Slice-selective excitation with B₁⁺-insensitive composite pulses

Spatially selective excitation pulses have been designed to produce uniform flip angles in the presence of the RF and static field inhomogeneities typically encountered in MRI studies of the human brain at 7 T. Pulse designs are based upon non-selective, composite pulses numerically optimized for the desired performance over prescribed ranges of field inhomogeneities. The non-selective pulses are subsequently transformed into spatially selective pulses with the same field-insensitive properties through modification of the spectral composition of the individual sub-pulses which are then executed in conjunction with an oscillating gradient waveform. An in-depth analysis of the performance of these RF pulses is presented in terms of total pulse durations, slice profiles, linearity of in-slice magnetization phase, sensitivity to RF and static field variations, and signal loss due to T(2) effects. Both simulations and measurements in phantoms and in the human brain are used to evaluate pulses with nominal flip angles of 45° and 90°. Target slice thickness in all cases is 2mm. Results indicate that the described class of field-insensitive RF pulses is capable of improving flip-angle uniformity in 7 T human brain imaging. There appears to be a subset of pulses with durations ?10 ms for which non-linearities in the magnetization phase are minimal and signal loss due to T(2) decay is not prohibitive. Such pulses represent practical solutions for achieving uniform flip angles in the presence of the large field inhomogeneities common to high-field human imaging and help to better establish the performance limits of high-field imaging systems with single-channel transmission.     

Reactivity Controlling Factors for an Aromatic Carbon‐Centered σ,σ,σ‐Triradical: The 4,5,8‐Tridehydroisoquinolinium Ion

The chemical properties of the 4,5,8‐tridehydroisoquinolinium ion (doublet ground state) and related mono‐ and biradicals were examined in the gas phase in a dual‐cell Fourier‐transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The triradical abstracted three hydrogen atoms in a consecutive manner from tetrahydrofuran (THF) and cyclohexane molecules; this demonstrates the presence of three reactive radical sites in this molecule. The high (calculated) electron affinity (EA=6.06 eV) at the radical sites makes the triradical more reactive than two related monoradicals, the 5‐ and 8‐dehydroisoquinolinium ions (EA=4.87 and 5.06 eV, respectively), the reactivity of which is controlled predominantly by polar effects. Calculated triradical stabilization energies predict that the most reactive radical site in the triradical is not position C4, as expected based on the high EA of this radical site, but instead position C5. The latter radical site actually destabilizes the 4,8‐biradical moiety, which is singlet coupled. Indeed, experimental reactivity studies show that the radical site at C5 reacts first. This explains why the triradical is not more reactive than the 4‐dehydroisoquinolinium ion because the C5 site is the intrinsically least reactive of the three radical sites due to its low EA. Although both EA and spin–spin coupling play major roles in controlling the overall reactivity of the triradical, spin–spin coupling determines the relative reactivity of the three radical sites.     

Silica-metal core-shell nanostructures

Silica-metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica-noble metal core-shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the St?ber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica-metal core-shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core-shell nanostructures. Silica-metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.     

Experimental and Computational Studies on the Formation of Three para‐Benzyne Analogues in the Gas Phase

Experimental and computational studies on the formation of three gaseous, positively‐charged para‐benzyne analogues in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon‐centered σ‐type mono‐ and biradicals are well understood. Cleavage of two iodine–carbon bonds in N‐deuterated 1,4‐diiodoisoquinolinium cation by collision‐activated dissociation (CAD) produced a long‐lived cation that showed nonradical reactivity, which was unexpected for a para‐benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N‐deuterated 2‐ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring‐opening before the second iodine atom loss to form an enediyne instead of a para‐benzyne. Similar results were obtained for the 5,8‐didehydroisoquinolinium cation and the 2,5‐didehydropyridinium cation. The findings for the 5,8‐didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7‐didehydro‐isoquinolinium cation and not the expected 5,8‐isomer.     

On Khalfin’s improvement of the LOY effective Hamiltonian for neutral meson complex

The general properties of the effective Hamiltonian for the neutral meson system improved by Khalfin in 1980 are studied. It is shown that contrary to the standard result of the Lee–Oehme–Yang (LOY) theory, the diagonal matrix elements of this effective Hamiltonian cannot be equal in a CPT invariant system. It is also shown that the scalar product of short, , and long, , living superpositions of neutral kaons cannot be real when CPT symmetry is conserved in the system under consideration, whereas within the LOY theory such a scalar product is real. PACS 03.65.Ca; 11.10.St; 11.30.Er; 13.20.Eb; 13.25.Es     

Effects of a hydroxyl substituent on the reactivity of the 2,4,6-tridehydropyridinium cation, an aromatic σ,σ,σ-triradical

The reactivity of 3-hydroxy-2,4,6-tridehydropyridinium cation was found to be drastically different from the reactivity of 2,4,6-tridehydropyridinium cation. While the latter triradical reacts with tetrahydrofuran, dimethyl disulfide and ally iodide via three consecutive atom or group abstractions, the former triradical exhibits this behavior only with tetrahydrofuran. Only a single atom or group abstraction was observed for the 3-hydroxy-2,4,6-tridehydropyridinium cation upon interaction with dimethyl disulfide and allyl iodide. This change in reactivity is caused by the hydroxyl group that strengthens the interactions between the two radical sites adjacent to it, thus reducing their reactivity. This explanation is supported by the observation of similar behavior for related biradicals.     

Reactivity of the 4,5-didehydroisoquinolinium cation

The chemical properties of a 1,8-didehydronaphthalene derivative, the 4,5-didehydroisoquinolinium cation, were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. This is an interesting biradical because it has two radical sites in close proximity, yet their coupling is very weak. In fact, the biradical is calculated to have approximately degenerate singlet and triplet states. This biradical was found to exclusively undergo radical reactions, as opposed to other related biradicals with nearby radical sites. The first bond formation occurs at the radical site in the 4-position, followed by that in the 5-position. The proximity of the radical sites leads to reactions that have not been observed for related mono- or biradicals. Interestingly, some ortho-benzynes have been found to yield similar products. Since ortho-benzynes do not react via radical mechanisms, these products must be especially favorable thermodynamically.