?, for ν_E = 0, 1, or 2 and for J_E = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν⁺ = ν_E, the detailed H₂⁺ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the J_E value of the intermediate state and the ν⁺ value of the ion.     

Excess enthalpy,excess heat capacity and excess volume of 1,2,4-trimethylbenzene +, and 1-methylnaphthalene + an n-alkane

A flow microcalorimeter of the Picker design was used to measure excess molar enthalpies H^E at 298.15 K as a function of mole fraction χ₁ for several mixtures belonging to series I: {χ₁1,2,4-C₆H₃(CH₃)₃ + χ₂n-C_ℓH_2ℓ+2}, and series II: {χ₁1-C₁₀H₇CH₃ + χ₂n-C_ℓH_2ℓ+2}. The chain length ℓ of the n-alkanes ranged between 7 and 16. 1,2,4-trimethylbenzene and 1-methylnaphthalene have about the same size and shape as the previously investigated chloro derivatives 1,2,4-C₆H₃Cl₃ and 1-C₁₀H₇Cl but a much smaller reduced dipole moment. The calorimeter was used in the discontinuous mode. A plot of H^E_max (i.e., the maximum value of H^E with respect to composition) against ℓ for series I shows a shallow minimum around ℓ = 11 with H^E_max (ℓ = 11) ≈ 250 J mol⁻¹, whereas H^E_max for series II decreases over the whole range 7 ⩽ ℓ ⩽ 16: H^E_max (ℓ = 7) ≈ 760 J mol⁻¹, and H^E_max (ℓ = 16) ≈ 595 J mol⁻¹. The corresponding enthalpic interaction parameters h₁₂, calculated from zeroth-order KGB (Kehiaian-Guggenheim-Barker) theory, decrease with increasing ℓ, and the rate of decrease, dh₁₂/dℓ, diminishes for larger chain lengths.For three mixtures belonging to series I (ℓ = 7, 10, 14), excess molar volumes V^E and excess molar heat capacities C^E_P at constant pressure were mesured at the same temperature. V^E was determined with a vibrating-tube densimeter (flow conditions), and C^E_P was obtained with another type of flow calorimeter (stepwise procedure). V^E(χ₁ = 0.5)/(cm³ mol⁻¹) = −0.207 for ℓ = 7, 0.060 for ℓ = 10, and 0.145 for ℓ = 14. The corresponding values for C^E_P x₁ = 0.5)/(J K⁻¹ mol⁻²) are 0.32, 0.66 and −0.09. Thus the chain length dependence of the excess molar heat capacity is qualitatively similar to that observed for the series with the homomorphic chloro derivative, (1,2,4-C₆H₃Cl₃ + n-C_ℓH_2ℓ+2), and to that of (1-C₁₀H₇Cl+n-C_ℓH_2ℓ+2).     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC₆H₄Sn(CH₃)₃, where X = para-N(CH₃)₂, para-OCH₃, para-OC₂H₅, para-CH₃, meta-CH₃, -H, para-F, meta-OCH₃, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH₃)₃. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [¹J(¹³C¹H)] and tin [²J(SnC¹H)]. Solvent effects and other extraneous factors invalidate comparisons of ? values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ?[C(1)Sn] > ? [SnC] > ? [CH]. Satisfactory correlations (r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, ²J(SnC¹H) and the constituent, one-bond ¹J (Sn¹³C) and J(¹³C¹H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, ¹J(Sn¹³C) and ¹J(¹³C¹H) is lower (r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and ¹¹⁹Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of ?0.14, and the significance of this value is discussed.