Near-barrier fusion of the ⁸B + ⁵⁸Ni proton-halo system

Fusion cross sections were measured for the exotic proton-halo nucleus ?B incident on a ??Ni target at several energies near the Coulomb barrier. This is the first experiment to report on the fusion of a proton-halo nucleus. The resulting excitation function shows a striking enhancement with respect to expectations for normal projectiles. Evidence is presented that the sum of the fusion and breakup yields saturates the total reaction cross section.     

A catalytic asymmetric allylation of aldehydes with allyl trichlorosilane activated by a chiral tetradentate bis-sulfoxide

Chiral homoallylic alcohols are easily accessible by asymmetric allylation of aldehydes with allyl trichlorosilane in the presence of catalytic amounts of a chiral tetradentate bis-sulfoxide, as organocatalyst, whose synthesis is reported.     

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions: Methanol and ethanol as solvents

The gas-phase reactions of F(-)(CH(3)OH) and F(-)(C(2)H(5)OH) with t-butyl bromide have been investigated to explore the effect of the solvent on the E2 transition state. Kinetic isotope effects (KIEs) were measured using a flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometer upon deuteration of both the alkyl halide and the alcohol. Kinetic isotope effects are significantly more pronounced than those previously observed for similar reactions of F(-)(H(2)O) with t-butyl halides. KIEs for the reaction of F(-)(CH(3)OH) with t-butyl bromide are 2.10 upon deuteration of the neutral reagent and 0.74 upon deuteration of the solvent. KIEs for the reaction of F(-)(C(2)H(5)OH) with t-butyl bromide are 3.84 upon deuteration of the neutral reagent and 0.66 upon deuteration of the solvent. The magnitude of these effects is discussed in terms of transition-state looseness. Additionally, deuteration of the neutral regent and deuteration of the solvent do not produce completely separable isotope effects, which is likely due to a crowded transition state. These results are compared to our previous work on S(N)2 and E2 solvated systems.     

Spectroscopic characterization of the isolated SF6- and C4F8- anions: observation of very long harmonic progressions in symmetric deformation modes upon photodetachment

Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.     

The Impact of Resonance Stabilization on the Intramolecular Hydrogen‐Atom Shift Reactions of Hydrocarbon Radicals

A series of intramolecular H‐atom shift reactions of both alkenyl and allylic radicals were investigated by using CBS‐QB3 electronic structure calculations. In the first set of reactions, an alkyl radical site was converted into an allylic radical site. In the second set, an allylic radical was converted into another allylic radical. The results are discussed in the context of a Benson‐type model to examine the impact of the transition‐state partial resonance stabilization on both the activation energies and the pre‐exponential factors. In most cases, the differences in the activation energies relative to those for the analogous alkyl radicals are primarily due to the barriers of the bimolecular reaction component of the activation energy. For the first set of reactions, there is additional entropy loss relative to the alkyl radical analogues. This additional loss of entropy may be smaller than some previous estimates. The pre‐exponential factors for the second set of reactions are generally similar to those of the analogous alkyl radical reactions (once the double bond in the transition state is accounted for).     

High-pressure rate rules for alkyl + O2 reactions. 1. The dissociation, concerted elimination, and isomerization channels of the alkyl peroxy radical

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).     

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.     

A convenient approach to δ-amino-β-ketoesters by vinylogous Mannich reaction of masked acetoacetates

SiCl₄ is an efficient and selective catalyst for the vinylogous Mannich reaction of linear and cyclic synthetic equivalents of acetoacetate dianion, leading to δ-amino-β-ketoesters in moderate to high yields and complete γ-selectivity; anti-diastereoselectivity was observed by using a γ-methyl-substituted cyclic silyloxydiene.