Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation,dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

The title radical (1) is generated in the gas-phase by collisional neutralization of carbonyl-protonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (approximately 56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one (2). The competing ring cleavages by O[bond]C-2 and C-4[bond]C-5 bond dissociations combined account for approximately 42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C(2)H(3)O and C(2)H(4)O cannot be explained as occurring competitively on the doublet ground (X) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-(1H)-oxolanium cation (3(+)) which upon collisional neutralization gives hypervalent 3-oxo-(1H)-oxolanium radical (3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at approximately 590 and approximately 400 nm.     

New liquid crystal compounds ( )-4-[5-(2-Methylbutyl)-1,3-dioxan-2-yl]phenyl 4-alkoxybenzoate

New liquid crystal compounds, (+)-4-[5-(2-methylbutyl)-1,3-dioxan-2-yl] phenyl 4-alkoxybenzoates (5), were synthesized. The mesomorphic behaviour of these compounds is compared with that of (+)-4-(5-alkyl-1,3-dioxan-2-yl)-phenyl 4-(2-methylbutoxy)benzoates (6). While compounds 6 exhibited a chiral smectic C phase, the corresponding compounds 5 did not. This might mean that for the appearance of a chiral smectic C phase in these types of compounds, it is necessary that the carbonyl and the chiral groups exist at nearby positions. Transition temperatures to those isotropic state for compounds 5 were lower than those for compounds 6. This result is common in both cases of (+)-4-alkoxycarbonylphenyl-4-[5-(2-methylbutyl)-1,3-dioxan-2-yl]benzoates (7), and (+)-4-(2-methylbutoxy-carbonyl)phenyl 4-(5-alkyl-1,3-dioxan-2-yl)-benzoates (8).     

An ab initio/RRKM study of product branching ratios in the photodissociation of buta-1,2- and -1,3-dienes and but-2-yne at 193 nm

Ab initio G2M(MP2)//B3LYP/6-311G** calculations have been performed to investigate the reaction mechanism of photodissociation of buta-1,2- and -1,3-dienes and but-2-yne after their internal conversion into the vibrationally hot ground electronic state. The detailed study of the potential-energy surface was followed by microcanonical RRKM calculations of energy-dependent rate constants for individual reaction steps (at 193 nm photoexcitation and under collision-free conditions) and by solution of kinetic equations aimed at predicting the product branching ratios. For buta-1,2-diene, the major dissociation channels are found to be the single Cbond;C bond cleavage to form the methyl and propargyl radicals and loss of hydrogen atoms from various positions to produce the but-2-yn-1-yl (p1), buta-1,2-dien-4-yl (p2), and but-1-yn-3-yl (p3) isomers of C(4)H(5). The calculated branching ratio of the CH(3) + C(3)H(3)/C(4)H(5) + H products, 87.9:5.9, is in a good agreement with the recent experimental value of 96:4 (ref. 21) taking into account that a significant amount of the C(4)H(5) product undergoes secondary dissociation to C(4)H(4) + H. The isomerization of buta-1,2-diene to buta-1,3-diene or but-2-yne appears to be slower than its one-step decomposition and plays only a minor role. On the other hand, the buta-1,3-diene-->buta-1,2-diene, buta-1,3-diene-->but-2-yne, and buta-1,3-diene-->cyclobutene rearrangements are significant in the dissociation of buta-1,3-diene, which is shown to be a more complex process. The major reaction products are still CH(3) + C(3)H(3), formed after the isomerization of buta-1,3-diene to buta-1,2-diene, but the contribution of the other radical channels, C(4)H(5) + H and C(2)H(3) + C(2)H(3), as well as two molecular channels, C(2)H(2) + C(2)H(4) and C(4)H(4) + H(2), significantly increases. The overall calculated C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is 24.0:49.6:4.6:6.1:15.2, which agrees with the experimental value of 20:50:8:2:2022 within 5 % margins. For but-2-yne, the one-step decomposition pathways, which include mostly H atom loss to produce p1 and, to a minor extent, molecular hydrogen elimination to yield methylethynylcarbene, play an approximately even role with that of the channels that involve the isomerization of but-2-yne to buta-1,2- or -1,3-dienes. p1 + H are the most important reaction products, with a branching ratio of 56.6 %, followed by CH(3) + C(3)H(3) (23.8 %). The overall C(4)H(5) + H/CH(3) + C(3)H(3)/C(2)H(3) + C(2)H(3)/C(4)H(4) + H(2)/C(2)H(2) + C(2)H(4) branching ratio is predicted as 62.0:23.8:2.5:5.7:5.6. Contrary to buta-1,2- and -1,3-dienes, photodissociation of but-2-yne is expected to produce more hydrogen atoms than methyl radicals. The isomerization mechanisms between various isomers of the C(4)H(6) molecule including buta-1,2- and -1,3-dienes, but-2-yne, 1-methylcyclopropene, dimethylvinylidene, and cyclobutene have been also characterized in detail.     

Neutralization-Reionization of alkenylammonium cations: An experimental and ab initio study of intramolecular N-H … C=C interactions in cations and hypervalent ammonium radicals

A series of isomeric hexenylammonium and hexenyldimethylammonium cations were neutralized by collisional electron transfer in the gas phase in an attempt to generate hypervalent ammonium radicals. The radicals dissociated completely on the 4.8–5.4 µs time scale. Radicals in which the hexene double bond was in the 3-, 4-, and 5-positions dissociated by competitive N-H and N=C bond cleavages. Allylic 2-hexen-1-ylammonium and 2-hexen-1-yldimethylammonium radicals underwent predominant cleavages of allylic N-C bonds. Deuterium labeling experiments revealed no intramolecular hydrogen transfer from the hypervalent ammonium group to the hexene double bond. Ab initio and density functional theory calculations showed that alkenylammonium and alkenylmethyloxonium ions preferred hydrogen bonded structures in the gas phase. The stabilization through intramolecular H bonding in 3-buten-1-ylammonium and 3-buten-1-yl methyloxonium ions was calculated by B3LYP/6-311G(2d,p) at 26 and 18 kJ mol^?1, respectively. No intramolecular hydrogen bonding was found for the allylammonium ion. The hypervalent 3-buten-1-yl-methyloxonium radical was calculated to be unbound and predicted to dissociate exothermically by O-H bond cleavage. This dissociation may provide kinetic energy for the hydrogen atom to overcome a small energy barrier for exothermic addition to the double bond. The 3-butten-1-ylammonium and allylammonium radicals were found to be bound and preferred gauche conformations without intramolecular hydrogen bonding. Vertical neutralization of alkenylammonium ions was accompanied by small Franck-Condon effects. The failure to detect stable or metastable hypervalent alkenylammonium radicals was ascribed to the low activation barriers to exothermic dissociations by N-H and N-C bond cleavages.     

Hypochlorite-mediated fragmentation of hyaluronan, chondroitin sulfates, and related N-acetyl glycosamines: evidence for chloramide intermediates, free radical transfer reactions, and site-specific fragmentation

Myeloperoxidase released from activated phagocytes reacts with H(2)O(2) in the presence of chloride ions to give hypochlorous acid. This oxidant has been implicated in the fragmentation of glycosaminoglycans, such as hyaluronan and chondroitin sulfates. In this study it is shown that reaction of HOCl with glycosaminoglycans and model compounds yields chloramides derived from the N-acetyl function of the glycosamine rings. The results of EPR spin trapping and product studies are consistent with the formation of amidyl radicals from these chloramides via both metal ion-dependent and -independent processes. In the case of glycosaminoglycan-derived amidyl radicals, evidence has been obtained in studies with model glycosides that these radicals undergo rapid intramolecular abstraction reactions to give carbon-centered radicals at C-2 on the N-acetyl glycosamine rings (via a 1,2-hydrogen atom shift) and at C-4 on the neighboring uronic acid residues (via 1,5-hydrogen atom shifts). The C-4 carbon-centered radicals, and analogous species derived from model glycosides, undergo pH-independent beta-scission reactions that result in glycosidic bond cleavage. With N-acetyl glucosamine C-1 alkyl glycosides, product formation via this mechanism is near quantitative with respect to chloramide loss. Analogous reactions with the glycosaminoglycans result in selective fragmentation at disaccharide intervals, as evidenced by the formation of "ladders" on gels; this selectivity is less marked under atmospheric oxygen concentrations than under anoxic conditions, due to competing peroxyl radical reactions. As the extracellular matrix plays a key role in mediating cell adhesion, growth, activation, and signaling, such HOCl-mediated glycosaminoglycan fragmentation may play a key role in disease progression and resolution, with the resulting fragments modulating the magnitude and quality of the immune response in inflammatory conditions.     

The reaction of phenyl radical with molecular oxygen: a G2M study of the potential energy surface

Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2. The reaction is shown to start with an exothermic barrierless addition of O2 to the radical site of C6H5 to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[2.5]octadienyl (dioxiranyl, 8) radicals. Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8. The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C5H5 + CO2, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C2H2. Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8. Thus, the branching ratio of C6H5O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10. According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H. Since C6H5O + O and C5H5 + CO2 are expected to be the major primary products of the C6H5 + O2 reaction and thermal decomposition of C6H5O leads to C5H5 + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring. Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions.     

A study of the unimolecular dissociation of the 2-buten-2-yl radical via the 193 nm photodissociation of 2-chloro-2-butene

This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces ground-state 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 +/- 2 kcal/mol, in agreement with the 32.8 +/- 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.     

1H- und 13C-NMR-spektroskopische Untersuchungen zur Ladungsverteilung in substituierten 1,3-Dioxan-2-ylium-Salzen

The ¹H and ¹³C NMR spectra of a series of 1,3-dioxan-2-ylium salts substituted at C-2 and C-4, C-5, C-6 have been measured. The atomic charge distribution in 1,3-dioxan- and 1,3-dioxolan-2-ylium cations has been calculated with the MINDO/3 method, leading to a linear correlation between π-charge and ¹³C chemical shift at C-2. The strong C-21-proton downfield shift is caused by the anisotropic influence of the mesomeric cation system. The conformations of substituted 1,3-dioxan-2-ylium cations are discussed.     

Mechanism of hydroxyl radical generation from a silica surface: molecular orbital calculations

The interaction of an H(2)O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (triple bond Si(*) and triple bond SiO(*)) and two representing heterolytic cleavage (triple bond Si(+) and triple bond Si-O(-)) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H(2)O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H(2)O. The reaction of H(2)O with triple bond SiO(*) was predicted to generate OH(*). Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction triple bond SiO(*) + H(2)O --> triple bond SiOH + OH(*). Stability of a finite number of triple bond SiO(*) sites at equilibrium in the above reaction with H(2)O was also predicted, which implies a long-term ability of silica surfaces to produce OH(*) radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.     

Reaction of the i-C4H5 (CH2CCHCH2) radical with O2

The resonantly stabilized radical i-C(4)H(5) (CH(2)CCHCH(2)) is an important intermediate in the combustion of unsaturated hydrocarbons and is thought to be involved in the formation of polycyclic aromatic hydrocarbons through its reaction with acetylene (C(2)H(2)) to form benzene + H. This study uses quantum chemistry and statistical reaction rate theory to investigate the mechanism and kinetics of the i-C(4)H(5) + O(2) reaction as a function of temperature and pressure, and unlike most resonantly stabilized radicals we show that i-C(4)H(5) is consumed relatively rapidly by its reaction with molecular oxygen. O(2) addition occurs at the vinylic and allenic radical sites in i-C(4)H(5), with respective barriers of 0.9 and 4.9 kcal mol(-1). Addition to the allenic radical form produces an allenemethylperoxy radical adduct with only around 20 kcal mol(-1) excess vibrational energy. This adduct can isomerize to the ca. 14 kcal mol(-1) more stable 1,3-divinyl-2-peroxy radical via concerted and stepwise processes, both steps with barriers around 10 kcal mol(-1) below the entrance channel energy. Addition of O(2) to the vinylic radical site in i-C(4)H(5) directly forms the 1,3-divinyl-2-peroxy radical with a small barrier and around 36.8 kcal mol(-1) of excess energy. The 1,3-divinyl-2-peroxy radical isomerizes via ipso addition of the O(2) moiety followed by O atom insertion into the adjacent C-C bond. This process forms an unstable intermediate that ultimately dissociates to give the vinyl radical, formaldehyde, and CO. At higher temperatures formation of vinylacetylene + HO(2), the vinoxyl radical + ketene, and the 1,3-divinyl-2-oxyl radical + O paths have some importance. Because of the adiabatic transition states for O(2) addition, and significant reverse dissociation channels in the peroxy radical adducts, the i-C(4)H(5) + O(2) reaction proceeds to new products with rate constant of around 10(11) cm(3) mol(-1) s(-1) at typical combustion temperatures (1000-2000 K). For fuel-rich flames we show that the reaction of i-C(4)H(5) with O(2) is likely to be faster than that with C(2)H(2), bringing into question the importance of the i-C(4)H(5) + C(2)H(2) reaction in initiating ring formation in sooting flames.     

Formation and decomposition of distonic o-, m-, and p-benzyne radical cations from photolysis of Mg(+)(o-, m-, p-C(6)H(4)F(2))

Distonic o-, m-, and p-benzyne radical cations (1-3) have been generated by a novel photolysis reaction of mass-selected Mg(+)-difluorobenzene complexes. The energy required for the formation of these radical cations is within 2.2 eV. The formation of o-benzyne cation is most facile. The benzyne radical cations dissociate further to yield ethyne and 1,3-butadiyne radical cation as major products given a sufficient amount of energy. The whole process involves only a single photon, and is very efficient. The calculated threshold for the formation of 1,3-butadiyne radical cation from Mg(+)(o-C(6)H(4)F(2)) is about 4.6 eV, quite comparable with the experimental estimate.     

An ab initio MO study on structures and energetics of CH2Si2, CH2Si2, and CH2Si2

The geometric structures and isomeric stabilities of various stationary points in CH₂Si₂ neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory. For the geometrical survey, the basis sets used are of the cc-pVTZ for the neutral and cation. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To the competitive two-anion isomers, the aug-cc-pVTZ basis sets are applied. The global minimum (N-1) of the CH₂Si₂ neutral has a quite different framework from those of the C₃H₂ (cyclopropenylidene) and Si₃H₂ (trisilacyclopropenylidene) neutrals. No competitive low-lying isomers are found in the CH₂Si₂ neutral. The attractive conformer (C-1) is predicted for the most stable cation, where its framework is quite different from that of the neutral N-1. Both H atoms are connected to the same C atom, but each C–H bond length is different from each other. Two competitive anion isomers with positive (real) electron affinities are predicted. The framework of the most stable anion A-1 is quite similar to that of the cation C-1, whereas both H atoms are equally connected to the same C atom. The framework of the anion isomer A-2 is the same as that in the neutral N-1. The vertical and adiabatic ionization potentials from the most stable neutral N-1 are 9.02 and 8.71 eV, respectively. The adiabatic electron affinity of the lowest lying isomer N-1 is only 0.43 eV and the vertical electron detachment energy form the global minimum anion (A-1) is 2.02 eV. The multi-centered Si–H–Si bonds are found in the neutral, cation, and anion.     

Isolation and Characterization of Two Furan-side Photoadducts of 7-Methylpyrido[3,4-c] Psoralen to the Sugar Moiety of 2-Deoxyadenosine†

Abstract. The UVA-mediated photoreaction of the monofunctional 7-methylpyrido[3,4-c]psoralen (MePyPs) with 2′-deoxy-adenosine was investigated in the dry state. Two main 7-methylpyrido[3,4-c]psoralen monoadducts to 2′-deoxy-adenosine were isolated by HPLC and characterized by soft ionization mass spectrometry (fast atom bombardment) and extensive ¹H NMR analyses including correlation spectroscopy, total correlation spectroscopy and nuclear Overhauser effect measurements. The two unusual photoadducts exhibit a covalent bond between the C-4′ of the furan moiety of the psoralen on one hand and either the C-l' or the C-4′ of the furanose ring of the purine nucleoside on the other hand. The furan-side monoadducts appear to be specific for both 2′-deoxy-adenosine and MePyPs. They are likely to result from the recombination of the 4′,5′-dihydrofuran-4′-yl radical of MePyPs with 2′-deoxyadenosyl carbon-centered sugar radicals at either the 1′ or the 5′ position.     

Oxidation of ethyne and But-2-yne. 2. Master equation simulations

The aim of this study is to improve understanding of the tropospheric oxidation of ethyne (acetylene, C2H2) and but-2-yne, which takes place in the presence of HO and O2. The details of the potential energy hypersurface have been discussed in a previous article [Maranzana et al., J. Phys. Chem. A 2008, 112, XXXX]. For both molecules, the initial addition of HO radical to the triple bond is followed by addition of O2 to form peroxyl radicals. In both reaction systems, the peroxyl radicals take two isomeric forms, E1 and E2 for ethyne and e1 and e2 for but-2-yne. Energy transfer parameters (alpha = 250 cm-1) for the ethyne system were obtained by simulating laboratory data for N2 buffer gas, where O2 was not present. In simulations of C2H2 + HO when O2 is present, E1 reacts completely and E2 reacts almost completely, before thermalization. Radical E1 produces formic acid ( approximately 44%) and E2 gives glyoxal ( approximately 53%), in quite good agreement with experiments. For but-2-yne, pressure-dependent laboratory data are too scarce to obtain energy transfer parameters directly, so simulations were carried out for a range of values: alpha = 200-900 cm-1. Excellent agreement with the available experimental yields at atmospheric pressure was obtained with alpha = 900 cm-1. Two reaction channels are responsible for acetic acid formation, but one is clearly dominant. Biacetyl is produced by reactions of e1 and, to a minor extent, e2. The peroxyl radical e2 leads to less than 8% of all products. Vinoxyl radical (which has been reported in experiments involving C2H2 + HO) and products of its reactions are predicted to be negligible under atmospheric conditions.     

Facile hydrometalation of alkynes by nido-1,2-(CpRuH)(2)B(3)H(7) yielding novel Ru-B edge-bridging alkylidenes. Stepwise conversion of HCtbd1;CC(O)OMe into nido-1,2-(CpRuH)(2)-3-HOB-4-MeC-5-MeOC-BH(3), Cp = eta(5)-C(5)Me(5)

The reaction of the alkyne HCCC(O)OMe with 7 sep 1,2-(Cp*RuH)2B3H7 leads to hydroboration plus hydroruthenation to produce nido-1,3-mu-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7, a compound with an exocluster ruthenium-boron mu-alkylidene that exists in two isomeric forms. Both isomers undergo rearrangement with intramolecular chelation of the carbonyl oxygen at a boron site, thereby opening the cluster and generating arachno-2,3,-mu(C)-5-eta1(O)-Me{C(O)OMe}C-1,2-(Cp*Ru)2B3H7. Further heating leads to deoxygenation of the carbonyl fragment by a boron center concurrent with insertion of the carbon atom into the metallaborane cage to give nido-1,2-(Cp*RuH)2-3-HOB-4-MeC-5-MeOC-BH3.     

Electron super-rich radicals in the gas phase. A neutralization-reionization mass spectrometric and ab initio/RRKM study of diaminohydroxymethyl and triaminomethyl radicals

Diaminohydroxymethyl (1) and triaminomethyl (2) radicals were generated by femtosecond collisional electron transfer to their corresponding cations (1+ and 2+, respectively) and characterized by neutralization-reionization mass spectrometry and ab initio/RRKM calculations at correlated levels of theory up to CCSD(T)/aug-cc-pVTZ. Ion 1+ was generated by gas-phase protonation of urea which was predicted to occur preferentially at the carbonyl oxygen with the 298 K proton affinity that was calculated as PA = 875 kJ mol-1. Upon formation, radical 1 gains vibrational excitation through Franck-Condon effects and rapidly dissociates by loss of a hydrogen atom, so that no survivor ions are observed after reionization. Two conformers of 1, syn-1 and anti-1, were found computationally as local energy minima that interconverted rapidly by inversion at one of the amine groups with a <7 kJ mol-1 barrier. The lowest energy dissociation of radical 1 was loss of the hydroxyl hydrogen atom from anti-1 with ETS = 65 kJ mol-1. The other dissociation pathways of 1 were a hydroxyl hydrogen migration to an amine group followed by dissociation to H2N-C=O* and NH3. Ion 2+ was generated by protonation of gas-phase guanidine with a PA = 985 kJ mol-1. Electron transfer to 2+ was accompanied by large Franck-Condon effects that caused complete dissociation of radical 2 by loss of an H atom on the experimental time scale of 4 mus. Radicals 1 and 2 were calculated to have extremely low ionization energies, 4.75 and 4.29 eV, respectively, which belong to the lowest among organic molecules and bracket the ionization energy of atomic potassium (4.34 eV). The stabilities of amino group containing methyl radicals, *CH2NH2, *CH(NH2)2, and 2, were calculated from isodesmic hydrogen atom exchange with methane. The pi-donating NH2 groups were found to increase the stability of the substituted methyl radicals, but the stabilities did not correlate with the radical ionization energies.     

The reactions of cytidine and 2'-deoxycytidine with SO4.- revisited. Pulse radiolysis and product studies

The reactions of SO4.- with 2'-deoxycytidine 1a and cytidine 1b lead to very different intermediates (base radicals with 1a, sugar radicals with 1b). The present study provides spectral and kinetic data for the various intermediates by pulse radiolysis as well as information on final product yields (free cytosine). Taking these and literature data into account allows us to substantiate but also modify in essential aspects the current mechanistic concept (H. Catterall, M. J. Davies and B. C. Gilbert, J. Chem. Soc., Perkin Trans. 2, 1992, 1379). SO4.- radicals have been generated radiolytically in the reaction of peroxodisulfate with the hydrated electron (and the H. atom). In the reaction of SO4.- with 1a (k = 1.6 x 10(9) dm3 mol-1 s-1), a transient (lambda max = 400 nm, shifted to 450 nm at pH 3) is observed. This absorption is due to two intermediates. The major component (lambda max approximately 385 nm) does not react with O2 and has been attributed to an N-centered radical 4a formed upon sulfate release and deprotonation at nitrogen. The minor component, rapidly wiped out by O2, must be due to C-centered OH-adduct radical(s) 6a and/or 7a suggested to be formed by a water-induced nucleophilic replacement. These radicals decay by second-order kinetics. Free cytosine is only formed in low yields (G = 0.14 x 10(-7) mol J-1 upon electron-beam irradiation). In contrast, 1b gives rise to an intermediate absorbing at lambda max = 530 nm (shifted to 600 nm in acid solution) which rapidly decays (k = 6 x 10(4) s-1). In the presence of O2, the decay is much faster (k approximately 1.3 x 10(9) dm3 mol-1 s-1) indicating that this species must be a C-centered radical. This has been attributed to the C(5)-yl radical 8 formed upon the reaction of the C(2')-OH group with the cytidine SO4(.-)-adduct radical 2b. This reaction competes very effectively with the corresponding reaction of water and the release of sulfate and a proton generating the N-centered radical. Upon the decay of 8, sugar radical 11 is formed with the release of cytosine. The latter is formed with a G value of 2.8 x 10(-7) mol J-1 (85% of primary SO4.-) at high dose rates (electron beam irradiation). At low dose rates (gamma-radiolysis) its yield is increased to 7 x 10(-7) mol J-1 due to a chain reaction involving peroxodisulfate and reducing free radicals. Phosphate buffer prevents the formation of the sugar radical at the SO4(.-)-adduct stage by enhancing the rate of sulfate release by deprotonation of 2b and also by speeding up the decay of the C(5)-yl radical into another (base) radical. Cytosine release in cytidine is mechanistically related to strand breakage in poly(C). Literature data on the effect of dioxygen on strand breakage yields in poly(C) induced by SO4.- (suppressed) and upon photoionisation (unaltered) lead us to conclude that in poly(C) and also in the present system free radical cations are not involved to a major extent. This conclusion modifies an essential aspect of the current mechanistic concept.     

Theoretical study on the mechanism of the CH2F + NO2 reaction

Despite the importance of the Fluoromethyl radicals in combustion chemistry, very little experimental information on their reactions toward stable molecules is available in the literature. Motivated by recent laboratory characterization about the reaction kinetics of Chloromethyl radicals with NO2, we carried out a detailed potential energy survey on the CH2F + NO2 reaction at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels as an attempt toward understanding the CH2F + NO2 reaction mechanism. It is shown that the CH2F radical can react with NO2 to barrierlessly generate adduct a (H2FCNO2), followed by isomerization to b1 (H2FCONO-trans) which can easily interconvert to b2 (H2FCONO-cis). Subsequently, Starting from b (b1, b2), the most feasible pathway is the C--F and N--O1 bonds cleavage along with N--F bond formation of b (b1, b2) leading to P1 (CH2O + FNO), or the direct N--O1 weak-bond fission of b (b1, b2) to give P2 (CH2FO + NO), or the 1,3-H-shift associated with N--O1 bond rupture of b1 to form P3 (CHFO + HNO), all of which may have comparable contribution to the reaction CH2F + NO2. Much less competitively, b2 either take the 1,4-H-shift and O1--N bond cleavage to form product P4 (CHFO + HON) or undergo a concerted H-shift to isomer c2 (HFCONOH), followed by dissociation to P4. Because the rate-determining transition state (TSab1) in the most competitive channels is only 0.3 kcal/mol higher than the reactants in energy, the CH2F + NO2 reaction is expected to be rapid, and may thus be expected to significantly contribute to elimination of nitrogen dioxide pollutants. The similarities and discrepancies among the CH2X + NO2 (X = H, F, and Cl) reactions are discussed in terms of the electronegativity of halogen atom. The present article may assist in future experimental identification of the product distributions for the title reaction, and may be helpful for understanding the halogenated methyl chemistry.     

Adenine radicals in the gas phase: an experimental and computational study of hydrogen atom adducts to adenine

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5). Radicals 1 that are formed in the fifth and higher electronically excited states of DeltaE > or = 2.5 eV can also undergo ring-cleavage dissociations resulting in expulsion of HCN. The relative stabilities, dissociation, and transition state energies for several hydrogen atom adducts to adenine have been established computationally at highly correlated levels of theory. Transition state theory calculations of 298 K rate constants in the gas phase, including quantum tunnel corrections, indicate the branching ratios for H-atom additions to C-8, C-2, N-3, N-1, and N-7 positions in adenine as 0.68, 0.20, 0.08, 0.03, and 0.01, respectively. The relative free energies of adenine radicals in aqueous solution point to the C-8 adduct as the most stable tautomer, which is predicted to be the predominating (>99.9%) product at thermal equilibrium in solution at 298 K.     

A new approach to the synthesis of strained cyclic systems: II. Mass spectrometric study of 2-dialkylamino-3-phenylthiophenes and their structural isomers, iminothietanes and iminocyclobutenes

Triads of isomeric N-alkyl-N-methyl-3-phenylthiophen-2-amines, N-methyl-3-alkyl-4-methylidene-3-phenylthietan-2-imines, and N-methyl-4-alkylsulfanyl-2-phenylcyclobut-2-en-1-imines (Alk = Me, Et, Bu) were synthesized from 1,3-dilithio-3-phenylpropyne, methyl isothiocyanate, and alkyl halides, and their fragmentation under electron impact was studied. Primary decomposition of the molecular ions of 2-aminothiophenes is determined by the localization of a radical cation center on the nitrogen atom, and it follows a path typical of alkyl(aryl)amines with elimination of hydrogen atom or methyl or propyl radical from the α-carbon atom in the N-alkyl substituent. Fragmentation of the iminothietanes involves cleavage of the four-membered ring in half to give neutral MeNCS molecule and 1-alkyl-1-phenylallene radical cation. Alkylsulfanyl(imino)-cyclobutenes undergo cleavage at the sulfur-containing side chain according to general relations holding in the fragmentation of alkyl sulfides.