Sergej Naumov

Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches

Abstract The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n -chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory (DTF) B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned.

Vacuum‐UV Irradiation‐Based Formation of Methyl‐Si‐O‐Si Networks from Poly(1,1‐Dimethylsilazane‐co‐1‐methylsilazane)

The vacuum-UV (VUV)-induced conversion of commercially available poly(1,1-dimethylsilazane-co-1-methylsilazane) into methyl-Si-O-Si networks was studied using UV sources at wavelengths around 172, 185, and 222 nm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS), X-ray photo electron spectroscopy (XPS), and Fourier transform infrared (FTIR) measurements, as well as kinetic investigations, were carried out to elucidate the degradation process. First-order kinetics were found for the photolytically induced decomposition of the Si-NH-Si network, the subsequent formation of the methyl-Si-O-Si network and the concomitant degradation of the Si-CH(3) bond, which were additionally independent of the photon energy above a threshold of about 5.5 eV (225 nm). The kinetics of these processes were, however, dependent on the dose actually absorbed by the layer and, in the case of Si-O-Si formation, additionally on the oxygen concentration. The release of ammonia and methane accompanied the conversion process. Quantum-chemical calculations on methyl substituted cyclotetrasilazanes as model compounds substantiate the suggested reaction scheme. Layers <100 nm in thickness based on mixtures of poly(1,1-dimethylsilazane-co-1-methylsilazane) and perhydropolysilazane (PHPS) were coated onto polyethylene terephthalate (PET) foils by a continuous roll to roll process and cured by VUV irradiation by using wavelengths <200 nm and investigated for their O(2) and water vapor-barrier properties. It was found that the resulting layers displayed oxygen and water vapor transmission rates (OTR and WVTR, respectively) of <1 cm(3) m(-2) d(-1) bar(-1) and <4 g m(-2) d(-1), respectively.

Encounter geometry determines product characteristics of electron transfer from 4-hydroxythiophenol to n-butyl chloride radical cations

Abstract The electron transfer reaction between the n -butyl chloride parent ion and 4-thiophenol was studied using pulse radiolysis in solutions of 4-thiophenol in n -butyl chloride. It was found to have a diffusion-controlled rate constant of 1.5×10 10 dm 3 mol −1 s −1 and to involve contributions from all functional groups, i.e. –SH, –OH and the aromatic ring. Consequently, thiyl and phenoxyl radicals and 4-hydroxythiophenol radical cations were observed as direct products of this ion–molecule reaction. This unexpected reaction behavior could be explained by the hypothesis that the encounter geometry of the reaction partners determines the product characteristics.

Electron transfer from phenolic compounds to parent n-butyl chloride radical cations—a quantum chemical study of product transient formation and stability

Abstract The Density Functional Theory (DFT) B3LYP and the semiempirical PM3 quantum chemical methods were used to describe the mechanism of electron transfer from aromatic solutes to n -butyl chloride radical cations. The influence of the electronic parameters, the distribution of the Mulliken charges for the radical cations, and the equilibrium geometries of the ground state and the radical cation on the chemical reactivity were analyzed. Pulse radiolysis experiments had shown that phenol radical cations and phenoxyl radicals appear as direct products of the electron transfer, indicating a product distribution determined by encounter geometry. The quantum chemical approaches introduced enabled both the geometry, the energy and the molecular orbitals of an encounter complex to be calculated, explaining the rapid phenoxyl radical formation by a second reaction channel. The experimentally observed lifetimes of the phenol radical cations were correlated with quantum chemical calculated spin and Mulliken charge distributions. It was found that the influence of the solvent type approximated by the Onsager reaction field model and the DFT method showed no essential differences in the atomic spin and Mulliken charge distributions. The semiempirical PM3-calculated parameters tally well with those observed at the B3LYP/6-31G(d) level. Furthermore, a reliable trend correlation between the atomic spin density on the oxygen atom and the lifetime of the radical cations was found. The configuration interaction with single excitation approximation at the semiempirical PM3 level was applied to calculate the low-energy electronic transitions of the aromatic transients. Finally, the electronic transitions are calculated for both the radical cations and the deprotonated neutral radicals.

Distonic dimer radical cation of 2,3-dihydrofuran: Quantum chemical calculations and low-temperature EPR results

Abstract The distonic dimer radical cation of the 2,3-dihydrofuran was radiolytically generated in Freon matrix and studied by low-temperature EPR spectroscopy. The structure of the dimer radical cation was also investigated using different quantum chemical methods. The BH&HLYP, similarly as ab initio HF and MP2 methods, predict in agreement with the experiment a distonic dimer radical cation, with strongly separated spin and charge. The widely used DFT B3LYP and B3PW91 methods fail in this case and lead to a wrong (delocalized) structure.

EPR study of methyl and ethyl acrylate radical cations and their transformations in low-temperature matrices

Using low-temperature EPR spectroscopy, transformations of radical cations of methyl (MeA) and ethyl acrylate (EtA) radiolytically generated in freon (CFCl2CF2Cl, CF3CCl3 and CFCl3) and in argon matrices were investigated. The assignment of the EPR spectra was made with the help of partially deuterated acrylates, namely ethyl-d2 (EtA-d2) and ethyl-d5 acrylates (EtA-d5) and methyl-d3 acrylate (MeA-d3). In addition, quantum chemical calculations were performed to obtain information on the electronic structure, the hfs constants and energy levels of the transient species observed. The primary radical cations show broad singlet spectra (ΔHpp ≈ 1.4 mT) and transform quickly by hydrogen transfer from the ester group to the carbonyl oxygen leading to distonic radical cations. This transformation can be observed directly at 77 K in the case of the MeA leading to CH2CH–(COH)+–OCH2˙ (a(2Hα) = 2.24 mT, first-order rate constant in CF3CCl3: 1 h−1). The primary cation of EtA could be trapped below 40 K in a CFCl3 matrix only. In all the freon matrices studied, the intramolecular rearrangement of this cation yields the CH2CH–(COH)+–OCH2CH2˙ terminal-type distonic radical cation. The conformation of the latter species in polycrystalline CFCl3 and CF3CCl3 matrices corresponds to the calculated minimum close to the transition state geometry. In the case of CF3CCl3 matrix, warming the samples to temperatures above 130 K results in the simultaneous formation of two new species, which were assigned to six- and five-member ring structures (a(H)/mT: 2.36 (Hα), 5.13 (Hβ1), 1.9 (Hβ2) and 2.27 (2Hα), 2.6 (Hβ), respectively) formed by intramolecular cycloaddition of the terminal radical to the vinyl double bond. The formation of the propagating radical –CH2–CH˙–R due to an ion-molecule reaction is observed in CFCl2CF2Cl at temperatures above 98 K; this process was also detected in other freons under the conditions of matrix softening. The primary radical cation of EtA is not trapped in an argon matrix even at 16 K due to the realisation of the “high-energy” reaction paths yielding methyl radicals and, probably, rearrangement products.

Hydride Transfer: A Dominating Reaction of Ozone with Tertiary Butanol and Formate Ion in Aqueous Solution

This article provides evidences that hydride transfer is an important primary step in ozone reactions of formate and tertiary butanol in aqueous media. In both systems, one argument is the fact that the free hydroxyl radical yields are relative low ((40 ± 4)% and (7 ± 0.8)% for formate and tertiary butanol, respectively). Another hint is the high exergonicity of these reactions: ΔG = –249 kJ mol−1 for formate/ozone system and ΔG = –114 kJ mol−1 for hydride transfer followed by a methyl shift in the reaction between tertiary butanol and ozone. In addition, the main product of tertiary butanol ozonolysis is butan-2-one [(89 ± 3)%], a compound that is formed only via hydride transfer. For the reaction of ozone with formate an activation energy of (54.6 ± 1.2) kJ mol−1 and a pre-exponential term of (2.5 ± 1.2) × 1011 were determined (in the presence of tertiary butanol as •OH scavenger) whereas for tertiary butanol the two activation parameters were (68.7 ± 1.9) kJ mol−1 and (2.0 ± 1.5) × 109, respectively.

Indication of molecular oscillations during free electron transfer: reaction of butyl chloride parent ions with benzyltrimethylsilanes

Molecular oscillations are accompanied with electron density fluctuations mainly of the higher occupied orbitals. This is particularly marked for rotational motions of large substituents of aromatic molecules. Hence, the extremely rapid and non-adiabatic free electron transfer from benzyltrimethylsilanes to n-butyl chloride parent radical cations affords the existence of different short-living molecular conformers. This is indicated by a characteristic pattern of transient products including metastable radical cations and fragment radicals derived from a dissociative and very unstable ionisation product.

Antithetical product situation in the femtosecond and nanosecond photoionization of sterically hindered phenols in non-protic solvents

Abstract The photoionization of sterically hindered phenols in acetonitrile or n-butyl chloride solution gives apparently contradictory results performing biphotonic ionization with femtosecond as well as nanosecond UV lasers: in the first case phenol radical cations were found whereas in the latter one only phenoxyl radicals were detected. This is explained by non-resonant and resonant ionization pathways. Furthermore, the deprotonation rate of the phenol radical cations in acetonitrile in the presence of water traces could be determined as to be diffusion controlled ((3 to 7 )×10 9 dm 3 mol −1 s −1 ) which could not be observed till now because of its extreme pKa value of ≪−1.

Discrete ionization of two different short-living conformers of selenophenol by rapid free electron transfer to solvent parent radical cations

Abstract In the pulse radiolysis of solutions of selenophenol in n -butyl chloride, selenophenol radical cations and phenylselenyl radicals are generated as direct products of the ion–molecule reaction between the solvent parent radical cations and selenophenol. This effect is explained with an electron-transfer phenomenon where in each encounter situation the electron jump proceeds so extremely rapid that different transient conformers of the scavenger molecule must be taken into account, differing in the phases of the molecular torsion vibration of the C–SeH bond. Quantum-chemical calculations resulted in a very low activation energy of less than 1 kcal mol −1 for the rotation of this bond and a favored geometry where the Se–H group is twisted against the aromatic moiety by an angle of 90°.