Effects of the ionization in the tautomerism of uracil: A reaction electronic flux perspective

The one‐step tautomerization processes of uracil and its radical cation and radical anion have been investigated in the light of the reaction force and reaction electronic flux (REF) formalisms. The relative energies of the different tautomers as well as the corresponding tautomerization barriers have been obtained through the use of the G4 high‐level ab initio method and by means of B3LYP/6‐311+G(3df,2p)//B3LYP/6‐311+G(d, p) calculations. Systematically, the enol radical cations are more stable in relative terms than the neutral, due to the higher ionization energy of the diketo forms with respect to the enolic ones. Conversely, the enol radical anions, with the only exception of the 2‐keto‐N1 anion, are found to be less stable than the neutral. The effects of the ionization are also sizable on the tautomerization barriers although this effect also depends on the particular tautomerization process. The reaction force analysis shows that all reactions are mainly activated through structural rearrangements that initiate the electronic activity. This electronic activity is monitored along the reaction coordinate through the REF that obeys a delicate balance between the acid and basic character of the atoms involved in the hydrogen transfer. © 2015 Wiley Periodicals, Inc.     

Ab initioSCF MO calculations on the reactions of hydroxyl radical with imidazole and monoprotonated imidazole

The addition reactions of hydroxyl radical with imidazole and its protonated form to yield radical adducts have been investigated by ab initio SCF MO methods using STO -3G and 4-31G basis sets. Analogous radical species are of importance in radiation damage to biological systems. Of the possible radical products, the calculations indicate that the allylic species are generally favored energetically over the nonallylic forms. On an energetic basis, the results show that the allylic adducts formed by addition at the C2 and C5 positions are about equally favorable. Although the C5 species is generally identified as the experimentally observed product in aqueous media for both protonated and unprotonated imidazole, some experimental evidence exists indicating the presence of other forms. Our results suggest that this other form is the C2 adduct. The calculations also point to the protonated form of imidazole being less reactive than imidazole, which is in accord with experimental observations.     

Theoretical study of the changes in the electronic structure of benzonitrile accompanying its conversion into a radical anion

The electronic structure of benzonitrile and its radical anion have been investigated at different levels of ab initio MO theory: STO-3G, 6-31G and 6-31G^**. The changes in the electronic structure of the neutral molecule accompanying its conversion into the corresponding radical anion have been estimated. It was established that the radicalization leads to significant changes in the bond lengths with double and triple bond character expressed in the conjugated system. The distribution of the total atomic charges on transition from the neutral molecule to the corresponding radical anion have been investigated using the Mulliken population analysis. The distribution of the odd electron density in the radical anion was estimated at the different basis sets: STO-3G, 6-31G and 6-31G^**. The ab initio calculations suggest that the quinoid structure is preferred for the radical anion.     

Density functional complete study of hydrogen bonding between the water molecule and the hydroxyl radical (H2O · HO)

The hydrogen bonding complexes formed between the H₂O and OH radical have been completely investigated for the first time in this study using density functional theory (DFT). A larger basis set 6‐311++G(2d,2p) has been employed in conjunction with a hybrid density functional method, namely, UB3LYP/6‐311++G(2d,2p). The two degenerate components of the OH radical ²Π ground electronic state give rise to independent states upon interaction with the water molecule, with hydrogen bonding occurring between the oxygen atom of H₂O and the hydrogen atom of the OH radical. Another hydrogen bond occurs between one of the H atoms of H₂O and the O atom of the OH radical. The extensive calculation reveals that there is still more hydrogen bonding form found first in this investigation, in which two or three hydrogen bonds occur at the same time. The optimized geometry parameter and interaction energy for various isomers at the present level of theory was estimated. The infrared (IR) spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. The estimates of the H₂O · OH complex's vibrational modes and predicted IR spectra for these structures are also made. It should be noted that a total of 10 stationary points have been confirmed to be genuine minima and transition states on the potential energy hypersurface of the H₂O · HO system. Among them, four genuine minima were located. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Synthesis of the perfluoroacetyldiisopropylmethyl radical

A stable trifluoroacetylperfluorodiisopropylmethyl radical has been prepared by interaction of the (-fluorosulfonyloxytetrafluoroethyl)perfluorodiisopropylmethyl radical with anhydrous CsF. This reaction is the first example of a chemical transformation of a C-centered radical which is not resonance-stabilized with retention of the radical center.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 226–227, January, 1993.     

Are the radical centers in peptide radical cations mobile? The generation, tautomerism, and dissociation of isomeric alpha-carbon-centered triglycine radical cations in the gas phase

The mobility of the radical center in three isomeric triglycine radical cations[G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(*)GG](+), [GY(*)G](+), and [GGY(*)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (> or = 44.7 kcal/mol), in support of the thesis that isomerically pure [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID spectra of [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.     

Kinetics of the hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with hydroxyl radical: an ab initio study

The hydrogen abstraction reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been investigated by the ab initio molecular orbital theory. The geometries of the reactants, products, and transition states have been optimized at the (U)MP2=full level of theory in conjunction with 6‐311G(d,p) basis functions. Single‐point (U)MP2=full with larger basis set, such as 6‐311G(3d,2p), and QCISD(T)=full/6‐311G(d,p) calculations have also been carried out to observe the effects of basis sets utilized and higher order electron correlation. Three and four reaction channels have been identified for 1,1‐ and 1,2‐difluoroethane, respectively. In the case of 1,1‐difluoroethane, hydrogen abstraction from the α‐carbon has been found to be easier than that from the β‐carbon. The barriers of the four reaction channels for 1,2‐difluoroethane are close to each other. Weak hydrogen bonding interactions have been observed between hydroxyl hydrogen and a fluorine atom in the transition states. Rate constants for the reactions of 1,1‐ and 1,2‐difluoroethane with the OH radical have been calculated using the standard transition state theory and found to be in good agreement with the experimental results. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1305–1318, 2000     

Stable Subporphyrin meso-Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

Most aminyl radicals studied so far are resonance-stabilized by neighboring heteroatoms, and those without such stabilization are usually short-lived. We report herein that subporphyrin meso-2,4,6-trichlorophenylaminyl radicals and a bis(5-subporphyrinyl)aminyl radical are fairly stable under ambient conditions without such stabilization. The subporphyrin meso-2,4,6-trichlorophenylaminyl radical crystal structure displays a characteristically short C_meso−N bond and a perpendicular arrangement of the meso-arylamino group. The stabilities of these radicals have been ascribed to extensive spin delocalization over the subporphyrin π-electronic network as well as steric protection around the aminyl radical center.     

Consecutive Photoinduced Electron Transfer (conPET): The Mechanism of the Photocatalyst Rhodamine 6G

The dye rhodamine 6G can act as a photocatalyst through photoinduced electron transfer. After electronic excitation with green light, rhodamine 6G takes an electron from an electron donor, such as N,N-diisopropylethylamine, and forms the rhodamine 6G radical. This radical has a reduction potential of around −0.90 V and can split phenyl iodide into iodine anions and phenyl radicals. Recently, it has been reported that photoexcitation of the radical at 420 nm splits aryl bromides into bromide anions and aryl radicals. This requires an increase in reduction potential, hence the electronically excited rhodamine 6G radical was proposed as the reducing agent. Here, we present a study of the mechanism of the formation and photoreactions of the rhodamine 6G radical by transient absorption spectroscopy in the time range from femtoseconds to minutes in combination with quantum chemical calculations. We conclude that one photon of 540 nm light produces two rhodamine 6G radicals. The lifetime of the photoexcited radicals of around 350 fs is too short to allow diffusion-controlled interaction with a substrate. A fraction of the excited radicals ionize spontaneously, presumably producing solvated electrons. This decay produces hot rhodamine 6G and hot rhodamine 6G radicals, which cool with a time constant of around 10 ps. In the absence of a substrate, the ejected electrons recombine with rhodamine 6G and recover the radical on a timescale of nanoseconds. Photocatalytic reactions occur only upon excitation of the rhodamine 6G radical, and due to its short excited-state lifetime, the electron transfer to the substrate probably takes place through the generation of solvated electrons as an additional step in the proposed photochemical mechanism.     

Restriction of Spin Probe Motions in Polymer Composites Due to Chemical Factors

Abstract

Hyperfine splitting constants of the nitroxyl radical, with and without hydrogen bonds to the surrounding molecules, have been calculated using the UHF method on a 6-31G* base. In polyethylene filled with silica, hydrogen bonds are formed between nitroxyl radicals and —OH groups of the filler. The formation of hydrogen bonds leads to a change in the A _zz value from 3.33 mT for an isolated nitroxyl radical to 3.83 mT for a radical with a hydrogen bond. The relevant values as measured experimentally are 3.4 and 4.0 mT, respectively. The same procedure was used to calculate the theoretical A _zz value for a nitroxyl radical interacting with polyamide via a hydrogen bond. The value was found to be 3.63 mT (experimental value = 3.6 mT). Hydrogen bond formation results in a restricted motion of the nitroxyl radical in a polymeric medium.     

Stable Subporphyrin meso‐Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

Most aminyl radicals studied so far are resonance‐stabilized by neighboring heteroatoms, and those without such stabilization are usually short‐lived. We report herein that subporphyrin meso ‐2,4,6‐trichlorophenylaminyl radicals and a bis(5‐subporphyrinyl)aminyl radical are fairly stable under ambient conditions without such stabilization. The subporphyrin meso ‐2,4,6‐trichlorophenylaminyl radical crystal structure displays a characteristically short C_meso −N bond and a perpendicular arrangement of the meso ‐arylamino group. The stabilities of these radicals have been ascribed to extensive spin delocalization over the subporphyrin π‐electronic network as well as steric protection around the aminyl radical center.     

Transition states for deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of acrylonitrile

The geometries and energetics of transition states (TS) for radical deactivation reactions, including competitive combination and disproportionation reactions, have been studied for the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐mediated free‐radical polymerization of acrylonitrile with quantum mechanical calculations at the DFT/UB3‐LYP/6‐311+G(3df,2p)//(U)AM1 level of theory (where DFT is density functional theory, AM1 is Austin model 1, and UAM1 is unrestricted Austin model 1). A method providing reasonable starting geometries for an effective search for TS between the TEMPO radical and 1‐cyanopropyl radical mimicking the growing polyacrylonitrile macroradical is shown. For the hydrogen atom abstraction reaction by the TEMPO radical from the 1‐cyanopropyl radical, practically one TS has been found, whereas for the combination reaction of the radicals, several TS have been found, mainly differing in out‐of‐plane angle α of the N? O bond in the TEMPO structure. α in the TS is correlated with the activation energy, ΔE, determined from the single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 level for the combination reaction of CH₃AN· with the TEMPO radical. The theoretical activation energy for the coupling reaction from DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 calculations has been estimated to be 11.6 kcal mol^?1, that is, only about 4.5 times smaller than ΔE for the disproportionation reaction obtained with the DFT UB3‐LYP/6‐311+G(3df, 2p)//(U)AM1 approach. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 914–927, 2006     

The theoretical reaction path for the cation radical vinylcyclobutane rearrangement: A concerted SR path

Ab initio reaction path calculations for the cation radical vinylcyclobutane rearrangement at the MP2/ 6-31G*//3-21G level reveal a concerted, sr reaction path with an activation energy of 9.4 kcal/mol. The vinylcyclobutane cation radical itself, at both the MP2 and MP3 levels of theory has predominant olefin cation radical character but with modest stretching of one of the adjacent ring carbon—carbon bonds.     

ESR single-crystal study of radical and radical-pair formation in some γ-irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low-temperature technique. After irradiation with γ-rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven-line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔM_s = 1 and ΔM_s = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five-line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔM_s = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ-radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.     

A theoretical study on the structure and properties of phenothiazine derivatives and their radical cations

Semiempirical CNDO, AM1, PM3 and ab initio HF/STO-3G, HF/3-21G(d), and HF/6-31(d) methods were employed in the geometry optimization of the phenothiazine and the corresponding radical cation. The results obtained from the PM3 performances were as good as those from the ab initio calculations in the structure optimization of both phenothiazine and phenothiazine radical cation. The PM3 method was used to optimize the structures of a series of N-substituted phenothiazine derivatives and their radical cations. The PM3-optimized results were then analyzed with the ab initio calculation at the 6-311G(d,p) level, which yielded the total energy, frontier molecular orbitals, dipole moments, and charge and spin density distributions of the phenothiazine derivatives and their radical cations.     

Theoretical Study on Reactivity of Electron Transfer in Model—System of Oxidation of α—Amino Carbon—centered Radical by O2

Electron transfer reaction between a simplified model model molecule of α－amino carbon-centered radical and O2 has studied with ab initio calculations at the MP2/6-31 G^**//UHF/6-31 G^** level,The reactant complex and the ion pair complex have been optimized and employed to perform calculation of the reaction heat and the reorganization energy,Solvent effects have been considered by applyning the conductor-like screening model,Theoretical results show that the highly endothermic charge separation process ,in which one electron transfers from the α－amino carbon-centered radical to O2,so as to form an ion pair complex,is difficult to occur in gas-phase,By apply-ing an external electronic field to prepare the charge-locallized molecular orbitals,the charge-separated state has been obtained using the initial-guess-induced self-consistent field technique,The theoretical investigations indicate that the solvent effect in the process of the oxidation of α-animo carbon-centered radical by O2 is remarkable.From the rate constant estima-tion ,it can be predicted that the oxidation of the model donor molecule by O2 can proceed,but not very fast.A peroxyl radi-cal compound has been found to be a competitive intermediate in the oxidation process.     

The mechanism of the electrochemical reduction of 5-nitrouracil

The electrochemical reduction of 5-nitrouracil (5NU) has been investigated on Ag-Hg cathode in DMSO by cyclic voltammetry and controlled potential electrolysis, in combination with UV and ESR spectroscopies. The results showed that 5NU proceeds in a four-electron reduction and simultaneously participates in the reaction as a proton donor. At a more negative potential, the 5NU anion (5NU^?) thus formed is reduced further through a one-electron process to a dianion whose ESR parameters have been determined as a_N = 14.6 G, a_H = 5.2 G and g = 2.005. The radical decay process might be conjectured through protonization by accepting protons from 5NU, of which the rate constant k₁ = 52 M^?1·s^?1.     

Electrospray-Ionization Mass Spectrometry: Detection of a radical cation present in solution: New results on the chemistry of (tetrahydropteridinone)-metal complexes

This paper marks the first reported detection of radical cations by Electrospray-Ionization Mass Spectrometry (ESI-MS). Electron Spin Resonance (ESR) measurements have proven that the detected radical cation existed already in solution and has not been generated by the electrospray ionization technique. However, we observed that the radical cation can be generated by changes in the ionization conditions. A molar mixture of 2-amino-5,6,7,8-tetrahydro-5-methylpterin-4(4H)-one dihydrochloride ( = 5,6,7,8-tetrahydro-N(5)-methylpterin-2 HCl, N(5)-MTHP-2 HCl), and tris(pentane-2,4-dionato)iron(III) in MeCN at pH 2–3 leads to the formation of a [bis(pentane-2,4-dionato)(2-amino-5,6,7,8-tetrahydro-5-methylpteridin-4 (4H)-one)]iron complex ( = [bis(pentane-2,4-dionato) (5,6,7,8-tetrahydro-N(5)-methylpteridin)]iron complex) which can be detected by ESI-MS. The results suggest that this complex might be an Fe^II radical cation, which could possibly be a suitable model complex for the active center of the phenylalanine hydroxylase. In the same solution, the stable radical cation of N(5)-MTHP is identified by ESI-MS and ESR.     

Substituent Effects on the C-H Bond Dissociation Energy of Toluene. A Density Functional Study

The bond dissociation energies of the benzylic C-H bond of a series of 16 para-substituted toluene compounds (p-X-C(6)H(4)CH(3)) have been calculated with the density functional method (BLYP/6-31G). The calculated substituent effects correlate well with experimental rates of dimerization of para-substituted alpha,beta,beta-trifluorostyrenes and rearrangement of methylenearylcyclopropanes. Both electron-donating and electron-withdrawing groups reduce the bond dissociation energy (BDE) of the benzylic C-H bond because both groups cause spin delocalization from the benzylic radical center. The calculated spin density variations at the benzylic radical centers correlate well with both the ESR hyperfine coupling constants determined by Arnold et al. and the calculated radical effects of the substituents. The relative radical stabilities are mainly determined by the spin delocalization effect of the substituents, and polar effect of the substituents are not important in the current situation. The ground state effect is also found to influence the C-H BDE.     

Theoretical study of the CCl3 radical

Employing the STO-3G and 4–31 G basis sets within the unrestricted Hartree-Fock method the equilibrium structure of the CCl₃ radical was calculated. The calculations predict the radical to be non-planar with both basis sets used. Using the UHF optimized geometries the SWX_α calculations have been performed to calculate energy levels, ionization potentials and electron affinities of CCl₃.