Theoretical study of spin–orbit coupling and kinetics in spin-forbidden reaction between Ta(NH2)3 and N2O

The activation mechanism of the nitrous oxide (N₂O) with the Ta(NH₂)₃ complex on the singlet and triplet potential energy surfaces has been investigated using the hybrid exchange correlation functional B3LYP. The minimum energy crossing point (MECP) is located by using the methods of Harvey et al. The rate-determining step of the N–O activation reaction is the intersystem crossing from ¹ 2 to ³ 2. The reacting system will change its spin multiplicities from the singlet state to the triplet state near MECP-1, which takes place with a spin crossing barrier of 32.5 kcal mol^?1, and then move on the triplet potential energy surface as the reaction proceeds. Analysis of spin–orbit coupling (SOC) using localized orbitals shows that MECP-1 will produce the significant SOC matrix element, the value of SOC is 272.46 cm^?1, due to the electron shift between two perpendicular π orbitals with the same rotation direction and the contribution from heavy atom Ta. The rate coefficients are calculated using Non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM). Results indicate that the coefficients, k(E), are exceedingly high, k(E) > 10¹² s^?1, for energies above the intersystem crossing barrier (32.5 kcal mol^?1); however, in the lower temperature range of 200–600 K, the intersystem crossing is very slow, k(T) < 10^?6 s^?1.     

Energy transfer between benzophenone and biacetyl

Energy transfer from benzophenone to biacetyl in the gas phase was studied by measuring the intensity and decay time of phosphorescence and fluorescence of both compounds as a function of pressure and composition of the mixture. We have established that benzophenone transfers energy along two parallel channels: singlet→singlet with high efficiency, k_SS? 2 ×10⁸ torr^?1s^?1, and triplet→triplet with lower efficiency, k_TT?8 × 10⁵ torr^?1s^?1.     

Singlet excited-state “energy hopping” in alkylbenzenes

The quenching of toluene fluorescence by cis-6-phenyl-2-hexene has been studied to determine the rate of singlet “energy hopping” in dilute solutions of alkylbenzenes. The singlet lifetime date have been analyzed by the Stern-Volmer method to give the quasi-isoenergetic rate, k_q, as 1.2 × 10¹⁰ M^?1?1. The result is consistent with an excimer formation dissociation mechanism for alkylaromatic singlet energy transfer in dilute solution.     

On the syntheses and singlet oxygen reactivity of oxodipyrromethene models for bilirubin

Various oxodipyrromethenes with varying β-substituents have been synthesized and their reaction with singlet oxygen studied. The rates of chemical reactivity (k_R) and physical quenching (k_Q) of singlet oxygen by those substrates approach the diffusion threshold in both chloroform and methanol solvents, with k_Q generally larger than k_R in chloroform but of comparable magnitude in methanol. The range of (k_Q+k_R) values is 0.2–4.2 × 10₉ M^?1 s^?1.     

Dye-sensitized photooxidation of chlorpromazine

Chlorpromazine efficently quenches singlet oxygen (¹O₂) with a k_q = 3.5 × 10⁷ M^?1 s^?1. The major result of the chemical interaction between these two species is the cleavage of the N-side chain.     

DETERMINATION OF THE ACRYLAMIDE QUENCHING CONSTANT FOR PROTEIN AND MODEL INDOLE TRIPLETS

Abstract— The decay of the indole triplet of single tryptophan-containing proteins and model compounds can be readily measured at room temperature in aqueous solution by monitoring the triplet-triplet absorption or phosphorescence emission following a 265 nm exciting laser pulse. The quenching action of acrylamide on the triplet excited state of indole side chains was studied in an analogous fashion to that previously done at the singlet level (Eftink and Ghiron, 1977). The acrylamide triplet quenching constant (^tk_q) ranged from a high of 7.8 times 10⁸M^-1 s^-1 for the exterior indole of corticotropin (ACTH) to a low of 2 times 10⁵ Af^-1 s^-1 for the interior indole of ribonuclease T, (RNase T,). The ratio (7) of these values with their respective acrylamide singlet quenching constants (^tk_q),(γ=^tk_q⁸K_q) ranged from a high of 0.22 for ACTH to a low of 0.001 for RNase T₁,. Acrylamide is also an inefficient quencher of model indoles in various solvents (i.e. it has a γ less than 1). The magnitude of γ varied from a high of 0.3 in H₂0 to a low of 0.02 in acetonitrile, but did not correlate with viscosity, dielectric constant or polarity. The lower efficiency observed for internal indole groups can not be explained by that class of models which predict the presence of static quenching at the triplet level, since none was observed. The present results confirm the observation of Calhoun et al. of a large discrepancy between acrylamide's singlet and triplet quenching constants for buried indole side chains, but suggest that it may be largely explained by the fact that acrylamide is an inefficient quencher of the indole triplet state (1983). The magnitude of this inefficiency is probably determined by specific microenvironmental factors. Thus, unlike ⁸K_q, the environmentally sensitive ^lk_H cannot be easily used to characterize the dynamics of proteins.     

Pyrene fluorescence quenching by aromatic azides

Pyrene fluorescence quenching by phenylazide derivatives with donor and acceptor substituents has been studied by fluorescence spectroscopy and flash photolysis. The rate constants of quenching (k _q) in acetonitrile ((0.2–1.2) × 10¹⁰ l mol^?1 s^?1) are found to be close to a diffusion limit; the rate constants were somewhat higher for perfluoro-substituted arylazides. It is found that k _q does not depend on solvent polarity; the formation of the pyrene cation in the course of pyrene fluorescence quenching by tolylazide was not detected. Pyrene fluorescence quenching occurred by an energy-transfer mechanism; this is supported by the coincidence of the quantum yields of the direct and sensitized photodecomposition of tolylazide. As estimated, energy transfer in rigid media occurs at characteristic distances of about 10 Å.     

Thermal isomerization and decomposition of 1,2-butadiene in shock waves

1,2-Butadiene diluted with Ar was heated behind reflected shock waves over the temperature and the total density range of 1100–1600 K and 1.36 × 10^?5 ? 1.75 × 10^?5 mol/cm³. The major products were 1,3-butadiene, 1-butyne, 2-butyne, vinylacetylene, diacetylene, allene, propyne, C₂H₆, C₂H₄, CH₄, and benzene, which were analyzed by gas chromatography. The UV kinetic absorption spectroscopy at 230 nm showed that 1,2-butadiene rapidly isomerizes to 1,3-butadiene from the initial stage of the reaction above 1200 K. In order to interpret the formation of 1,3-butadiene, 1-butyne, and 2-butyne, it was necessary to include the parallel isomerizations of 1,2-butadiene to these isomers. The present data were successfuly modeled with a 82 reaction mechanism. From the modeling, rate constant expressions were derived for the isomerization 1,2-butadiene = 1,3-butadiene to be k₃ = 2.5 × 10¹³ exp(?63 kcal/RT) s^?1 and for the decomposition 1,2-butadiene = C₃H₃ + CH₃ to be k₆ = 2.0 × 10¹⁵ exp(?75 kcal/RT) s^?1, where the activation energies, 63 kcal/mol and 75 kcal/mol, were assumed. These rate constants are only applicable under the present experimental conditions, 1100–1600 K and 1.23–2.30 atm. © 1995 John Wiley & Sons, Inc.     

Quenching of triplet states of diazines by H-atom donors. Formation of azyl radicals

The laser flash photolysis of pyrazine in water and in organic solvents has been examined. The ³(n, π^*) state in water has absorption bands at 230, ≈ 260, ≈ 295, ≈ 640, 700 and 810 nm, and decays with k = 2.2 × 10⁵ sec^?1. It is quenched by oxygen with k_q = 3.2 × 10⁹ M^?1 sec^?1 and by various H-atom donors, e.g., k_q = 1.3 × 10⁸ M^?1 sec^?1 for isopropyl alcohol. On reaction with H-atom donors, the chemistry of ³(n, π^*) pyrazine produces the neutral pyrazyl-radical and the dihydro radical cation, whose characteristic absorption spectra have been identified. These results are discussed by comparison with ³(π, π^*) diazines and with ³(n, π^*) aromatic carbonyl compounds.     

proton transfers of substituted ammonium salts—XIII : N-inversion of piperidines in aqueous acidic solutions

The kinetics of the nitrogen inversion of 1,2-dimethyl-(2); 1,4-dimethyl-(8); 1-trans(2,6)-trimethyl-(4); 1,4-cis(2,6)-tetramethyl-(7) and 1,2,2,6,6-pentamethyl-(5)piperidines have been investigated in aqueous acidic solution (pH = 6.5–8.5) at 33° by dynamic NMR. In all cases, two isomeric cations AH⁺ and BH⁺ are observed in acidic conditions (pH<6?), and the nitrogen inversion is brought to the NMR time scale as a result of a progressive deprotonation of the cations into their conjugate amines on increasing the pH. Low rate constants K_A are obtained for α-substituted or unsubstituted compounds (k_A ?10³ s^?1), except for piperidine 5 where the rate constant k_A = 4.3 × 10⁵ s^?1 is of the same order of magnitude as the one found for tertiary acyclic amines.     

SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF CURCUMIN

Curcumin, bis(4-hydroxy-3-methoxyphenyl)-l,6-heptadiene-3,5-dione, is a natural yellow-orange dye derived from the rhizome of Curcuma longa, an East Indian plant. In order to understand the photobiology of curcumin better we have studied the spectral and photochemical properties of both curcumin and 4-(4-hydroxy-3-methoxy-phenyl)-3-buten-2-one (hC, half curcumin) in different solvents. In toluene, the absorption spectrum of curcumin contains some structure, which disappears in more polar solvents, e.g. ethanol, acetonitrile. Curcumin fluorescence is a broad band in acetonitrile (λ_max= 524 nm), ethanol (λ_max= 549 nm) or micellar solution (λ_max= 557 nm) but has some structure in toluene (λ_max= 460, 488 nm). The fluorescence quantum yield of curcumin is low in sodium dodecyl sulfate (SDS) solution (φ= 0.011) but higher in acetonitrile (φ= 0.104). Curcumin produced singlet oxygen upon irradiation (φ > 400 nm) in toluene or acetonitrile (Φ= 0.11 for 50 μM curcumin); in acetonitrile curcumin also quenched ¹O₂ (k_q, = 7 × 10⁶ M^?1 s^?1). Singlet oxygen production was about 10 times lower in alcohols and was hardly detectable when curcumin was solubilized in a D₂O micellar solution of Triton X-100. In SDS micelles containing curcumin no singlet oxygen phosphorescence could be observed. Curcumin photogenerates superoxide in toluene and ethanol, which was detected using the electron paramagnetic resonance/spin-trapping technique with 5,5-dimethyl-pyrroline-.N-oxide as a trapping agent. Unidentified carbon-centered radicals were also detected. These findings indicate that the spectral and photochemical properties of curcumin are strongly influenced by solvent. In biological systems, singlet oxygen, superoxide and products of photodegradation may all participate in curcumin phototoxicity depending on the environment of the dye.     

Through bond mechanism versus exciplex formation in the photochemistry of fullerene / ferrocene donor-bridge-acceptor dyads

A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 (Φ_rel = 0.17 × 10^?4, 3 (Φ_rel = 0.78 × 10^?4), 4 (Φ_rel = 1.5 × 10^?4), 5 (Φ_rel = 0.7 × 10^?4), and 6 (Φ_rel = 2.9 × 10^?4) were substantially quenched, relative to N-methyl fulleropyrrolidine (1) (Φ_rel = 6.0 × 10^?4). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28×10⁹ s^?1, 6.9×10⁹ s^?1, and 3.4×10⁹ s^?1, 14×10⁹ s^?1, 2.3×10⁹ s^?1 respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic π-radical anion bands at λ_max = 400 and 1055 nm in degassed benzonitrile with τ_1/2 = 1.8 μs (3) and 2.5 μs (4).     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

Solvent effect and temperature dependence in the interaction of singlet oxygen with α-phenyl-N-tert-butyl nitrone

The solvent effect on the quenching of singlet oxygen by -phenyl-N-tert-butyl-nitrone /PBN/ has been investigated by laser flash photolysis technique registrating luminescence kinetics of¹O₂. The values of the rate constant /k_q/ of the quenching were at 293 K: /9.0±0.4/×10⁶, /4.4±0.3/×10⁶ and /18.3±0.5/×10^{6 –}M^–1 s^–1 in toluene, chloroform and acetonitrile, respectively. The rate constant for the chemical interaction between¹O₂ and PBN, was k_r<1×10⁵ M^–1 s^–1k_q independently of the solvent. At temperatures between 223 and 293 K in toluene E_q=0.4±0.4 kJ mol^–1.     

Photophysical and photochemical studies of thifensulfuron-methyl herbicide in aqueous solution

The photophysical and photochemical studies of a sulfonylurea herbicide, thifensulfuron-methyl (THM), have been investigated in a buffered aqueous solution. In the first part, the influence of pH on the spectroscopic properties was studied. This allowed the determination of the ground and excited state acidity constants, pK_a = 4 and 4.4, respectively, thus exhibiting the potential existence of a photoinduced protonation in the singlet state. In the second part, the photolysis kinetics was studied at different pH and varying oxygen concentrations, using an HPK 125 W lamp and followed up by the identification of photoproducts formed under continuous photo-irradiation. The kinetics results suggest that the photolysis process is faster in acidic (k = 3 × 10^?4 s^?1) than in basic medium (k = 9.8 × 10^?5 s^?1). The photolysis products were identified by high performance liquid chromatography HPLC-DAD, HPLC–MS and HPLC–MS–MS. In order to obtain a better understanding of the photodegradation mechanism, a laser flash photolysis study was performed. By comparing the quenching rate constant (k_q = 9.64 × 10⁸ mol^?1 l s^?1) obtained from triplet state quenching by molecular oxygen and from the Stern–Volmer relation (k_q = 0.41 × 10⁸ mol^?1 l s^?1), the role of the singlet state in the photodegradation process was demonstrated. The photoproducts originating from both singlet and triplet excited states have been identified and hypothetical photodegradation pathways of the thifensulfuron-methyl in aqueous solution are proposed.     

Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC2H 5 +· isomers

Eleven isomers with the PyC₂H ₅ ^+· composition, which include three conventional (1–3) and eight distonic radical cations (4–11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS² and MS³ experiments. The three conventional radical cations, that is, the ionized ethylpyridines C₂H₅-C₅H₄N^+· (1–3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H₂C^·?C₅H₄N⁺?CH₃ (4–6), CH₃?C₅H₄N⁺?CH ₂ ^· (7–9), C₅H₅N⁺?CH₂CH ₂ ^· (10), and C₅H₅N⁺?CH^·?CH₃ (11). Unique features of the low-energy (15-eV) CID and ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6–31G(d, p)//6–31G(d, p)+ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC₂H₅ ^+· system, which provided theoretical support for facile 4?7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-distonic ions 4–9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin derealization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical “spatially separated” ab initio charge and spin densities of 4–11 were used to classify them as distonic ions, whereas 1–3 show, as expected, “localized” electronic structures characteristic of conventional radical ions.     

Kinetic studies of the reactions of pentacyanoferrate(III) complexes with L‐ascorbic acid

The reduction of Fe(CN)₅L^2? (L = pyridine, isonicotinamide, 4,4′‐bipyridine) complexes by ascorbic acid has been subjected to a detailed kinetic study in the range of pH 1–7.5. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the ascorbic acid in the form of H₂A(k₀), HA^?(k₁), and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Fe(III) complex. With given Ka₁ and Ka₂, the rate constants are k₀ = 1.8, 7.0, and 4.4 M^?1 s^?1; k₁ = 2.4 × 10³, 5.8 × 10³, and 5.3 × 10³ M^?1 s^?1; k₂ = 6.5 × 10⁸, 8.8 × 10⁸, and 7.9 × 10⁸ M^?1 s^?1 for L = py, isn, and bpy, respectively, at μ = 0.10 M HClO₄/LiClO₄, T = 25°C. The kinetic results are compatible with the Marcus prediction. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 126–133, 2005     

Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile-1-methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ? pyrene < phenanthrene, fluorene ≈ 2-methoxynaphthalene ≈ biphenyl < anthracene < 2-methylnaphthalene < 1-methoxynaphthalene < 2,3,6-trimethylnaphthalene < 2,3-dimethylnaphthalene ≈ naphthalene < 1-methylnaphthalene < 2,6-dimethylnaphthalene < p-terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k_q for the formation of exciplex as well as the self-quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k_q were calculated from the Stern-Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k_q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2-methylnaphthalene ≈ 1-methylnaphthalene ≈ 2,3-dimethylnaphthalene < 2,6-dimethylnaphthalene < acenaphthene < p-terphenyl < 1-methoxynaphthalene. The study of the concentration effect reflecting the self-quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self-quenching efficiency of aromatic hydrocarbons: 2-methoxynaphthalene ≈ pyrene > anthracene > 1-methoxynaphthalene > fluorene > 2,6-dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6-trimethylnaphthalene > 2,3-dimethylnaphthalene > 1-methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above-mentioned sequences.     

Kinetics of the reaction of NO2 with a macrocyclic nickel complex

The kinetics of the reaction between NO₂ and ([14]aneN₄)Ni²⁺ were determined by laser flash photolysis. The NO₂ was generated in two independent reactions, one of which is based on the photochemistry of (NH₃)₅CoNO₂²⁺, and the other on the photochemistry of HNO₂/NO₂^?. The results from both sets of experiments yielded a consistent value for the rate constant, k₁ = 1.2 × 10⁸ M^?1 s^?1 in aqueous solutions at pH 1–4. There was no evidence for the reverse reaction. NO₂ reacts with Fe_aq²⁺ more slowly, k_Fe ～ 2 × 10⁵ M^?1 s^?1. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 278–281, 2002