Steric influence of E- or Z-monosubstituted and terminally disubstituted vinyl groups in matrix-isolated cyclopentane-1,3-diyl triplet diradicals as probed by the zero-field EPR D parameter

The zero-field D parameter of the localized E, Z, and disubstituted vinyl cyclopentane-1,3-diyl triplet diradicals V was determined at 77 K in a 2-methyltetrahydrofuran (2-MTHF) matrix. Good linear correlations were obtained with the semiempirically (PM3; r(2) = 0. 991, n = 19) and DFT (B3LYP; r(2) = 0.998, n = 7) calculated spin densities of the triplet diradicals. The D values for the disubstituted triplet diradicals V are generally larger than the corresponding monosubstituted ones and, thus, the former are less well-delocalized and thereby more poorly stabilized. For the E- and Z-diastereomeric pairs V, only marginal changes in the theoretical assessed spin densities as well as in the D values have been found. Steric effects operate and distort the conformation of the vinyl substituent in the triplet diradical V. This is adequately reproduced by theoretical calculations. For the diphenyl-substituted triplet diradical Vl, for example, they show a more or less planar alignment of the E-phenyl group and the allylic pi system (torsion angle 12 degrees ) and thus optimal delocalization of spin, whereas the Z-phenyl group is twisted about 78 degrees out of plane and therefore is not involved in the delocalization and stabilization of spin. This results in a slightly higher D value (0.0368 cm(-)(1)), and the spin is delocalized less than in the monosubstituted E-Vm one (0.0357 cm(-)(1)).     

EPR spectral determination of electronic substituent effects on the D values of hydrocarbon polyradicals (Quintet and septet spin states) composed of localized 1,3-cyclopentanediyl spin-carrying units linked by 1,3-Di- and 1,3,5-trimethylenebenzene ferromagnetic couplers

The parent and p-nitrophenyl-substituted diradicals D-3a,b (triplets), tetraradicals T-3a,b (quintets), and hexaradicals H-3a,b (septets) were photochemically generated in matrix-isolated form (toluene, 77 K) by successive denitrogenation of the trisazoalkanes 3a,b and EPR spectrally characterized. In these high-spin polyradicals the spin-spin interaction within the localized spin-carrying 1,3-cyclopentanediyl diradical unit is much stronger than within the cross-conjugated ferromagnetic coupling unit. Accordingly, a change of the electronic properties in the cyclopentanediyl unit affects decisively the D value of the whole polyradical. Therefore, the spin-accepting p-nitro group reduces the D value of the tetra- and hexaradical in the same amount as that of the diradical. Thus, irrespective of the spin multiplicity, the substituent stabilizes electronically the triplet (D-3a,b), quintet (T-3a,b), and septet (H-3a,b) species with equal efficacy.     

Guaiazulene-based phenolic radical scavengers: synthesis, properties, and EPR studies of their reaction with oxygen-centred radicals

A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies.     

Formation of high-spin complexes in reactions of oxides of the γ-Al2O3 type with radical pairs of sterically shielded semiquinones: A new procedure for the preparation of organometallic polyradicals and their ESR spectra

A new simple method was developed for the preparation of organometallic bi- and triradicals by the heterogeneous reaction of γ-Al₂O₃ or analogous oxides with free radicals generated in solution through the interaction of 3,6-di-tert-butyl-o-benzoquinone with 3,6-di-tert-butylpyrocatechol. An explanation was offered for the plus sign of the dipoledipole coupling constant observed previously in the ESR spectra recorded in a superstrong field at low temperatures. Deceased. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 298–302, February, 1997.     

Photodynamic action of amino substituted hypocrellins: EPR studies on the photogenerations of active oxygen and free radical species

A novel method has been employed to prepare 2-butylamino-2-demethoxy hypocrellin A (BADMHA) and 2-butylamino-2-demethoxy hypocrellin B (BADMHB). Both compounds exhibit stronger absorption at the phototherapeutic window (600-900 nm). The spin trapping and spin counteraction studies have shown that they are both efficient generators of the active oxygen (1O2, O2*-) in the aerobic condition. Under the anaerobic condition they generate non-oxygen free radical (semiquinone radical anion), and the active oxygen mechanism of photosensitization can be converted into non-oxygen free radical mechanism with the depletion of oxygen. The quantum yields of 1O2 generation of BADMHA and BADMHB are 0.46 and 0.44, respectively. Both are lower than those of their parent compounds HA and HB. But the productions of superoxide anion are enhanced significantly compared with HA and HB, indicating they are both favorable Type I phototherapeutic agents.     

On the structure of vanadium oxide supported on aluminas: UV and visible raman spectroscopy, UV-visible diffuse reflectance spectroscopy, and temperature-programmed reduction studies

Vanadia species on aluminas (delta- and gamma-Al2O3) with surface VOx density in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temperature-programmed reduction in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymerized VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm(-1), observed on dehydrated V/delta-, gamma-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidation reactions catalyzed by supported vanadia. Two types of frequency shifts of the V=O stretching band (1013-1035 cm(-1)) have been observed in the Raman spectra of V/Al2O3: a shift as a function of surface VOx density and a shift as a function of excitation wavelength. The shift of the V=O band to higher wavenumbers with increasing surface VOx density is due to the change of VOx structure. The V=O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymerized VOx species while visible Raman is more sensitive to highly polymerized VOx species and crystalline V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.     

Characterization of a nematic mixture by reversed-phase HPLC and UV spectroscopy: an application to phase behaviour studies in liquid crystal-CO2 systems

In the present work a simple but selective reversed-phase high performance liquid chromatographic method (HPLC) was developed for the analysis of the nematic liquid crystal mixture E7 to determine precisely the composition of the liquid crystal mixture used in PDLC film preparation. Ultraviolet absorption spectrophotometry experiments were carried out to determine the HPLC detection wavelength and to characterize the absorptivity constants of E7 constituents. The technique developed is applied in the case of equilibrium solubility studies for E7 in supercritical carbon dioxide (scCO₂). The results indicate unambiguously that scCO₂ can fractionate the mixture towards the E7 components. The four single component peaks of the E7 mixture were distinctively separated by this method, which enabled the determination of the solubility of E7 constituents in the scCO₂.     

Theoretical studies of the reactions of CF3CHCLOCHF2/CF3CHFOCHF2 with OH radical and Cl atom and their product radicals with OH

The mechanisms and dynamics studies of the OH radical and Cl atom with CF(3)CHClOCHF(2) and CF(3)CHFOCHF(2) have been carried out theoretically. The geometries and frequencies of all the stationary points are optimized at the B3LYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. For each reaction, two H-abstraction channels are found and four products (CF(3)CHFOCF(2), CF(3)CFOCHF(2), and CF(3)CHClOCF(2), CF(3)CClOCHF(2)) are produced during the above processes. The rate constants for the CF(3)CHClOCHF(2)/CF(3)CHFOCHF(2) + OH/Cl reactions are calculated by canonical variational transition-state theory (CVT) within 200-2000 K, and the small-curvature tunneling is included. The total rate constants calculated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained. Our calculation shows that the substitution of Cl by F decreases the reactivity of CF(3)CHClOCHF(2) toward OH and Cl. In addition, the mechanisms of subsequent reactions of product radicals and OH radical are further investigated at the G3(MP2)//B3LYP/6-311G(d,p) level, and the main products are predicted in the this article.     

Catalytic asymmetric synthesis of both syn- and anti-3,5-dihydroxy esters: application to 1,3-polyol/alpha-pyrone natural product synthesis

[reaction: see text] We describe a catalytic asymmetric synthesis of both syn- and anti-3,5-dihydroxy esters. The method relies upon catalytic asymmetric epoxidation of alpha,beta-unsaturated imidazolides and amides, using lanthanide-BINOL complexes, and diastereoselective reduction of ketones. The method was applied to the enantioselective syntheses of 1,3-polyol/alpha-pyrone natural products 9a, 9b, and strictifolione (10). The absolute stereochemistry of 9a and 9b was also determined.     

Stereoselective synthesis of anti-1,3-diol units via Prins cyclisation: application to the synthesis of (−)-sedamine

The scope of the Prins cyclisation, the higher stereoselective synthesis of multisubstituted tetrahydropyrans from aldehydes and homoallylic alcohols, is expanded. A new approach for the stereoselective synthesis of polyketide precursors containing anti-1,3-diols, flanked by a variety of alkyl branches and functional groups is described. The approach is successfully exploited for the synthesis of (−)-sedamine.     

Low-temperature fluorination of fluoro-containing polymers: EPR studies of polyvinylidenefluoride and the copolymer of tetrafluoroethylene with ethylene

The direct fluorination of polyvinylidenefluoride (PVDF) and the copolymer of tetrafluoroethylene with ethylene (CTE) was studied at 35-300 K. The dependence of radical formation on temperature and reaction time was obtained by use of electron paramagnetic resonance (EPR) spectroscopy. Primary alkyl radicals formed as a result of the reaction of fluorine abstracting a hydrogen from the polymer were detected at 35 K. These radicals rapidly react with molecular oxygen producing long-lived (∼48 h at 300 K) peroxy radicals. The peroxy radicals when subjected to UV-irradiation (λ < 280 nm) give rise to other radicals that are not stable at T > 77 K. The concentration of the radicals produced during fluorination of PVDF at 77-200 K is one order of magnitude less than that formed from CTE under similar conditions. A mechanism based on the abstraction of the H and the energies of the C-H bonds is given. Density functional theory was used to predict the structures and EPR parameters for a number of fluorinated radicals to explain the observed spectra. The FOO radical was detected at low temperatures.     

Characterization of the phenoxyl radical in model complexes for the Cu(B) site of cytochrome c oxidase: steady-state and transient absorption measurements, UV resonance raman spectroscopy, EPR spectroscopy, and DFT calculations for M-BIAIP

Physicochemical properties of the covalently cross-linked tyrosine-histidine-Cu(B) (Tyr-His-Cu(B)) unit, which is a minimal model complex [M(II)-BIAIPBr]Br (M = Cu(II), Zn(II)) for the Cu(B) site of cytochrome c oxidase, were investigated with steady-state and transient absorption measurements, UV resonance Raman (UVRR) spectroscopy, X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations. The pH dependency of the absorption spectra reveals that the pK(a) of the phenolic hydroxyl is ca. 10 for the Cu(II) model complex (Cu(II)-BIAIP) in the ground state, which is similar to that of p-cresol (tyrosine), contrary to expectations. The bond between Cu(II) and nitrogen of cross-linked imidazole cleaves at pH 4.9. We have successfully obtained UVRR spectra of the phenoxyl radical form of BIAIPs and have assigned bands based on the previously reported isotope shifts of Im-Ph (2-(1-imidazoyl)-4-methylphenol) (Aki, M.; Ogura, T.; Naruta, Y.; Le, T. H.; Sato, T.; Kitagawa, T. J. Phys. Chem. A 2002, 106, 3436-3444) in combination with DFT calculations. The upshifts of the phenoxyl vibrational frequencies for 8a (C-C stretching), 7a' (C-O stretching), and 19a, and the Raman-intensity enhancements of 19b, 8b, and 14 modes indicate that UVRR spectra are highly sensitive to imidazole-phenol covalent linkage. Both transient absorption measurements and EPR spectra suggest that the Tyr-His-Cu(B) unit has only a minor effect on the electronic structure of the phenoxyl radical form, although our experimental results appear to indicate that the cross-linked Tyr radical exhibits no EPR. The role of the Tyr-His-Cu(B) unit in the enzyme is discussed in terms of the obtained spectroscopic parameters of the model complex.     

Molecular orbital studies on the spin states of nitroxide species: Bis- and trisnitroxymetaphenylene, 1,1-bisnitroxyphenylethylene,and 4,6-dimethoxy-1,3-dialkylnitroxy-benzenes

We describe by approximate MO calculation a number of species for which high spin states are either the ground state or lie very low in energy. These are models for the spin coupling in projected ferromagnetic organic materials. The theory guiding their construction is based on planar conjugated systems, while experimentally realized systems are often far from planarity. We can by appeal to steric decoupling explain the failure of the prediction that methoxy subsitution on metaphenylene-coupled nitroxides should stabilize the triplet. However, we find in general that the qualitative rules derived from discussion of planar systems are robust; drastic departures from planarity are required before they lose their value. © 1996 John Wiley & Sons, Inc.     

EPR studies of amine radical cations, part 1: thermal and photoinduced rearrangements of n-alkylamine radical cations to their distonic forms in low-temperature freon matrices

The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed.     

A study of the ethene-ozone reaction with photoelectron spectroscopy: measurement of product branching ratios and atmospheric implications

The ozone-ethene reaction has been investigated at low pressure in a flow-tube interfaced to a u.v. photoelectron spectrometer. Photoelectron spectra recorded as a function of reaction time have been used to estimate partial pressures of the reagents and products, using photoionization cross-sections for selected photoelectron bands of the reagents and products, which have been measured separately. Product yields compare favourably with results of other studies, and the production of oxygen and acetaldehyde have been measured as a function of time for the first time. A reaction scheme developed for the ozone-ethene reaction has been used to simulate the reagents and products as a function of time. The results obtained are in good agreement with the experimental measurements. For each of the observed products, the simulations allow the main reaction (or reactions) for production of that product to be established. The product yields have been used in a global model to estimate their global annual emissions in the atmosphere. Of particular interest are the calculated global annual emissions of formaldehyde (0.96 ± 0.10 Tg) and formic acid, (0.05 ± 0.01 Tg) which are estimated as 0.04% and 0.7% of the total annual emission respectively.     

Assignment of the ferriheme resonances of the high-spin forms of nitrophorins 1 and 4 by 1H NMR spectroscopy: comparison to structural data obtained from X-ray crystallography

In this work, we report the assignment of the majority of the ferriheme resonances of high-spin nitrophorins (NPs) 1 and 4 and compare them to those of NP2, published previously. It is found that the structures of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the histidine imidazole ligand, can be described with good accuracy by NMR techniques and that the angle plot proposed previously for the high-spin form of the NPs (Shokhireva, T. Kh.; Shokhirev, N. V.; Walker, F. A. Biochemistry 2003, 42, 679-693) describes the angle of the effective nodal plane of the axial histidine imidazole in solution. There is an equilibrium between the two heme orientations (A and B), which depends on the heme cavity shape, which can be altered by mutation of amino acids with side chains (phenyl vs tyrosyl) near the potential position where a heme vinyl group would be in one of the isomers. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.     

The search for bishomoaromatic semibullvalenes and barbaralanes: computational evidence of their identification by UV/Vis and IR spectroscopy and prediction of the existence of a blue bishomoaromatic semibullvalene

Time-dependent B3LYP/6-31G calculations have been performed at the optimized C(2) or C(2v) geometries of several substituted semibullvalenes (1(deloc)) and barbaralanes (2(deloc)), to compare the computed vertical electronic excitation energies with the temperature-dependent, long-wavelength absorptions that have been observed in the UV/vis spectra of some of these compounds by Quast and co-workers. The excellent agreement between the calculated vertical excitation energies and the observed values of lambda(max) provides strong support for the identification of the bishomoaromatic species 1(deloc) and 2(deloc) as the source of these absorptions. Furthermore, the CN stretching frequencies, computed for the C(2) geometry of 1,5-dimethyl-2,6-dicyano-4,8-diphenylsemibullvalene (1f(deloc)), fit the low-frequency absorptions seen in the IR spectrum of 1f, thus furnishing independent evidence that bishomoaromatic geometries of semibullvalenes have, in fact, been observed spectroscopically. B3LYP/6-31G calculations predict that 2,6-dicyano-4,8-diphenylsemibullvalene 1c has a C(2) equilibrium geometry (1c(deloc)) and that the long-wavelength UV/vis absorption (lambda(max) = 585 nm) and CN stretching frequencies (2192 and 2194 cm(-1)) computed for 1c(deloc) should serve to identify this bishomoaromatic semibullvalene when it is synthesized.     

DFT studies of Pt/Au bimetallic clusters and their interactions with the CO molecule

Density functional theory (DFT) calculations were performed to study Pt/Au clusters of different size, structure, and composition as well as their interactions with a CO molecule. Among the Pt/Au isomers studied here, the planar structure is the most stable structure in many Pt compositions, although three-dimensional structures become more stable with increasing Pt composition. Furthermore, structures with the Pt atoms surrounded by Au atoms are more stable among homotops. However, these conclusions will be altered if ligands are attached to the Pt/Au bimetallic clusters, as evidenced from the results of CO adsorption. When both Au and Pt sites are exposed, CO adsorption at the Pt site is stronger. If only a Au site is available for CO adsorption, the strongest adsorption occurs at approximately 25% Pt composition, which may correlate with the experimentally observed reactivity of the core-shell structured Pt/Au nanoparticles.     

Cobalt-catalyzed sequential cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives with Grignard reagents and their application to the synthesis of 1,3-diols

Cobalt/N-heterocyclic carbene system or cobalt/diamine combination effectively catalyzes sequential cyclization/cross-coupling reactions of 6-halo-1-hexene derivatives with trialkylsilylmethyl, 1-alkynyl, and aryl Grignard reagents. The sequential cyclization/cross-coupling reactions are applied to the synthesis of 1,3-diols starting from siloxy-tethered 6-halo-1-hexene derivatives.