Stanislav Biskupič

EPR study of photoinduced reduction of nitroso compounds in titanium dioxide suspensions

Abstract The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5N OH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5N OD was established. Additionally, an identical radical C6H5N OH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals ( CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.

The potential pitfalls of using 1,1-diphenyl-2-picrylhydrazyl to characterize antioxidants in mixed water solvents

Approaching living systems, aqueous solutions are appropriate to characterize antioxidants, whereas the frequently used standard 1,1-diphenyl-2-picrylhydrazyl (DPPH) is insoluble in water. Therefore, mixed water–ethanol solvents were investigated using the electron paramagnetic resonance (EPR) spectroscopy. Two forms of DPPH were identified: at higher ethanol ratios a quintet spectrum characteristic of solutions, and at lower ratios, a singlet spectrum typical for solid DPPH, were found. Mixed solvents with 0–50% (v/v) water reproduced the same antioxidant equivalent points well and the reaction rate between DPPH and the antioxidant may increase considerably with increasing water ratios, as demonstrated using vitamin E as an antioxidant. But at still higher water ratios (70–90% (v/v)) the antioxidant activities dropped, since a part of the DPPH in the aggregated form does not react sufficiently with the antioxidants. Characteristics of the most common antioxidants were determined in ethanol or its 50% (v/v) aqueous solution.

X-ray constrained unrestricted Hartree-Fock and Douglas-Kroll-Hess wavefunctions.

The extension of the X-ray constrained (XC) wavefunction approach to open-shell systems using the unrestricted Hartree-Fock formalism is reported. The XC method is also extended to include relativistic effects using the scalar second-order Douglas-Kroll-Hess approach. The relativistic effects on the charge and spin density on two model compounds containing the copper and iron atom are reported. The size of the relativistic effects is investigated in real and reciprocal space; in addition, picture-change effects are investigated and discussed for the isolated Cu atom. It is found that the relativistic terms lead to changes in the densities that are much smaller than those from the X-ray constraint. Nevertheless, the use of the relativistic corrections in the ab initio model always leads to an improvement in the agreement statistics. An interesting result of the unrestricted XC technique is the possibility of obtaining experimentally derived spin densities from X-ray data.

Electron transfer: A primary step in the reactions of sodium hydrosulphide, an H2S/HS− donor

Endogenously produced H2S/HS−, a newly found gasotransmitter, is well represented by NaHS. In deoxygenated media it terminated semi-stable oxidant radicals up to stoichiometric ratios of 1:1. In the presence of oxygen the antioxidant activities of NaHS were impaired considerably due to its competitive reactions with molecular oxygen. The primary reaction steps of NaHS were investigated using two different spin traps, 5,5-dimethylpyrroline-N-oxide and sodium 3,5-dibromo-4-nitrosobenzenesulphonate (DBNBS), in protolytic and aprotic solvents (water and dimethylsulphoxide, DMSO) under argon and oxygen. Sulphhydryl radicals (HS•/S• −) were primarily formed (S• − in water and HS• in DMSO), probably coupled to the formation of superoxide radical anions. The DBNBS spin trap acted also as an electron acceptor and formed its radical anions in the presence of NaHS. Hence, one of the primary steps in the reactions of sulphides is the electron transfer from H2S/HS− species to a suitable acceptor, which may play a fundamental role in their biological functions.

Wigner's (2n + 1) rule in MBPT

Wigners (2n + 1) rule (stating that the 2nth and/or (2n + 1)th order perturbation contributions to a non-degenerate energy can be evaluated from a knowledge of the wavefunction through nth order) is applied to many-body perturbation theory (MBPT). This approach essentially simplifies the calculation of correlation energy to an order higher than the third and reduces the number of required diagrams.

Reactive radical intermediates formed from illuminated nifedipine.

Nifedipine, (1,4-dihydro-2,6,dimethyl-4-(2-nitrophenyl)-3, 5-pyridinedicarboxylic acid dimethyl ester) a calcium channel blocker widely used in treatment of hypertension, is strongly photolabile. This may represent a problem in patients taking nifedipine and in handling of nifedipine samples. Reactive radical intermediates were determined and characterized in the process of nifedipine illumination using EPR spectroscopy. On illumination of nifedipine by daylight or by a mercury lamp, a nitroxide radical, RIIL-NIFNO.X was observed (in the first step), in various solvents like benzene, cyclohexane, methanol, acetonitrile, dimethylsulphoxide, or aqueous suspensions of liposomes. RIIL-NIF represents the nifedipine skeleton centered with phenyl group, and X is an EPR silent substituent. The generation of RIIL-NIFNO.X is coupled with the formation of nitroso compound, RIIL-NIFNO, as characterized by UV-visible spectroscopy. In a further step, RIIL-NIFNO abstracts hydrogen from nifedipine skeleton under the formation of RIIL-NIFNO.H radical. In addition to this, in system containing RIIL-NIFNO and unsaturated lipids, nitroxide radicals RIIL-NIFNO.RLIPIDS are formed probably via a pseudo Diels-Alder mechanism (RLIPIDS represents lipidic skeleton). The unusually easy photochemical activation of nifedipine is probably stimulated by photosensitization of its nitro group interacting with suitably positioned hydrogen or carboxylic methyl ester group from the pyridinyl ring.

On relativistic effects in ground state potential curves of Zn2, Cd2, and Hg2 dimers. A CCSD(T) study

The ground state potential curves of the Zn2, Cd2, and Hg2 dimers calculated at different levels of theory are presented and compared with each other as well as with experimental and other theoretical studies. The calculations at the level of Dirac‐Coulomb Hamiltonian (DCH), 4‐component spin‐free Hamiltonian, nonrelativistic Lévy–Leblond Hamiltonian and at the level of simple Coulombic correction to DCH are presented. The potential curves are calculated in an all‐electron supermolecular approach including the correction to basis set superposition error (BSSE). Electron correlation is treated at the coupled cluster level including single and double excitations and noniterative triple corrections, CCSD(T). In addition, simulations of the temperature dependence of dynamic viscosities in the low‐density limit using the obtained ground state potential curves are presented.

Ab initio calculation of stationary points on the HF2 potential energy surface

Abstract Ground and excited states of an HF 2 system were investigated. Spectroscopic quantities such as dissociation energy, equilibrium bond distances, and harmonic frequencies, assessed from analysis of dissociation fragments (HF and F 2 ), were used as suitability criteria for the given computational approach. Accurate calculations of the stationary points on the HF 2 potential energy surface were performed using DZP and aug-cc-pVTZ basis sets; the exothermicity of the H+F 2 xa0→xa0HF+F reaction was predicted at the multireference single-double configuration interaction (MRCI-SD) level, including the Davidson correction. Our results are compared with previous ab initio calculations, and with experimental data.

Anion radicals of [60]-fullerene and its ortho-quinodimethane adducts generated by cathodic reduction and photochemical electron transfer: an EPR and vis-near-IR study

Abstract [60]-Fullerene and its four ortho-quinodimethane adducts, one of them in oligomeric form, were reduced by three different methods, (a) by cathodic reduction, (b) in photoexcited TiO2 suspension, and (c) quenching photoexcited fullerene by triethylamine donor. The radicals obtained were mainly investigated by EPR and also characterized by vis—near-IR measurements. All three methods of generation gave two identical radical products: the primary radical A, which is a fullerene mono-anion, with EPR spectral parameters gA = 2.0000 and peak-to-peak width ppA = 0.09 mT for [60]-fullerene, ppA = 0.105 for its adducts and ppA = 0.19 mT for the fullerene oligomer. The near-IR band of A at 1077 nm for [60]-fullerene shiftd into the region of 1010 nm for its ortho-quinodimethane adducts. As a consecutive product of A, radical B was found with gB = 2.0006 and ppB = 0.07 mT for all [60]-fullerene adducts. The EPR signal of B converts to A after stopping photogeneration, resulting in a total increase of the spin susceptibility in the form of fullerene mono-anion A. The formation of fullerene di-anion or fullerene associates is considered as an alternative.

The role of fourth-order terms in MBPT calculations of the correlation energy

Abstract The closed-shell version of the Coester-Kummel theory is studied from the standpoint of complete fourth-order calculation of the correlation energy by MBPT. From the example of the LiH and H 2 O molecules it is demonstrated that the largest part of the fourth-order correlation energy is accounted for by “linked” terms T 2 . Contributions from the other “linked” T 1 , T 3 and “unlinked” T 2 2 terms varies for the studied molecules.