We observe chlorine radical dynamics in solution following two-photon photolysis of the solvent, dichloromethane. In neat CH(2)Cl(2), one-third of the chlorine radicals undergo diffusive geminate recombination, and the rest abstract a hydrogen atom from the solvent with a bimolecular rate constant of (1.35 +/- 0.06) x 10(7) M(-1) s(-1). Upon addition of hydrogen-containing solutes, the chlorine atom decay becomes faster, reflecting the presence of a new reaction pathway. We study 16 different solutes that include alkanes (pentane, hexane, heptane, and their cyclic analogues), alcohols (methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol), and chlorinated alkanes (cyclohexyl chloride, 1-chlorobutane, 2-chlorobutane, 1,2-dichlorobutane, and 1,4-dichlorobutane). Chlorine reactions with alkanes have diffusion-limited rate constants that do not depend on the molecular structure, indicating the absence of a potential barrier. Hydrogen abstraction from alcohols is slower than from alkanes and depends weakly on molecular structure, consistent with a small reaction barrier. Reactions with chlorinated alkanes are the slowest, and their rate constants depend strongly on the number and position of the chlorine substituents, signaling the importance of activation barriers to these reactions. The relative rate constants for the activation-controlled reactions agree very well with the predictions of the gas-phase structure-activity relationships. 相似文献
Three model dyads have been synthesized by esterification of beta- and alpha-cholesterol (Ch) with (S)- and/or (R)-ketoprofen (Kp). The alpha-dyads are efficient photogenerators of the 7-allyl Ch radicals by intramolecular H abstraction. Subsequent cyclization via C-C coupling occurs in a stereoselective way. 相似文献
The vertical electron affinity is demonstrated to be a key factor in controlling the selectivity of charged phenyl radicals in hydrogen atom abstraction from isopropanol in the gas phase. The measurement of the total reaction efficiencies (hydrogen and/or deuterium atom abstraction) for unlabeled and partially deuterium-labeled isopropanol, and the branching ratios of hydrogen and deuterium atom abstraction, by using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, allowed the determination of the selectivity for each site in the unlabeled isopropanol. Examination of hydrogen atom abstraction from isopropanol by eight structurally different radicals revealed that the preferred site is the CH group. The selectivity of the charged phenyl radicals correlates with the radical's vertical electron affinity and the reaction efficiency. The smaller the vertical electron affinity of a radical, the lower its reactivity, and the greater the preference for the thermodynamically favored CH group over the CH3 group or the OH group. As the vertical electron affinity increases from 4.87 to 6.28 eV, the primary kinetic isotope effects decrease from 2.9 to 1.3 for the CD group, and the mixture of primary and alpha-secondary kinetic isotopes decreases from 6.0 to 2.4 for the CD3 group. 相似文献
The gas phase hydrogen abstraction reaction between OH and CY(2)XH, where X = H, F, OH, or NH(2) and Y = H, CH(3) or F, in the absence and presence of a single water molecule is investigated using both density function theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. We find that a single water molecule could have a catalytic effect at low temperatures possible in laboratory experiments, but does not seem to catalyze these reactions at 298 K, and will not play a role under relevant atmospheric conditions. 相似文献
Kinetic solvent effects on hydrogen abstractions, namely, acceleration in nonpolar solvents, have been presumed to be restricted to O-H hydrogen donors. We demonstrate that also abstractions from C-H and even Sn-H bonds by cumyloxyl radicals and n,pi*-excited 2,3-diazabicyclo[2.2.2]oct-2-ene are fastest in the gas phase and nonpolar solvents but slowest in acetonitrile. Accordingly, solvent effects on hydrogen abstractions are more general, presumably due to stabilization of the reactive oxygen or nitrogen species in polar solvents. 相似文献
n-Hexyl and s-octyl radical isomerizations by intramolecular hydrogen atom shift have been studied in the presence of high methyl radical concentration where isomerized alkyl radicals reacted predominantly by combination and disproportionation reactions with methyl radicals. By assuming the rate coefficient of 1-hexyl radical recombination to be equal to that of ethyl self-combination, the rate coefficient of log(k1/s?1) = (9.5 ± 0.3) – (11.6 ± 0.3) kcal mol?1/RT ln 10 has been derived for the 6sp isomerization of n-hexyl radicals, 1-hexyl → 2-hexyl (1). Investigation of s-octyl radical isomerization was complicated by fast interconversion between 3-octyl, 2-octyl, and 4-octyl radicals. Use of the methyl trapping technique and systematic variation of methyl radical concentration made possible the determination of log(k2/s?1) = (9.4 ± 0.7) ? (11.2 ± 1.0) kcal mol?1/RT ln 10 for the 6ss isomerization of 3-octyl and the estimation of log(k3/s?1) = 10.5–17 kcal mol?1/RT ln 10 for the 5ss isomerization of 2-octyl radicals, where 3-octyl → 2-octyl (2), and 2-octyl → 4-octyl (3). 相似文献
Low-energy molecular ions of gas-phase primary and secondary aliphatic amines undergo reciprocal NH/CH exchange of hydrogen atoms prior to fragmentation on the microsecond time-scale; the subsequent decomposition reactions are significantly different from those observed in the mass spectrometer ion source. 相似文献
The gas phase photolysis of CCl4 in the presence of several alkanes has been used to obtain Arrhenius parameters for the abstraction of hydrogen atoms by the CCl3 radical: The following log k4 values were obtained:
RH
log k4
c-C5H10,
n-C6H14
2,3-Dimethylbutane
c-C7H14
Methylcyclohexane
c-C8H16
The results are compared to those for CH3 and CF3 radicals. 相似文献
Measurements of relative rates and rate constants for inter- and intramolecular hydrogen transfer reactions of polymer-bound radicals are reported. The relative rate of reaction of resin-bound primary alkyl radical with tributyltin hydride is about 2 times slower than that of the benchmark reaction in solution. The data do not reveal whether this is due to a reduced rate constant or a lower concentration of tin hydride in the resin phase. Yet the difference between solid and solution reactions is small enough to be neglected, and it appears that rate constants measured in solution can be applied directly to resin-bound radicals. A resin-bound aryl radical abstracts a hydrogen atom rapidly (k = 3 x 10(6) s(-1)) from its own polymer backbone and linker, and a simplified view of the resin as a "solvent" is suggested for predicting such effects with other polymers and linkers. Rapid cyclizations of resin-bound aryl radicals will be possible, but slower cyclizations and most bimolecular reactions will be difficult due to the competing polymer/linker hydrogen transfer. 相似文献
The enthalpies, activation energies, and rate constants for the reactions of 15 phenoxyl radicals derived from natural bioantioxidants
with hydroperoxides, C-H bonds of linoleic acid, SH-groups of l-cysteine, and O-H bonds of α-tocopherol (60 reactions) were calculated. The activation energies were calculated using the
model of intersecting parabolas. The interatomic distances in the reaction sites of the transition states of the studied reactions
were calculated. The factors affecting the reactivity of these radicals are discussed. The activation energy of the reaction
of oxygen with the O-H bond of the 1,2-dihydroxybenezene semiquinone radical was estimated. 相似文献
The 17-electron species [M(CO)5χLχ] (M Mn, Re, χ 0; M Mn, Re; L Ph3P, χ 1, 2; M Mn, Re; L (o-MeC6H4O)3P, χ 2; M Mn; L (p-ClC6H4O)3P, (PhO)3P, χ 2; M Mn; L P(OMe)3, χ 3) have been generated by one electron oxidation of the corresponding anions and show typical radical reactivity, undergoing dimerisation or hydride abstraction in reactions controlled by steric effects. Evidence is presented for the source of the hydrogen atom. The 19-electron species [M(CO)3(η7-C7H7)]? (M Cr, Mo) and [Fe(CO)3(η5-C6H7)]?, generated by reduction of the corresponding cations, undergo dimerisation at the organic ligand. Similar treatment of [Fe(CO)2-L(η-cp)]+ (L CO, PPh3, P(OPh)3, Me2CO) yields [Fe2(CO)4(η-cp)2] and these reduction reactions are rationalised in terms of the nature of the HOMO in the intermediate radical. Similar reduction of [Rh(diphos)2]+ yield the 17-electron intermediate [Rh(diphos)2] and this also undergoes hydrogen abstraction. 相似文献
The photochlorinations of the -butyl, -pentyl, and -hexyltrimethyl-ammonium chlorides, using molecular chlorine in hexachloroacetone or 15% CD3CO2D/85% H2SO4, or using N-chlorodimethylamine in the acid solvent are described. The ammonium group exerted a strong polar directing effect upon the site of substitution. This effect was found to be more pronounced in the more polar protic solvent. The reagent, N-chlorodimethylamine, generated the dimethylamminium radical, whose reaction showed a polar sensitivity toward hydrogen abstraction similar to that of the chlorine atom, but exhibiting a much greater secondary/primary selectivity. Comparison of the isomer distributions obtained from the self photochlorination reactions of N-chloro--hexylmethylamine and N-chloro--pentylmethylamine in the acid solvent, with the distribution pattern obtained for the chlorinations of the ammonium salts with N-chlorodimethylamine, suggested that the self chlorinations of the N-chloroamines proceed by the intramolecular hydrogen abstraction mechanism suggested previously. 相似文献
Intramolecular hydrogen atom (H-atom) abstraction from the o-OCH3 group effectively intercepts the p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-1-methoxybenzene (1) before this intermediate undergoes either retro-Bergman ring opening or external H-atom abstraction. This process leads to the formation of a new diradical and renders the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies. 相似文献
Vulcanizates of five elastomers having different chemical structures were prepared with recipes comprising dicumyl peroxide. Cumyl alcohol/acetophenone molar ratios were determined by gas–liquid chromatography (GLC) on acetone extracts of vulcanizates. Diphenyl ether was used as the internal standard. The ease of hydrogen abstraction by cumyloxy radicals, reflected by the cumyl alcohol/acetophenone ratio, decreases in the following order: cis-1,4-polyisoprene > poly(propylene oxide) > poly(vinyl n-butyl ether) > poly-1-heptene ? EPR. This order is in qualitative agreement with the order based on calculated relative rates of hydrogen abstraction by methyl and tert-butoxy radicals and literature data on oxygen absorption of some of these polymers as a result of hydrogen abstraction by peroxy radicals. 相似文献
The reaction of CF3 radicals with NH3 has been studied over a wide temperature range 298–673 K, using the photolysis and the thermal decomposition of CF3I as the free radical source. It was found that the reaction could not be explained in terms of a simple mechanism in the whole temperature range because a marked pressure dependence on the rate of products formation and the presence of a dark reaction complicate the system at low temperatures. Thus, Arrhenius parameters for reaction (1) have been calculated relative to the CF3 recombination from data in the range 523–673 K where pure hydrogen transfer occurs. The rate constant expression is given by where kH/k is in units of cm3/2/mol1/2 s1/2 and θ = 2.303 RT/kJ/mol. 相似文献
High-level ab initio electronic structure calculations have been carried out with respect to the intermolecular hydrogen-transfer reaction HCOOH+.OH-->HCOO.+H(2)O and the intramolecular hydrogen-transfer reaction .OOCH2OH-->HOOCH(2)O.. In both cases we found that the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An analysis of the atomic spin population, computed within the framework of the topological theory of atoms in molecules, suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. It is suggested that, in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible. 相似文献
The rate coefficients of H-abstraction reactions of butene isomers by the OH radical were determined by both canonical variational transition-state theory and transition-state theory, with potential energy surfaces calculated at the CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311G(d,p) level and CCSD(T)/6-311++G(d,p)//BH&HLYP/cc-pVTZ level and quantum mechanical tunneling effect corrected by either the small-curvature tunneling method or the Eckart method. While 1-butene contains allylic, vinylic, and alkyl hydrogens that can be abstracted to form different butene radicals, results reveal that s-allylic H-abstraction channels have low and broad energy barriers, and they are the most dominant channels which can occur via direct and indirect H-abstraction channels. For the indirect H-abstraction s-allylic channel, the reaction can proceed via forming two van der Waals prereactive complexes with energies that are 2.7-2.8 kcal mol(-1) lower than that of the entrance channel at 0 K. Assuming that neither mixing nor crossover occurs between different reaction pathways, the overall rate coefficient was calculated by summing the rate coefficients of the s-allyic, methyl, and vinyl H-abstraction paths and found to agree well with the experimentally measured OH disappearance rate. Furthermore, the rate coefficients of p-allylic H abstraction of cis-2-butene, trans-2-butene, and isobutene by the OH radical were also determined at 300-1500 K, with results analyzed and compared with available experimental data. 相似文献