Thermochemistry of arylselanyl radicals and the pertinent ions in acetonitrile

Reduction and oxidation potentials of a series of parasubstituted phenylselanyl radicals, XC(6)H(4)Se(*), have been measured using photomodulated voltammetry in acetonitrile. The thermodynamic significance of these data was substantiated through a study of the oxidation process of the pertinent selenolates in linear sweep voltammetry. Both the reduction and the oxidation potentials correlate linearly with the Hammett substituent coefficients sigma and sigma(+) leading in the latter case to slopes, rho(+), of 2.5 and 3.8, respectively. Through comparison of these slopes with those published previously for the O- and S-centered analogues, it is revealed that the pi-interaction becomes progressively smaller as the size of the radical center increases in the order O, S, and Se. Solvation energies of the pertinent selenolates and selanylium ions have been extracted from thermochemical cycles incorporating the measured electrode potentials for XC(6)H(4)Se(*) as well as electron affinities and ionization potentials obtained from theoretical calculations at the B3LYP/6-31+G(d) level. The extracted data show the expected overall substituent dependency for both kinds of ions; that is, the absolute value of the solvation energy decreases as the charge becomes more delocalized. The data have also been compared with solvation energies computed using the polarizable continuum model (PCM). Interestingly, we find that, while the model seems to work well for selenolates, it underestimates the solvation of selanylium ions in acetonitrile by as much as 25 kcal mol(-)(1). These large deviations are ascribed to the fact that the PCM method does not take specific solvent effects into account as it treats the solvent as a continuum described solely by its dielectric constant. Gas-phase calculations show that the arylselanylium ions can coordinate covalently to one or two molecules of acetonitrile in strong Ritter-type adducts. When this strong interaction is included in the solvation energy calculations by means of a combined supermolecule and PCM approach, the experimental data are reproduced within a few kcal mol(-)(1). Although the energy difference of the singlet and triplet spin states of the arylselanylium ions is small for the gas-phase structures, the singlet cation is undoubtedly the dominating species in solution because the triplet cation lacks the ability to form covalent bonds.     

The heat of formation of the acetonyl radical VLPP studies of acetonyl bromide,isopropenylmethylether, and hexanedione-2,5 in a search for acetonyl stabilization energy

The decomposition of acetonyl bromide, isopropenylmethylether, and hexanedione-2,5 was studied using the very-low-pressure pyrolysis (VLPP) technique. The acetonyl radical is a product of each reaction. Arrhenius parameters determined are or acetonyl bromide ← acetonyl + Br: and for isopropenylmethylether ← acetonyl + CH₃: These lead to values of acetonyl stabilization energy (SE) of 0.8 and ?4.0 kcal/mol, respectively. Comparison of the pyrolyses of hexanedione-2,5 and 2,5-dimethylhexane indicate a value of SE ～ 2 kcal/mol. The total of these results is taken, along with previous work, to indicate that 0 ? SE ? 2 kcal/mol.     

Photoinitiated copolymerization of acetonyl methacrylate

Synthesis of two methacrylate monomers derivatives of carbonyl compounds: vanillin and acetone are presented. The obtained acetonyl methacrylate was used for copolymerization with methyl methacrylate. Copolymerizations were carried out in the presence of commercially available photoinitiator Irgacure 651. Thermal and mechanical properties of the copolymers with different concentration of acetonyl methacrylate were studied.     

Thermochemistry of chlorine‐containing hydrocarbons related to waste combustion

The thermochemistry of 25 chlorinated compounds potentially involved in chemical reactions related to waste combustion has been calculated using ab initio methods. Some of these species have never been reported in the literature. To check the validity of our calculations, 11 additional brominated analogues were also calculated. The thermochemical properties of these compounds were used in reaction kinetic models designed to estimate the performance of waste incinerators. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 113–122, 2009     

Rearrangements of free radicals x : C7H7 - radicals and related systems

7-Norbornadienyl radical rearranges in matrix to tropylium radical. Deuterated and cyano substituted bicyclo(3.2.0)heptadienyl radicals do not undergo 1.2-vinyl shifts prior to electrocyclic ring opening.     

Thermochemistry of Ketene

A new value, -100±10 kJ mol^{- 1}, was obtained for the enthalpy of formation of gaseous ketene, H _f ⁰(g)(CH₂ = C = O), from the data obtained by the authors in combination with certain published experimental and calculation data. The suggested value is considerably lower than the value accepted in the literature, -48 kJ mol^{- 1}.     

Formation of free radicals in photoirradiated cellulose and related compounds

Free-radical formation in various modifications of celluloses, namely, wood cellulose I, II, III, and IV, rayon cellulose, amorphous cellulose, cotton linters, and absorbent cotton, irradiated with ultraviolet light, has been studied by means of ESR spectroscopy at 77°K. Various types of free radicals were generated from these samples. The line shapes and the signal intensities of the ESR spectra depended greatly upon the degree of crystallinity, the lattice type, and the arrangement of molecules in cellulose. The effect of degree of crystallinity and the amount of sensitizer (Fe³⁺) absorbed revealed that photochemical reactions in cellulose occurred exclusively in the amorphous regions of the polymer. Free radicals formed in these samples behaved distinctively upon a warm-up process. Phenomena of radical migration and formation of new radicals were observed from the sensitized samples of rayon cellulose and amorphous cellulose.     

Thermochemistry of lignin

Thermochemical reactions occurring in various stages of structural transformations of native lignin in its thermal treatment in a wide temperature range are considered and classified. Attention is given to the initial state of lignin in its primary isolation without heating. The terminology of lignin products, used in the literature, is put in order to a certain extent. The thermochemical reactions in which lignins are transformed in processing of raw wood materials and the structure of isolated lignins undergoes changes in the course of the target thermal treatment are differentiated. The applied aspect of the directed thermochemical synthesis of new lignin-based low- and high-molecular-mass compounds is discussed.     

Thermochemistry of explosives

The kinetics constants for the decomposition reaction of an explosive can be used to calculate the lowest temperature (critical temperature, T_m) at which any specific size and shape of explosive can self heat to explosion; however, the accuracy of the calculation is in doubt without an independent experimental determination of a critical temperature for a known size and shape of the explosive. A method is presented for the experimental determination of critical temperatures on a routine basis, and it is shown that agreement between calculated and experimental values is excellent for most common explosives.     

Thermochemistry of paracetamol

Combustion calorimetry, Calvet-drop sublimation calorimetry, and the Knudsen effusion method were used to determine the standard (p ^o = 0.1 MPa) molar enthalpies of formation of monoclinic (form I) and gaseous paracetamol, at T = 298.15 K: \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text cr I ) = - ( 4 10.4 ±1. 3)\text kJ  \textmol ^{- 1} \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ cr I}}} \right) = - ( 4 10.4 \pm 1. 3){\text{ kJ}}\;{\text{mol}}^{ - 1} and \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text g ) = - ( 2 80.5 ±1. 9)\text kJ  \textmol ^{- 1} . \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ g}}} \right) = - ( 2 80.5 \pm 1. 9){\text{ kJ}}\;{\text{mol}}^{ - 1} . From the obtained \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text cr I ) \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ cr I}}} \right) value and published data, it was also possible to derive the standard molar enthalpies of formation of the two other known polymorphs of paracetamol (forms II and III), at 298.15 K: \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text crII ) = - ( 40 8.4 ±1. 3)\text kJ  \textmol ^{- 1} \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ crII}}} \right) = - ( 40 8.4 \pm 1. 3){\text{ kJ}}\;{\text{mol}}^{ - 1} and \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text crIII ) = - ( 40 7.4 ±1. 3)\text kJ  \textmol ^{- 1} . \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ crIII}}} \right) = - ( 40 7.4 \pm 1. 3){\text{ kJ}}\;{\text{mol}}^{ - 1} . The proposed \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textO ₂ \textN,\text g ) \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{O}}_{ 2} {\text{N}},{\text{ g}}} \right) value, together with the experimental enthalpies of formation of acetophenone and 4′-hydroxyacetophenone, taken from the literature, and a re-evaluated enthalpy of formation of acetanilide, \Updelta_\textf H_\textm^\texto ( \textC ₈ \textH ₉ \textON,\text g ) = - ( 10 9. 2 ± 2. 2)\text kJ  \textmol ^{- 1} , \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} \left( {{\text{C}}_{ 8} {\text{H}}_{ 9} {\text{ON}},{\text{ g}}} \right) = - ( 10 9. 2\,\pm\,2. 2){\text{ kJ}}\;{\text{mol}}^{ - 1} , were used to assess the predictions of the B3LYP/cc-pVTZ and CBS-QB3 methods for the enthalpy of a isodesmic and isogyric reaction involving those species. This test supported the reliability of the theoretical methods, and indicated a good thermodynamic consistency between the \Updelta_\textf H_\textm^\texto \Updelta_{\text{f}} H_{\text{m}}^{\text{o}} (C₈H₉O₂N, g) value obtained in this study and the remaining experimental data used in the \Updelta_\textr H_\textm^\texto \Updelta_{\text{r}} H_{\text{m}}^{\text{o}} calculation. It also led to the conclusion that the presently recommended enthalpy of formation of gaseous acetanilide in Cox and Pilcher and Pedley’s compilations should be corrected by ~20 kJ mol⁻¹.     

Synthesis and HIV-1 RNase H-activity of new alizarin acetonyl derivatives

Alkylation of 1,2-dihydroxyanthraquinone (alizarin) by α-bromoacetone was studied and its β-acetonyl derivative was chemically modified. The composition and structure of the products were confirmed by elemental analysis; UV, IR, PMR, and ¹³C NMR spectroscopy; and mass spectrometry. The synthesized derivatives were tested as inhibitors of HIV-1 RNase H.     

EPR of free radicals formed from 3-hydroxyesculetin and related derivatives

EPR spectroscopy of 3-hydroxyesculetin (1, solid and in solution) and of the radicals formed during the aerobic oxidation of alkaline solutions of 1 and related compounds was investigated. 1 in the solid state was studied by pulsed EPR experiments and showed a radical character. The aerobic oxidation of alkaline solutions of 1 was also followed by EPR spectroscopy. A ring contraction occurred leading to a 5,6-dihydroxybenzofuran-2-carboxylate radical. The autoxidation of an alkaline solution of (Z)-3-(3,4-dihydroxyphenyl)-2-hydroxypropenoic acid allowed the observation of a spectrum attributable to 5,6-dihydroxybenzofuran-3-one-2-carboxylate radical. The formation mechanisms of these radicals are discussed.     

Electron spin resonance spectra of some phosphonyl radicals and related species

The isotropic ESR spectra of a number of phosphonyl radicals (X₂$\text{P}$O), the dimethylphosphinyl radical, and the phosphoranyl radical (MeO)₃$\text{P}$OBu-t, are described, and accurate values of the phosphorus hyperfine splittings and g-factors are reported. For X₂$\text{P}$O, the value of a(P) increases and the g-factor decreases as the electronegativity of X increases. There is a linear relationship between a(P) for X₂$\text{P}$O and ¹J(PH) for X₂P(O)H, but the same relationship does not hold for Me₂P- and Me₂PH. The spectrum of the di-n-hexylphosphonyl radical shows coupling to two pairs of α-methylene protons, and this non-equivalence is attributed to the pyramidal structure of the phosphonyl radical.