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A study is presented to demonstrate that two tautomers of one-electron-oxidized guanine are produced by the protonation of an 8-haloguanine electron adduct. The reaction of hydrated electrons [e.sub.aq.sup.-] with 8-haloguanosine or 8-halo-2'-deoxyguanosine produces the first observable transient species that decay unimolecularly to give the one-electron oxidized guanosine or 2'deoxyguanosine.

Ostwald ripening in which the size of nanoparticles increases at the expense of the smaller ones dues to atomization and diffusion between particles could be caused by the process of refluxing the nanoparticles during the Suzuki reaction. The nanoparticles increase in size during the beginning of the reaction and level off toward the end of the first cycle.

The potential cycloaddition reactions between cyclopentadiene and cycloheptariene are explored theoretically. B3LYP/6-31G* was used to locate the transition states, intermediates, and products for concerted pathways and stepwise pathways passing through diradical intermediates.

The experimental and computational study presented demonstrates that hydrogen bond donors can significantly change the spectroscopic and thermochemical properties of phenoxyl radicals. A strong H-bond donor such as hexafluoropropanol produces large variations of the hyperfine splitting constants indicative of a large redistribution of electron spin density which can be accounted for by the increased importance of the mesomeric structures (E-H) with electric charge separation.

The chemistry of model C5' DNA radicals are studied to determine the structural and reactivity effects in the formation of 5',8-cyclonucleoside. For this purpose, pulse radiolysis, product studies and density functional theory calculations are used.

Research is presented concerning the properties of the 2'-deoxyuridin-1'-yl radical and the thyminylmethyl radical. The use of bulk photolysis and laser flash photolysis to produce the two radicals is discussed.

Research was conducted to examine the impact of diffuse functions on geometry and isotropic hyperfine splitting (hfs) constants of radical anions at the UQCISD/6-311G(d,p) level. Unreliable geometry and hfs constants for radical anions that are kinetically unstable in the gas phase are provided by the utilization of standard diffuse functions. A proposed method for computing reliable hfs constants in radical cations is presented.

Research was conducted to examine the conformation and the electronic structure of various alpha-heterosubstituted acetone oximes using a multidisciplinary approach based on ultraviolet photoelectron, electron transmission and dissociative electron attachment spectroscopies and fully optimized ab initio 6-31G** and MP2/6-31G calculations. Results indicate that the characteristics of the acetone oximes are mainly governed by the mixing between the orbitals localized at the X and C = N fragments and by electrostatic interactions between the electronegative atoms and hydrogen.

The 2- and 3-furyl and the 2- and 3-thienyl forms of arylhydroxymethane undergo conformational analysis by electron paramagnetic resonance spectroscopy of their photoinduced radicals. The study shows that the radicals experience restricted rotation about the Ar-C(sub alpha) bond, leading to rotamers with unequal stabilities. In the 2-substituted forms, the E-isomer of the furyl derivatives and the Z-isomer of the thienyl derivatives prove to be the more stable rotamers. An intramolecular hydrogen bond in the furyls accounts for the higher rotation barrier in the Z conformer.

Ab initio molecular orbital calculations and electron configuration analysis revealed that beta-substituted ethyl radicals bearing second- and third-row elements have structural parameters resembling those of classical radicals with nonplanar radical centers. The presence of second-row elements on the ethyl substituents formed a nearly free C(alpha)-C(beta) bond rotation which facilitated the formation of a gauche structure. In addition, third-row elements produced a sizable rotation barrier which limited bond motion resulting in an eclipsed conformation.

Research has been conducted on the [3,3]-sigmatropic rearrangement. The radical-anion mechanism for this rearrangement has been developed and discussed.

The one-electron oxidation and one-electron reduction reactions of 8-oxo-7,8-dihydroguanine (8-oxo-G) and 8-aminoguanine (8-N[H.sub.2]-G) derivatives are examined. The one-electron reduction of 8-azidoguanine (8-[N.sub.3]-G) has produced the same transient of one-electron oxidation of 8-N[H.sub.2]-G, which has represented another example of generation of one-electron oxidized guanine derivatives under reducing conditions.

A detailed chemical radiation study of 8-bromo-2-aminoadenosine (8-Br-2AA) with reducing species and 2-aminoadenosine (2AA) with oxidizing species, in aqueous solution at various pH values is presented.

The effect of water on the O-H bond dissociation enthalpy (BDE) of para-substituted phenols using DFT calculations is examined. The experimental BDE values are fairly well reproduced by simple calculations carried out on the phenol/phenoxyl-water complexes, taking into account only hydrogen-bonding (HB) interactions of water molecules with molecular sites (HB model).

An attempt is made to intercept and identify free radicals from 5H-Pyridophenoxazin-5-one (PPH) generated under bioreductive conditions. It is proposed that the monoelectronic reduction of PPH in the proximity of DNA may eventually lead to radicals that could cause considerable damage to DNA and that PPH is able to suppress a large number of lymphoblastoid and solid tumor-derived cells at submicromolar concentrations.

Relative kinetic data were determined for the 5-endo-trig cyclization of radical compared to hydrogen abstraction. The 5-endo-trig cyclization to form the five membered rings was strongly favored over exo cyclization to form the four-membered ring on thermodynamic grounds as well as kinetically.

Presents a theoretical study of the effect of alpha-substituents on the silicon (Si) hyperfine splitting (hfs) constant in silyl radicals. Arrangement of atoms; Magnitude and trends of Si hfs constant from ESR spectra; Relation between the structure at the radical site and Si hybridization in the singly occupied molecular orbital.

8-Oxo-7,8-dihydroguanine (8-oxo-G) is the major lesion of oxidatively generated DNA damage. Despite two decades of intense study, several fundamental properties remain to be defined. Its isoelectronic 8-aminoguanine (8-NH2-G) has also received considerable attention from a biological point of view, although its chemistry involving redox processes remains to be discovered. We investigated the one-electron oxidation and one-electron reduction reactions of 8-oxo-G and 8-NH2-G derivatives. The reactions of hydrated electrons (eaq-) and azide radicals (N3*) with both derivatives were studied by pulse radiolysis techniques, and the transient absorption spectra were assigned to specific tautomers computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The protonated electron adducts of 8-NH2-G and 8-oxo-G showed a substantial difference in their absorption spectra, the unpaired electron being mainly delocalized in the imidazolyl ring and in the six-membered ring, respectively. On the other hand, the deprotonated forms of one-electron oxidation of 8-NH2-G and 8-oxo-G showed quite similar spectral characteristics. In a parallel study, the one-electron reduction of 8-azidoguanine (8-N3-G) afforded the same transient of one-electron oxidation of 8-NH2-G, which represents another example of generation of one-electron oxidized guanine derivatives under reducing conditions. Moreover, the fate of transient species was investigated by radiolytic methods coupled with product studies and allowed self- and cross-termination rate constants associated with these reactions to be estimated. [ABSTRACT FROM AUTHOR]

Relative kinetic data were determined for the 5-endo-trig cyclization of radical 12 compared to hydrogen abstraction from (TMS)[sub 3]SiH in the temperature range of 344-430 K, which allows for the estimation of a rate constant of 2 × 10[sup 4] s[sup -1] at 298 K with an activation energy of ca. 9 kcal/mol for the cyclization process. The 5-endo-trig cyclization of a variety of radicals that afford five-membered nitrogen-containing heterocycles was addressed computationally at the UB3LYP/6-31G[sup *] level. The 5-endo vs 4-exo mode of cyclication and the effect of delocalization of the unpaired electron in the transition state were investigated. Because the ring formed during cyclization contains five sp² centers, electrocyclization via a pentadienyllike resonance form was also considered. For comparison, similar calculations were performed for 4-penten1-yl and related radicals. The factors that affect the activation energies of homolytic 5-endo-trig cyclization were determined. In the absence of steric or conformational effects, the endo cyclization to form the fivemembered ring was strongly favored over exo cyclization to form the four-membered ring not only on thermodynamic grounds but also kinetically. When a substituent on the double bond was able to delocalize the unpaired electron in the transition state of the 4-exo path, the two modes of cyclization became kinetically comparable. These results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process. [ABSTRACT FROM AUTHOR]

Photooxygenations of PhSMe and Bu[sub2]S sensitized by N-methylquinolinium (NMQ[sup+]) and 9,10-dicyano- anthracene (DCA) in O[sub2]-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ[sup+] promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu[sub2]S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu[sub2]S, only the dimeric form of the radical cation, (Bu[sub2]S)[sub2][sup+], was observed, while the absorptions of both PhSM[sup+•] and (PhSMe[sub2][sup+•] were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O[sub2], was attributed to the reaction of the radical cation (presumably in the monomeric form) with O[sub2]-• generated in the reaction between NMQ• or DCA[sup-•] and O[sub2]. The fluorescence quenching of both NMQ[sup+] and DCA was also investigated, and it was found that the fluorescence of the two sensitizers is efficiently quenched by both sulfides (rates controlled by diffusion) as well by O[sub2] (k[subq] = 5.9 × 10[sup9] M[sup-1] s[sup-1] with NMQ[sup+] and 6.8 × 10[sup9] M[sup-1] s[sup-1] with DCA). It was also found that quenching of 1NMQ* by O[sub2] led to the production of [sup1]O[sub2] in significant yield (∅Δ = 0.86 in O[sub2]-saturated solutions) as already observed for 1DCA*. The steady- state photolysis experiments showed that the NMQ[sup+]- and DCA-sensitized photo oxygenation of PhSMe afford exclusively the corresponding sulfoxide. [ABSTRACT FROM AUTHOR]

The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH[sub 2] groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH[sub 2] groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the... [ABSTRACT FROM AUTHOR]

Here, we show that two tautomers are produced by the protonation of the guanine-electron adduct. The fate of electron adducts of a variety of substituted guanosines was investigated by radiolytic methods and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq-) with guanosine and 1-methylguanosine produces two transient species, whereas the reaction with N2-ethylguanosine and N2,N2-diethylguanosine produces only one. The two short-lived intermediates, which show a substantial difference in their UV-visible spectra, are recognized to be two purine tautomers (i.e., iminic 18 and aminic 19 forms). The tautomerization 18 --> 19 occurs with a rate constant of ca. 1.5 x 106 s(-1) , and theory suggests that it is a water-assisted process.

The direct EPR detection of thioacyl radicals has been reported only once, while thioacyl nitroxides remain an elusive species. We failed to detect the thioacyl radicals from two thioaldehydes and from phosphoryldithioformates but have obtained EPR evidence that thioacyl radicals react with 2-methyl-2-nitrosopropane to give thiocarbonyloxyaminyls rather than thioacyl nitroxides. The results of DFT calculations support this unexpected reactivity of thioacyls, while making questionable their previous EPR identification.