Solid State C NMR of 30-Crown-10 Ether and 30-Crown-10.4H2O

The ¹³C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state ¹³C NMR spectrum of the 30-crown-10.4H₂O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable ¹³C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic ¹³C shieldings for simpler model compounds. Results of dipolar dephased ¹³C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H₂O, indicating that it also is relatively rigid in the solid.     

Highp T hadrons as leading particles in jets produced at the ISR

The density of charged particles near a single highp _T trigger particle in proton-proton collisions has been studied with the SFM detector at the CERN-ISR. It is shown that:—the secondaries cluster in a jet-like manner about the trigger particle; —the momentum distributions in this trigger jet can be unambiguously separated from the spectator background distributions;—the momentum distributions in this trigger jet can be unambiguously separated from the spectator background distributions;—the momentum component transverse to the jet direction is exponentially damped;—the width of the transverse distributions increases with the momentum component along the jet;—the fractional longitudinal momentum density depends strongly on the trigger transverse momentum and scales with \(x_T = {{2p_T } \mathord{\left/ {\vphantom {{2p_T } {\sqrt s }}} \right. \kern-0em} {\sqrt s }}\) . All these findings support the hard constituent scattering origin of highp _T hadrons.     

Nature of the 3.8 eV plasmon in x-ray photoemission from silver

Overlayer experiments show that the 3.8 eV satellite in X-ray photoemission from silver is due almost entirely to the excitation of extrinsic surface plasmons.     

Photodegradation of poly(vinyl esters)—III. Photolysis mechanism for both polymeric and low molecular mass esters

Acid, aldehyde and carbon dioxide were the major products of the vacuum photodegradations of poly(vinyl acetate) (PVAC), poly(vinyl propanoate) (PVPR) and poly(ethylene-co-vinyl acetate) (PEVAC). Aldehydes have not previously been reported as formed from the acid portions of esters undergoing photolysis. The generality of aldehyde formation has been demonstrated by the detection of the respective aldehydes in the liquid-phase photolysis of butane-1,3-diol diacetate (BDAC) and butane-1,3-diol dipropanoate (BDPR) in this study, which is the first to report the quantum yields for acid and aldehyde formation in low molecular mass esters. Two mechanisms, one involving hydrogen abstraction by the acyl radical formed in a Norrish Type I cleavage process and the other involving an intramolecular hydrogen abstraction by an excited carbonyl group followed by fragmentation, have been proposed to account for aldehyde formation. The absence of propanal from the product distribution in the photo-oxidation of PVPR could be explained by both mechanisms.     

Dyotropic rearrangement of alpha-lactone to beta-lactone: a computational study of small-ring halolactonisation

Transition structures have been optimised using the B3LYP/6-31+G* density functional level method, in vacuum and in implicit (PCM) and explicit (DFT/MM) aqueous solvation, for the degenerate rearrangement of the alpha-lactone derived by the formal addition of Cl(+) to acrylate anion and for the dyotropic rearrangement of this to the beta-lactone. Despite being lower in energy than the alpha-lactone, there is no direct pathway to the beta-lactone from the acrylate chloronium zwitterion, which is the transition structure for the degenerate rearrangement. This may be rationalised by consideration of the unfavorable angle of attack by the carboxylate nucleophile on the beta-position; attack on the alpha-position involves a less unfavorable angle. Formation of the beta-lactone may occur by means of a dyotropic rearrangement of the alpha-lactone. This involves a high energy barrier for the acrylate derived alpha-lactone, but dyotropic rearrangement of the beta,beta-dimethyl substituted alpha-lactone to the corresponding beta-lactone involves a much lower barrier, estimated at about 46 kJ mol(-1) in water, and is predicted to be a facile process.     

Ligand and Linkage Isomers of Bis(ethylthiocarbamato) Copper Complexes with Cyclic C6H8 Backbone Substituents: Synthesis,Characterization, and Antiproliferation Activity

A series of isomeric bis(alkylthiocarbamate) copper complexes have been synthesized, characterized, and evaluated for antiproliferation activity. The complexes were derived from ligand isomers with 3-methylpentyl (H₂L²) and cyclohexyl (H₂L³) backbone substituents, which each yield a pair of linkage isomers. The thermodynamic products CuL^2a/3a have two imino N and two S donors resulting in three five-member chelate rings (555 isomers). The kinetic isomers CuL^2b/3b have one imino and one hydrazino N donor and two S donors resulting in four-, six-, and five-member rings (465 isomers). The 555 isomers have more accessible Cu^II/I potentials (E_1/2=−811/−768 mV vs. ferrocenium/ferrocene) and lower energy charge transfer bands than their 465 counterparts (E_1/2=−923/-854 mV). Antiproliferation activities were evaluated against the lung adenocarcinoma cell line (A549) and nonmalignant lung fibroblast cell line (IMR-90) using the MTT assay. CuL^2a was potent (^A549EC₅₀=0.080 μM) and selective (^IMR-90EC₅₀/^A549EC₅₀=25) for A549. Its linkage isomer CuL^2b had equivalent A549 activity, but lower selectivity (^IMR-90EC₅₀/^A549EC₅₀=12.5). The isomers CuL^3a and CuL^3b were less potent with ^A549EC₅₀ values of 1.9 and 0.19 M and less selective with ^IMR-90EC₅₀/^A549EC₅₀ ratios of 2.3 and 2.65, respectively. There was no correlation between reduction potential and A549 antiproliferation activity/selectivity.     

Asymmetric aza-[3+3] annulation in the synthesis of indolizidines: an unexpected reversal of regiochemistry

An enantioselective and diastereoselective aza-[3+3] annulation of pyrrolidine-based exo-cyclic vinylogous amides and urethanes with chiral vinyl iminium salts is described. This asymmetric annulation manifold is possible because of an unexpected regiochemical reversal whereby head-to-tail annulations dominated over the predicted head-to-head. It should find prevalent synthetic applications in the enantioselective synthesis of indolizidines.     

Computational study of bond dissociation enthalpies for substituted β-O-4 lignin model compounds

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant β-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.     

Single-conformation infrared spectra of model peptides in the amide I and amide II regions: Experiment-based determination of local mode frequencies and inter-mode coupling

Single-conformation infrared spectra in the amide I and amide II regions have been recorded for a total of 34 conformations of three α-peptides, three β-peptides, four α∕β-peptides, and one γ-peptide using resonant ion-dip infrared spectroscopy of the jet-cooled, isolated molecules. Assignments based on the amide NH stretch region were in hand, with the amide I∕II data providing additional evidence in favor of the assignments. A set of 21 conformations that represent the full range of H-bonded structures were chosen to characterize the conformational dependence of the vibrational frequencies and infrared intensities of the local amide I and amide II modes and their amide I∕I and amide II∕II coupling constants. Scaled, harmonic calculations at the DFT M05-2X∕6-31+G(d) level of theory accurately reproduce the experimental frequencies and infrared intensities in both the amide I and amide II regions. In the amide I region, Hessian reconstruction was used to extract local mode frequencies and amide I∕I coupling constants for each conformation. These local amide I frequencies are in excellent agreement with those predicted by DFT calculations on the corresponding (13)C = (18)O isotopologues. In the amide II region, potential energy distribution analysis was combined with the Hessian reconstruction scheme to extract local amide II frequencies and amide II∕II coupling constants. The agreement between these local amide II frequencies and those obtained from DFT calculations on the N-D isotopologues is slightly worse than for the corresponding comparison in the amide I region. The local mode frequencies in both regions are dictated by a combination of the direct H-bonding environment and indirect, "backside" H-bonds to the same amide group. More importantly, the sign and magnitude of the inter-amide coupling constants in both the amide I and amide II regions is shown to be characteristic of the size of the H-bonded ring linking the two amide groups. These amide I∕I and amide II∕II coupling constants remain similar in size for α-, β-, and γ-peptides despite the increasing number of C-C bonds separating the amide groups. These findings provide a simple, unifying picture for future attempts to base the calculation of both nearest-neighbor and next-nearest-neighbor coupling constants on a joint footing.