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41.
David K. Geiger Dylan E. Parsons 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(7):681-688
The structures of 4‐nitrobenzene‐1,2‐diamine [C6H7N3O2, (I)], 2‐amino‐5‐nitroanilinium chloride [C6H8N3O2+·Cl−, (II)] and 2‐amino‐5‐nitroanilinium bromide monohydrate [C6H8N3O2+·Br−·H2O, (III)] are reported and their hydrogen‐bonded structures described. The amine group para to the nitro group in (I) adopts an approximately planar geometry, whereas the meta amine group is decidedly pyramidal. In the hydrogen halide salts (II) and (III), the amine group meta to the nitro group is protonated. Compound (I) displays a pleated‐sheet hydrogen‐bonded two‐dimensional structure with R22(14) and R44(20) rings. The sheets are joined by additional hydrogen bonds, resulting in a three‐dimensional extended structure. Hydrohalide salt (II) has two formula units in the asymmetric unit that are related by a pseudo‐inversion center. The dominant hydrogen‐bonding interactions involve the chloride ion and result in R42(8) rings linked to form a ladder‐chain structure. The chains are joined by N—H...Cl and N—H...O hydrogen bonds to form sheets parallel to (010). In hydrated hydrohalide salt (III), bromide ions are hydrogen bonded to amine and ammonium groups to form R42(8) rings. The water behaves as a double donor/single acceptor and, along with the bromide anions, forms hydrogen bonds involving the nitro, amine, and ammonium groups. The result is sheets parallel to (001) composed of alternating R55(15) and R64(24) rings. Ammonium N—H...Br interactions join the sheets to form a three‐dimensional extended structure. Energy‐minimized structures obtained using DFT and MP2 calculations are consistent with the solid‐state structures. Consistent with (II) and (III), calculations show that protonation of the amine group meta to the nitro group results in a structure that is about 1.5 kJ mol−1 more stable than that obtained by protonation of the para‐amine group. DFT calculations on single molecules and hydrogen‐bonded pairs of molecules based on structural results obtained for (I) and for 3‐nitrobenzene‐1,2‐diamine, (IV) [Betz & Gerber (2011). Acta Cryst. E 67 , o1359] were used to estimate the strength of the N—H...O(nitro) interactions for three observed motifs. The hydrogen‐bonding interaction between the pairs of molecules examined was found to correspond to 20–30 kJ mol−1. 相似文献
42.
A single multiblock copolymer chain in poor solvent undergoes microphase separation within its own globule, driven by the same kind of forces operating in the bulk system. However, the necessity of packing a large AB interface into a small volume leads to novel convoluted geometries. Long block lengths form a double droplet. Very short block lengths exhibit bulk behavior, forming a lamellar globule. With intermediate block lengths, the AB interface buckles to form a hand shake or spiral dicluster. An order-disorder transition is reported for short block lengths. 相似文献
43.
Unprecedented 1,14‐seco‐Crotofolanes from Croton insularis: Oxidative Cleavage of Crotofolin C by a Putative Homo‐Baeyer–Villiger Rearrangement
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Dr. Lidiya A. Maslovskaya Dr. Andrei I. Savchenko Carly J. Pierce Dr. Victoria A. Gordon Dr. Paul W. Reddell Prof. Peter G. Parsons Prof. Craig M. Williams 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(44):14226-14230
EBC‐162 isolated from Croton insularis, obtained from the northern rainforest of Australia, was structurally affirmed as crotofolin C ( 4 ). Novel oxidative degradation products, EBC‐233 and EBC‐300, which are the first crotofolane endoperoxides, were also isolated. Both endoperoxides were found to be stable intermediates, which are proposed to undergo an unprecedented homo‐Baeyer–Villiger biosynthetic rearrangement to give a new class of 1,14‐seco‐crotofolane diterpenes. Prolonged storage of all isolates assisted in authenticating their natural product status. Anticancer activities of reported compounds are presented. 相似文献
44.
Josiane Arnaud Isabelle Coté Patrick J. Parsons Marina Patriarca Andrew Taylor Cas Weykamp 《Accreditation and quality assurance》2014,19(3):169-174
Proficiency testing schemes monitor laboratory performance and provide a stimulus for improvement in accuracy. Where several schemes operate in the same analytical sector, there are risks that assessments of performance may be in conflict. Performance assessment for the determination of trace elements such as aluminum in serum is particularly important due to the high risk of contamination and therefore erroneous results. The objectives of this work were (1) to compare several mathematical models to establish a predefined standard deviation for proficiency assessment and (2) to evaluate the influence of instrumental methods and proficiency testing scheme on the assessment of performance for serum aluminum measurements. For this purpose, three samples were sent to the participants of four proficiency testing schemes. Assigned values were calculated according to algorithm A according to ISO 13528 and standard deviation for proficiency assessment according to three methods based on individual variability, state of the art or previous proficiency testing results. The method based on individual variability produced a more stringent standard deviation compared to analytical imprecision based on the state of the art. The instrumental methods gave similar results, whereas significant differences were observed between the four proficiency testing schemes indicating that harmonization of the standard deviation for proficiency assessment fails to allow transferability from one proficiency testing scheme to another and that additional factor(s) contribute to variability in performance assessment. 相似文献
45.
R.J. Brown M.L. Parsons 《Journal of Quantitative Spectroscopy & Radiative Transfer》1979,21(6):553-561
Relative atomic transition probabilities (Aji), for Na, Sc, Ti, Cr, Fe, Co, and Ni, using flame atomic absorption spectroscopy have been determined. Absorption measurements in air-acetylene and nitrous oxide-acetylene flames using both line emission and continuum excitation sources were employed. A comprehensive discussion and critical evaluation of Aji values for this work and others of comparable scope and design is included. The effect of the absorption line profile including hyperfine structure is discussed. 相似文献
46.
47.
R.R. Parsons 《Solid State Communications》1979,29(11):763-766
The origin of the photoluminescence in heavily-doped silicon is examined. Transient photoluminescence data for Si(P) are presented and used to identify the “Low Level” emission bands in terms of recombination of impurity band electrons with holes bound to acceptor sites. The “High Level” bands are attributed to recombination of impurity band electrons with free holes. The energies of the band gap and optical band gap in heavily-doped silicon are determined from the photoluminescence measurements. 相似文献
48.
49.
Albone DP Challenger S Derrick AM Fillery SM Irwin JL Parsons CM Takada H Taylor PC Wilson DJ 《Organic & biomolecular chemistry》2005,3(1):107-111
Amination of C-H bonds activated by ether oxygen atoms is facile with chloramine-T as nitrene source and copper(I) chloride in acetonitrile as catalyst. For cyclic ethers the hemiaminal products are generally stable and can be isolated pure. For acyclic ethers, the hemiaminal products, as expected, fragment with elimination of alcohol to yield imines. When activation of benzylic positions is remote through a conjugated system, stable benzylamine derivatives are isolated. Mechanistic studies are consistent with concerted insertion of an electrophilic nitrenoid into the C-H bond in the rate-determining step, though in an asynchronous manner with a more activated substrate. 相似文献
50.
Timms PL Norman NC Pardoe JA Mackie ID Hinchley SL Parsons S Rankin DW 《Dalton transactions (Cambridge, England : 2003)》2005,(3):607-616
The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods. 相似文献