Size-dependent electron chemical potential: Effect on particle charging

Electrostatic charging of particles of identical composition, but different sizes, is a poorly understood phenomenon that may be of importance in dust storms, generation of lightning, numerous technological applications involving solid particulates, and in the agglomeration of lunar dust and inter-stellar dust clouds. We show that under optical excitation, the relative magnitude of surface to volume de-excitation gives size-dependent electron and hole concentrations. The consequent differences in chemical potentials can lead to charge transfer between particles of different size. The direction of charge transfer, from large to small or vice versa, depends critically on the properties of the materials.     

Mathematical analysis of the dynamics of COVID-19 in Africa under the influence of asymptomatic cases and re-infection

Coronavirus pandemic (COVID-19) hit the world in December 2019, and only less than 5% of the 15 million cases were recorded in Africa. A major call for concern was the significant rise from 2% in May 2020 to 4.67% by the end of July 15, 2020. This drastic increase calls for quick intervention in the transmission and control strategy of COVID-19 in Africa. A mathematical model to theoretically investigate the consequence of ignoring asymptomatic cases on COVID-19 spread in Africa is proposed in this study. A qualitative analysis of the model is carried out with and without re-infection, and the reproduction number is obtained under re-infection. The results indicate that increasing case detection to detect asymptomatically infected individuals will be very effective in containing and reducing the burden of COVID-19 in Africa. In addition, the fact that it has not been confirmed whether a recovered individual can be re-infected or not, then enforcing a living condition where recovered individuals are not allowed to mix with the susceptible or exposed individuals will help in containing the spread of COVID-19.     

ORGANOPHOSPHORUS COMPOUNDS AS POTENTIAL FUNGICIDES. PART I. N-(ω-GUANI-DINOALKYL)AMINOALKANEPHOSPHONIC ACIDS AND THEIR AMINOPHOSPHONIC PRECURSORS: PREPARATION,NMR SPECTROSCOPY,AND FAST ATOM BOMBARDMENT MASS SPECTROMETRY

Abstract

N-(ω-Aminoalkyl)- and N-(ω-guanidinoalkyl)-aminoalkanephosphonic acids have been prepared from α, ω-diaminoalkanes by reaction with chloromethanephosphonic acid (or an ester of a halogeno-alkanephosphonic acid), followed by treatment with S-methylisothiouronium chloride. Ethylene diamine yielded 1-phosphonomethyl-2-iminoimidazolidine. A number of 1:1 salts of the α, ω-diamines and chloromethanephosphonic acid are also reported. Doubly charged zwitterionic structures are assigned to both ω-amino and ω-guanidino compounds on the basis of ³¹P and ¹³C nmr data. Thus the addition of an excess of acid (D₂SO₄) causes the ³¹P chemical shift to move to higher field, from ca. 8 to 14 ppm, whilst ¹ Jpc increases from ca. 130 to 150 Hz. The ¹H and ¹³C chemical shifts of the terminal methylene groups in the polymethylene chain are unaffected by acidification.

Fast atom bombardment mass spectrometry gives rise to characteristic [M + H]⁺ ions, frequently as the base peak, and to fragmentations involving the loss of phosphorous acid, or the formation of ions resulting from carbon-nitrogen or carbon-carbon cleavage. The compounds show activity against a number of fungal pathogens and other microbial organisms.     

Sterically congested macrobicycles with heteroatomic bridgehead functionality

A series of cyclophanes composed of two triarylelement caps linked by two-atom bridges has been synthesized. The bridgehead functional groups include phosphines in combination with amines, hydrosilanes, methylsilanes, and ethoxysilanes. Computational studies accurately predicted that when the bridgehead substituents are small (lone pairs or protons), an in_,in bridgehead stereochemistry is strongly favored, but larger bridgehead substituents favor the formations of in_,out stereoisomers. The X-ray structures, spectra, and reactivity of these compounds are discussed, as well as the resolution of one of the cyclophanes into pure enantiomers.     

Optimization of matrix solid-phase dispersion conditions for UV filters determination in biota samples

A low solvent consumption method for the determination of eight ultraviolet (UV) filters, displaying low to medium polarities, in freeze-dried samples of marine bivalves and fish is proposed. Matrix solid-phase dispersion (MSPD) and gas chromatography with mass spectrometry (GC-MS) were used as sample preparation and determination techniques, respectively. This work describes the influence of several parameters (type and amount of dispersant and clean-up sorbents, as well as elution solvent) on the yield and the selectivity of the MSPD extraction. Under optimized conditions, samples (0.5?g) were ground with 2?g of Florisil in a mortar with a pestle and transferred into a polypropylene syringe, which contained 1?g of C18 as clean-up sorbent. Analytes were eluted with 5?mL of acetonitrile. This extract was concentrated to dryness, re-constituted with 1?mL of ethyl acetate and injected in the GC-MS system without any further clean-up. The global average recoveries, measured for three different biota samples, spiked at three different levels (between 50 and 1000?ng?g^?1), ranged from 80% to 101% with associated standard deviations below 10%. The inter-day precision of the method varied from 4% to 15% and the achieved LOQs (defined for a signal to noise ratio of 10) ranged from 4 to 28?ng?g^?1, referred to the freeze-dried matrix. Octocrylene (OCR) was found in some samples of fish and mussels at concentrations between 15 and 20?ng?g^?1, referred to dry mass.     

Self‐Assembled Tetragonal Prismatic Molecular Cage Highly Selective for Anionic π Guests

The metal‐directed supramolecular synthetic approach has paved the way for the development of functional nanosized molecules. In this work, we report the preparation of the new nanocapsule 3? (CF₃SO₃)₈ with a A₄B₂ tetragonal prismatic geometry, where A corresponds to the dipalladium hexaazamacrocyclic complex Pd‐1 , and B corresponds to the tetraanionic form of palladium 5,10,15,20‐tetrakis(4‐carboxyphenyl)porphyrin ( 2 ). The large void space of the inner cavity and the supramolecular affinity for guest molecules towards porphyrin‐based hosts converts this nanoscale molecular 3D structure into a good candidate for host–guest chemistry. The interaction between this nanocage and different guest molecules has been studied by means of NMR, UV/Vis, ESI‐MS, and DOSY experiments, from which highly selective molecular recognition has been found for anionic, planar‐shaped π guests with association constants (K_a) higher than 10⁹ M ^?1, in front of non‐interacting aromatic neutral or cationic substrates. DFT theoretical calculations provided insights to further understand this strong interaction. Nanocage 3? (CF₃SO₃)₈ can not only strongly host one single molecule of M(dithiolene)₂ complexes (M=Au, Pt, Pd, and Ni), but also can finely tune their optical and redox properties. The very simple synthesis of both the supramolecular cage and the building blocks represents a step forward for the development of polyfunctional supramolecular nanovessels, which offer multiple applications as sensors or nanoreactors.