We propose a quarter-wave plate based on nanoslits and analyze it using a semianalytical theory and simulations. The device comprises two nanoslits arranged perpendicular to one another where the phases of the fields transmitted by the nanoslits differ by λ/4. In this way, the polarization state of the incident light can be changed from linear to circular or vice versa. The plasmonic nanoslit wave plate is thin and has a subwavelength lateral extent. We show that the predictions for the phase shift obtained from a semianalytical model are in very good agreement with simulations by the finite difference time domain method. 相似文献
We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic 13C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state 13C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs.A new polymorph of l-tyrosine is reported, with the crystal structure determined by combined analysis of 3D-ED data and powder XRD data, augmented by information from periodic DFT-D calculations and solid-state 13C NMR data.相似文献
Functional and anatomical neuroimaging has had a dramatic effect on the evaluation of patients for seizure surgery. The demonstration by PET that the epileptogenic focus has interictal metabolic abnormalities has allowed a greater number of patients to come to seizure surgery, with fewer of these patients requiring intracranial electrode evaluations. Metabolic changes have also been demonstrated utilizing single voxel and whole brain 1H and 31P MRS imaging techniques with the interictal focus characterized by increased Pi, pH, and decreased PME and NAA. These findings can be used to accurately lateralize temporal lobe as well as frontal lobe epilepsy. Furthermore, there is evidence that these findings can be used to localize the seizure focus with the changes specific for the epileptogenic region; although, more diffuse changes both ipsilaterally and contralaterally have been seen. In patients with anterior hippocampal seizure foci the pH is significantly alkaline only in the ipsilateral hippocampus, whereas the increased Pi and decreased PME can be seen throughout the ipsilateral temporal lobe. When compared to controls the contralateral hemisphere is acidotic. Decreased NAA concentrations as well as NAA/Cr ratios have been demonstrated in the epileptogenic region in temporal and frontal lobe epilepsy. The decreased NAA has been correlated with the severity of cell loss, and may be a more sensitive measure than qualitative or quantitative measures of the hippocampal atrophy; however, the NAA decrease is more widespread than just the epileptogenic focus but may be maximal at the site of seizure initiation. In preliminary work, NAA maps of deviation from normality have suggested the maximal change to coincide with the epileptogenic region. These results suggest that in focal epilepsy there is abnormal metabolic activity throughout the brain detectable by MRS, with patterns of metabolic asymmetry that are useful for seizure localization. 相似文献
Fe–N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe–NHC complexes to date. Here, we have employed [Fe(iii)(btz)3]3+ (btz = (3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe–NHC complexes, [Fe(iii/ii)(btz)3]3+/2+ benefits from sizable charge transfer excited state lifetimes ≥0.1 ns in both oxidation states, and the Fe(iii) 2LMCT and Fe(ii) 3MLCT states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(iii) 2LMCT state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing 3MLCT state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) 2LMCT state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.An iron complex with N-heterocyclic carbene ligands engages in efficient photoredox catalysis via excited state electron transfer reactions of its Fe(ii) and Fe(iii) oxidation states.相似文献
Water-dispersible two-dimensional (2D) materials are desirable for diverse applications. Aqueous dispersions make processing safer and greener and enable evaluation of these materials on biological and environmental fronts. To evaluate the effects of 2D materials with biological systems, obtaining dispersions without additives is critical and has been a challenge. Herein, a method was developed for obtaining additive-free aqueous dispersions of 2D materials like transition metal dichalcogenides and hexagonal boron nitride (h-BN). The nanosheet dispersions were investigated through spectroscopic and microscopic methods, along with the role of size on stability. The aqueous media enabled investigations on cytocompatibility and enzymatic degradation of molybdenum disulphide (MoS2) and h-BN. Cytocompatibility with mixed glial cells was observed up to concentrations of 100 μg mL−1, suggesting their plausible usage in bioelectronics. Besides, biodegradation using human myeloperoxidase (hMPO) mediated catalysis was investigated through Raman spectroscopy and electron microscopy. The findings suggested that additive-free 2H-MoS2 and h-BN were degradable by hMPO, with 2H-phase exhibiting better resistance to degradation than the 1T-phase, while h-BN exhibited slower degradation. The findings pave a path for incorporating 2D materials in the burgeoning field of transient bioelectronics. 相似文献
Electrophilic 11C-labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross-coupling of aryl halides with [11C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [11C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP-ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [11C]benzamides, the current method paves the way for future application of [11C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis. 相似文献
Science China Chemistry - Rechargeable aluminum batteries (RABs) have attracted great interest as one of the most promising candidates for large-scale energy storage because of their high... 相似文献
The ophiobolin sesterterpenes are notable plant pathogens which have recently elicited significant chemical and biological attention because of their intriguing carbogenic frameworks, reactive functionalities, and emerging anticancer profiles. Reported herein is a total synthesis of (+)-6-epi-ophiobolin A in 14 steps, a task which addresses construction of the synthetically challenging spirocyclic tetrahydrofuran motif as well as several other key stereochemical problems. This work demonstrates a streamlined synthetic platform to complex ophiobolins leveraging disparate termination modes of a radical polycyclization cascade for divergent elaboration and functionalization. 相似文献
Dual detection systems are of interest for rapid, accurate data collection in sensing systems and in vitro testing. We introduce an IrIII complex with a boronic acid receptor site attached to the 2-phenylpyridine ligand as an ideal probe with photo- and electrochemical signals that is sensitive to monosaccharide binding in aqueous solution. The complex displays orange luminescence at 618 nm, which is reduced by 70 and 40 % upon binding of fructose and glucose, respectively. The electro-chemiluminescent signal of the complex also shows a direct response to monosaccharide binding. The IrIII complex shows the same response upon incorporation into hydrogel matrices as in solution, thus demonstrating the potential of its integration into a device, as a nontoxic, simple-to-use tool to observe sugar binding over physiologically relevant pH ranges and saccharide concentrations. Moreover, the complex's luminescence is responsive to monosaccharide presence in cancer cells. 相似文献