Scalar dynamic nuclear polarization of protons in trifluoroacetic acid solutions

Strongly scalar ¹H intermolecular dynamic nuclear polarization (DNP) of solvent ¹H nuclei in solutions of organic free radicals has now been observed at room temperature. Transient protonation of the CF₃COOH/pyridyl imidazoline-1-oxyl radical system accounts for the observed effects.     

Effect of isotope substitution on the magnitude of nonequilibrium nuclear polarization in photolysis of acetone in methanol

Polarization of nuclei in both the products of the reactions and in the CHD₂OD proton without polarization in the CH₃OH protons is observed in irradiation of a solution of acetone in CD₃OD in the presence of CHD₂OD and CH₃OH. Polarization of the protons of the products is strongly dependent on the temperature of the solution and arises in radical pairs; polarization of the proton of partially deuterated methyl alcohol is due to a mechanism of optical nuclear polarization. It was hypothesized that the isotope effect is due to a difference in proton and electron relaxation and to a difference in the rates of cross-relaxation transitions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2683–2687, December, 1989.     

Dynamic nuclear polarization of tin-119 nuclei in free radical solutions

Dynamic nuclear polarization of tin-119 in free radical solutions has been studied at 175 G. Scalar interactions are predominant for both bivalent and tetravalent tin compounds.     

Nonequilibrium solvent polarization in kinetics of SN2 reactions

The solvent effect on the experimental activation barriers for the reactions of methyl iodide with chloride and thiocyanate ions was analyzed according to the Marcus and Shaik theories, considering S_N2 mechanism in terms of a single electron shift. The linear increase in the solvent reorganization energy of the Marcus theory (after removing contributions from the specific solvation) with the solvent Pekar factor, describing the effect of the nonequilibrium solvent polarization, was observed for six aprotic solvents. The direct support of the title effect based on the Shaik theory was less evident; however, in general, the calculated activation barriers in 10 solvents change parallel with the experimental ones. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 61–66, 2003     

Mechanism of the photolysis of substituted tetraphenylporphin with p-benzoquinones in vitro according to chemical nuclear polarization (CNP) data

It is established by means of chemical nuclear polarization that the photointeraction of quinones with porphyrin leads to the transfer of an electron with the formation of ion-radical pairs, and then to the transfer of a proton with the reversible reduction of quinone.     

Flash photolysis of s-trinitrobenzene solutions

Flash photolysis of s-trinitrobenzene (TNB) aerated solutions in alcohols generates a transient species with absorption maxima at 430 and 510 nm. The yield of the transient is a function of oxygen concentration, and its rate of formation is viscosity dependent. In deaerated solutions instead of the transient, a brown permanent product, identified as a charge transfer (CT) complex with absorption maxima at 470 and 550 nm, is produced. These species are formed only in polar solvents with relatively high proton affinity. The data suggest an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion The anion thereby produced interacts with oxygen in aerated alcohols to form the transient attributed to CT complex while in deaerated alcohols the anion reacts with the solvent to produce RO^?, which leads to the CT complex RO…^?TNB. This mechanism is supported by detailed kinetic and spectroscopic studies.     

Chemically induced dynamic nuclear polarization in the photolysis of tetrafluoro-1,4-benzoquinone with plane polarized light evidence of phototriplet mechanism

CIDNP in the photolysis of tetrafluoro-1,4-benzoquinone was studied by using plane polarized light for excitation. Experiments show that the magnitude of CIDNP depends upon the polarization of the excitation light and thus provides evidence for the phototriplet mechanism in the initial electron spin polarization which subsequently leads to nuclear polarization by cross relaxation.     

Flash photolysis of solutions of sodium in ethers

Flash photolysis of ethereal solutions of sodium results in the almost complete disappearance of the original absorption band at 15000 cm⁻¹ and in the formation of solvated electrons. After the flash an intermediate with an absorption maximum at 12000 cm⁻¹ is formed. This absorption is assigned to a species which has the stoichiometric composition of a sodium atom. Although it may be described as a metal “monomer” we propose as an alternative model the concept of an “ion pair” of a solvated electron and a sodium cation. In analogy to the 12000 cm⁻¹ transient we propose that the blue species can be described as an associate of a sodium cation with two solvated electrons.     

Pulse photolysis of PbCl4 in non-aqueous solutions

Lead(IV) chloride solutions in CCl₄, CHCl₃, cyclohexane, benzene, toluene, chlorobenzene, N,N-dimethylformamide and pyridine have been irradiated with an approximatelyy 1.5 kJ pulse of UV radiation lasting 0.5 ms. The course of the photochemical processes has been followed by measuring the loss of PbCl₄, the changes of Cl₂ concentration and by identifiction of chlorination products. In a similar way, chlorine solutions in cyclohexane and benzene have been investigated. Primary processes are discussed in detail. The most probable process in solvents that do not form complexes with PbCl₄ is predissociation of the Pb-Cl bond

whereas in those forming complexes with PbCl₄ electron transfer takes place:

The results are compared with those obtained earlier under steady irradiation. This allows us to determine new pathways of secondary processes that are unobservable with the continuous illumination technique. The pathways include processes photoinduced by the phase of fresh finely dispersed PbCl₂ crystals:

     

Pulse photolysis of PbCl4 in non-aqueous solutions

Lead(IV) chloride solutions in CCl₄, CHCl₃, cyclohexane, benzene, toluene, chlorobenzene, N,N-dimethylformamide and pyridine have been irradiated with an approximately 1.5 kJ pulse of UV radiation lasting 0.5 ms. The course of the photochemical processes has been followed by measuring the loss of PbCl₄, the changes of Cl₂ concentration and by identification of chlorination products. In a similar way, chlorine solutions in cyclohexane and benzene have been investigated. Primary processes are discussed in detail. The most probable process in solvents that do not form complexes with PbCl₄ is predissociation of the PbCl bond

whereas in those forming complexes with PbCl₄ electron transfer takes place:

The results are compared with those obtained earlier under steady irradiation. This allows us to determine new pathways of secondary processes that are unobservable with the continuous illumination technique. The pathways include processes photoinduced by the phase of fresh finely dispersed PbCl₂ crystals:

     

Investigation of lead thiosulfate photolysis in aqueous solutions

A model of photolysis of PbS₂O₃ aqueous solutions has been proposed on the basis of identified photolysis products and semiempirical quantum-chemical calculations. The degradation of PbS₂O₃ starts with the dissociation of the sulfur-sulfur bond in the thiosulfate group via photochemical excitation and transition of the system a whole to the activated state, which is decomposed by the solvent. The interaction of the primary photolysis products with PbS₂O₃ results in the formation of final products.     

Optical nuclear polarization in the gas phase

Optical nuclear polarization (absorption) has been detected experimentally for the first time in benzaldehyde under UV irradiation in a magnetic field of 4.7 T. The phenomenon has been interpreted theoretically under the assumption of a non-equilibrium electron (optical) polarization transfer to the nuclear subsystem by a scalar cross relaxation mechanism of the second kind.     

Laser flash photolysis of aqueous acetaminophen solutions

The N-acetyl-4-aminophenoxyl radical, a supposed intermediate of the enzymatic oxidation of acetaminophen in living organisms, was prepared and studied by means of nanosecond laser flash photolysis. A number of important spectral-kinetic parameters of this species were determined, namely, the absorption coefficient at 440 nm ((4.2±0.2)×10³ l mol^?1cm^?1), the quantum yield of acetaminophen photoionization at 266 nm (φ= 0.03), and the rate constants for recombination (2k= (2.4±0.3))×10⁹ l mol^?1s^?1) and the reaction with the superoxide radical (k= (9±2))×10⁹ l mol^?1s^?1).     

Solid effect dynamic nuclear polarization and polarization pathways

Using dynamic nuclear polarization (DNP)/nuclear magnetic resonance instrumentation that utilizes a microwave cavity and a balanced rf circuit, we observe a solid effect DNP enhancement of 94 at 5 T and 80 K using trityl radical as the polarizing agent. Because the buildup rate of the solid effect increases with microwave field strength, we obtain a sensitivity gain of 128. The data suggest that higher microwave field strengths would lead to further improvements in sensitivity. In addition, the observation of microwave field dependent enhancements permits us to draw conclusions about the path that polarization takes during the DNP process. By measuring the time constant for the polarization buildup and enhancement as a function of the microwave field strength, we are able to compare models of polarization transfer, and show that the major contribution to the bulk polarization arises via direct transfer from electrons, rather than transferring first to nearby nuclei and then transferring to bulk nuclei in a slow diffusion step. In addition, the model predicts that nuclei near the electron receive polarization that can relax, decrease the electron polarization, and attenuate the DNP enhancement. The magnitude of this effect depends on the number of near nuclei participating in the polarization transfer, hence the size of the diffusion barrier, their T(1), and the transfer rate. Approaches to optimizing the DNP enhancement are discussed.     

Products of photolysis of cadmium thiosulfate aqueous solutions

On the basis of X-ray diffraction and chemical analysis data, it was found that cadmium sulfide exhibiting the ‘size effect’ is formed during the UV irradiation of aqueous solutions of cadmium thiosulfate. Sulfate ions and hydrogen ions accumulate in the aqueous solution. It was shown that the yield of cadmium sulfide is affected by the concentration of thiosulfate ions in the solution. The overall equation of photolysis of cadmium thiosulfates is proposed.     

Laser photolysis study of the transient products of 4-carboxybenzophenone-sensitized photolysis of chlorophenoxyacetic acid-based herbicides in aqueous micellar solutions

The kinetics of the reaction of the triplet states and radicals of 4-carboxybenzophenone (CB) in aqueous sodium dodecyl sulfate micellar solutions in the absence of additives and in the presence of 2,4-dichlorophenoxyacetic acid (DCPA) or 2-methyl-4-chlorophenoxyacetic acid (MCPA) were studied. The rate constants of ³CB quenching by DCPA and MCPA were found to be 8.5 × 10⁵ and 1.4 × 10⁶ s⁻¹, respectively (as calculated for a reactant pair in the micelle). As a result of quenching, radical pairs are produced in the micelle, as well as the ketyl radicals CBH^· that lost their partners because of the fast disappearance of some of the DCPA and MCPA radicals. An applied external magnetic field substantially retards the recombination of radical pairs in the micelle.