The fleeting existence of the classical vinyl cation structure

ABSTRACT

Using a hyperbolic function type-potential and Schrödinger’s equation transformed into a hypergeometric differential equation along with the results presented from an ab-initio CASSCF(10,11)/cc-pVTZ calculation we show that the potential well of the classical vinyl cation minimum is incapable of supporting a zero point vibrational motion for the protons. Furthermore, as opposed to the previously found transition state linking the classical and bridged vinyl cations, we suggest that it becomes a transition state for the interconversion or tautomerization of the bridged vinyl cation.     

Can the symmetrical Dewar pyridine be observed experimentally? A theoretical study

ABSTRACT

In this work we study the possibility of the photochemical formation of the symmetrical Dewar pyridine (1-azabicyclo-[2,2,0]-hexa-2,5-diene), by applying the complete active space self-consistent field method and the multiconfigurational second-order perturbation theory to explore the corresponding ground and excited state potential energy surfaces. According to our theoretical calculations there are three possible paths that can be followed, one is a biphotonic process which involves irradiating pyridine in its ground state with a 358?nm laser guiding the system to an intersystem crossing S₁/T₁/S₀ of triple character whereby deactivation to the ground state, S₀, Dewar minimum occurs, the second one, which is a ground state thermal reaction involves the use of a far-Infra-Red laser where planar pyridine is vibrationally excited to a very high vibrational level whose energy is comparable to that of the ground state transition structure, S₀(TS), connecting the symmetrical S₀ Dewar pyridine and the ground state of planar pyridine. The third process is also a biphotonic one involving excitation of planar pyridine with an energy which is in the limits of its ionisation potential. In this case there is a theoretically accessible S₁/S₀ Conical Intersection which leads directly to the ground state of the symmetrical Dewar pyridine.     

Preparation of hybrid triple‐stimuli responsive nanogels based on poly(L‐histidine)

A series of novel multi‐responsive disulfide cross‐linked polypeptide nanogels has been synthesized by a one‐step ring‐opening polymerization process. The pH‐responsive core of the prepared nanogels was based on poly(L‐histidine), the difunctional N‐carboxy anhydride of l ‐cystine (l ‐Cys‐NCA) was used as a reduction‐cleavable cross‐linking agent, while the outer hydrophilic corona was comprised of a poly(ethylene oxide) block. Extensive molecular characterization studies were conducted in order to confirm the formation of the desired polymeric nanostructures and also to prove their responsiveness to external stimuli within the physiological values of healthy and cancer tissues. Furthermore, the disruption of the disulfide‐bond linkages between the polymeric chains was achieved by the presence of the reductive tripeptide glutathione (GSH), leading to size variations that were monitored by dynamic light scattering (DLS) and size‐exclusion chromatography (SEC). “Stealth” properties of the formed nanostructures were examined by zeta potential measurements. The described nanogels are clearly promising candidates for drug delivery applications. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1278–1288     

Heart ablation using a planar rectangular high intensity ultrasound transducer and MRI guidance

The aim of this study was to evaluate a flat rectangular (3 × 10 mm²) MRI compatible transducer operating at 5 MHz. The main task was to explore the feasibility of creating deep lesions in heart at a depth of at least 15 mm. The size of thermal necrosis in heart tissue was estimated as a function of power and time using a simulation model. The system was then tested in an excised lamb heart. In this study, we were able to create lesions of 15 mm deep with acoustic power of 6 W for an exposure of approximately 1 min. The contrast to noise ratio (CNR) between lesion and heart tissue was evaluated using fast spin echo (FSE). The CNR value was approximately 22 using T1 W FSE. Maximum CNR was achieved with repetition time (TR) between 300 and 800 ms. Using T2W FSE, the corresponding CNR was approximately 13 for the 14 in vivo experiments. The average lesion depth was 11.93 mm with a standard deviation of 0.62 mm. In vivo irradiation conditions were 6 W for 60 s. The size of the lesion in the other two dimensions was close to 3 × 10 mm² (size of the transducer element).     

Improvement of image quality using BLADE sequences in brain MR imaging

The purpose of this study is to compare two types of sequences in brain magnetic resonance (MR) examinations of uncooperative and cooperative patients. For each group of patients, the pairs of sequences that were compared were two T2-weighted (T2-W) fluid attenuated inversion recovery sequences with different k-space trajectories (conventional Cartesian and BLADE) and two T2-TSE weighted with different k-space trajectories (conventional Cartesian and BLADE). Twenty-three consecutive uncooperative patients and 44 cooperative patients, who routinely underwent brain MR imaging examination, participated in the study. Both qualitative and quantitative analyses were performed based on the signal-to-noise ratio, contrast-to-noise ratio (CNR), and relative contrast (ReCon) measures of normal anatomic structures. The qualitative analysis was performed by experienced radiologists. Also, the presence of motion, other (e.g., Gibbs, susceptibility artifacts, phase encoding from vessels) artifacts and pulsatile flow artifacts was evaluated.     

Heavier alkaline earth catalysts for the intermolecular hydroamination of vinylarenes, dienes, and alkynes

The heavier group 2 complexes [M{N(SiMe(3))(2)}(2)](2)(1, M = Ca; 2, M = Sr) and [M{CH(SiMe(3))(2)}(2)(THF)(2)] (3, M = Ca; 4, M = Sr) are shown to be effective precatalysts for the intermolecular hydroamination of vinyl arenes and dienes under mild conditions. Initial studies revealed that the amide precatalysts, 1 and 2, while compromised in terms of absolute activity by a tendency toward transaminative behavior, offer greater stability toward polymerization/oligomerization side reactions. In every case the strontium species, 2 and 4, were found to outperform their calcium congeners. Reactions of piperidine with para-substituted styrenes are indicative of rate-determining alkene insertion in the catalytic cycle while the ease of addition of secondary cyclic amines was found to be dependent on ring size and reasoned to be a consequence of varying amine nucleophilicity. Hydroamination of conjugated dienes yielded isomeric products via η(3)-allyl intermediates and their relative distributions were explained through stereoelectronic considerations. The ability to carry out the hydroamination of internal alkynes was found to be dramatically dependent upon the identity of the alkyne substituents while reactions employing terminal alkynes resulted in the precipitation of insoluble and unreactive group 2 acetylides. The rate law for styrene hydroamination with piperidine catalyzed by [Sr{N(SiMe(3))(2)}(2)](2) was deduced to be first order in [amine] and [alkene] and second order in [catalyst], while large kinetic isotope effects and group 2 element-dependent ΔS(++) values implicated the formation of an amine-assisted rate-determining alkene insertion transition state in which there is a considerable entropic advantage associated with use of the larger strontium center.     

A Birkhoff-James cosine function for normed linear spaces

Aequationes mathematicae - The cosine function is a classical tool for measuring angles in inner product spaces, and it has various extensions to normed linear spaces. In this paper, we investigate...     

Synthesis of 7-aryl/heteraryl-1,3-diphenyl-1,2,4-benzotriazinyls via palladium catalyzed Stille and Suzuki-Miyaura reactions

Stille and Suzuki-Miyaura reactions of 7-iodo-1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl are presented as rare examples of cross-coupling reactions with stable organic radicals. Both the Stille and Suzuki-Miyaura reactions are in most cases high yielding but the latter are cleaner while the former are faster and are accompanied by 1,3-diphenyl-1,2,4-benzotriazin-7(H)-one as by-product. A range of 7-aryl and 7-heteroaryl-1,2,4-benzotriazinyls have been synthesized and characterized using standard spectroscopic and spectrometric means.     

Reversible spherical organic water microtraps

Spherical water microtraps were prepared via a two steps process, which comprises the preparation of cross-linked poly(methacrylic acid) spheres by distillation–precipitation polymerization and the subsequent conversion of carboxylic groups to the corresponding sodium salts by treatment with aqueous sodium hydroxide solution. The influence of the cross-linker amount was investigated in order to achieve greater water absorption and a reversibility of the initial spheres after drying the formed hydrogel or washing with an excess of organic solvent. We achieved the highest water absorbency of about 70 times their weight occurring rapidly and unlimited times reversible after going through repeatable cycles of absorption and drying. This new material, called microtraps, was developed for its incorporation into multilayer anticorrosive coatings to prevent contact of water with metallic substrates.