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71.
Perepichka IF Popov AF Orekhova TV Bryce MR Andrievskii AM Batsanov AS Howard JA Sokolov NI 《The Journal of organic chemistry》2000,65(10):3053-3063
2,4,5,7-Tetranitro-9-fluorenone (1b) reacts readily with n-butanethiol in dipolar aprotic solvents with selective substitution of nitro groups by butylsulfanyl groups in positions 2 and 7 (2, 3); the 2,5-isomer 4 was formed only as a minor product (<1%). Condensation of fluorenones 2-4 with malononitrile yielded 9-dicyanomethylene derivatives 5-7, which showed strong intramolecular charge transfer (lambda approximately 510-560 nm) and were found to sensitize the photoconductivity of carbazole-containing polymer films. Oxidation of sulfides 2-4 gave sulfoxide 8 or sulfones 9-11, which then were converted into their corresponding dicyanomethylene derivatives 12-15. All these novel acceptors showed three reversible single-electron reduction waves (cyclic voltammetry) yielding radical anion, dianion, and radical trianion; moreover, acceptors 13-15 showed also a fourth reduction wave, representing reversible tetraanion formation. Substitution of the oxygen of the carbonyl group in the fluorenones by a dicyanomethylene group increased the thermodynamic stability (K(SEM) growth) of the radical anion; K(SEM) ranged from 3 x 10(5) to 3 x 10(9) M(-1). CV measurements characterize compounds 3, 4 (EA = 1. 86-1.89 eV) as poor acceptors, 2, 6-11 (EA = 2.13-2.31 eV) as moderate acceptors, and 5, 12-15 (EA = 2.53-2.66 eV) as strong electron acceptors. Charge-transfer complex (CTC) formation between acceptors 9, 10, 13, 14, and anthracene as a donor was monitored by the appearance of additional low-energy bands in the visible region (CTC bands) of their electron absorption spectra. Increasing the EA of the acceptors from 9-fluorenones to the corresponding 9-dicyanomethylenefluorenes increases the complexation constants K(CTC) by 2.5-3 times, while sulfonyl substituents present substantial steric hindrance for complexation (as compared to the nitro group), decreasing K(CTC) values. Two CTCs for acceptors 14 and 17 with tetrathiafulvalene (TTF) were obtained, and their structures were solved by single-crystal X-ray diffractometry, giving the stoichometries 14:TTF, 2:3, and 17:TTF:PhCl, 1:1:0.5. In the former complex the packing motif is a mixed.DDAD'A. stack; in the latter complex the D and A moieties form unusually close CT pairs, which pack in a herringbone motif. 相似文献
72.
A Fujita type global existence-global nonexistence theorem for a weakly coupled system of reaction-diffusion equations 总被引:3,自引:0,他引:3
Howard A. Levine 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1991,42(3):408-430
LetDR
N be a region with smooth boundaryD. Letp·q>1,p, q1. We consider the system:u
t=u+v
p,v
t=u+u
q inD×[0, ) withu=v=0 inD×[0, ) andu
0,v
0 nonnegative. Let=max(p, q). We show that ifD isR
N, a cone or the exterior of a bounded domain, then there is a numberpc(D) such that (a) if (+1)/(pq–1)>pc(D) no nontrivial global positive solutions of the system exist while (b) if (+1)/(pq–1)<pc(D) both nontrivial global and nonglobal solutions exist. In caseD is a cone orD=R
N, (a) holds with equality. An explicit formula forpc(D) is given.This research was supported in part by NSF Grant DMS-8822788 and in part by the Air Force Office of Scientific Research. 相似文献
73.
Using petroleum ether as a precipitant and chloroform–acetone mixture as solvent, six fractions of cellulose triacetate were obtained by fractional precipitation. The molecular weight of each fraction was obtained from osmotic pressure measurements carried out on chloroform solutions. It was shown that virtual nonfractionability of cellulose triacetate with regard to molecular weight occurred in this system. This behavior was also observed in attempts to fractionate it from tetrachloroethane or acetic acid solutions. These results are explained by the hypothesis that specific polymer–solvent interaction takes place due to hydrogen bonding. 相似文献
74.
Keith P. Brierley Joseph Howard Clifford J. Ludman Keith Robson Thomas C. Waddington John Tomkinson 《Chemical physics letters》1978,59(3):467-471
Inelastic neutron scattering spectra of KHCO3 and CsH(NO3)2 have been obtained in the region 400 → 2400 and 400 → 2800 cm?1 respectively. The in- and out-of-phase bending vibrations of the hydrogen bonds have been observed and assigned. For CsH(NO3)2 the two bending modes are closer in frequency than in KHCO3 and they are not resolved from the antisymmetric stretch. 相似文献
75.
The complexes [1,3-diphenyl-1,3-propanedionato]tricyclohexyltin(IV), (tropolonato)triphenyltin(IV), and (tropolonato)tricyclohexyltin(IV) have been prepared for the first time and have been found to be five-coordinate in the solid state. These and related five-coordinate complexes prepared previously have been studied by a variety of physical methods; 13C NMR, UV, IR, Raman, dipole moments and the Kerr effect. While all structures are demonstrably five-coordinate, and all chelates bidentate in the solid state, the geometries of two of the complexes in solution appear to vary somewhat from the expected fac or mer. There is evidence from the solution Kerr effect and 13C NMR that cyclohexyl derivatives may disproportionate. 相似文献
76.
Blackwell HE Sadowsky JD Howard RJ Sampson JN Chao JA Steinmetz WE O'Leary DJ Grubbs RH 《The Journal of organic chemistry》2001,66(16):5291-5302
Heptapeptides containing residues with terminal olefin-derivatized side chains (3 and 4) have been treated with ruthenium alkylidene 1 and undergone facile ring-closing olefin metathesis (RCM) to give 21- and 23-membered macrocyclic peptides (5 and 6). The primary structures of peptides 3 and 4 were based upon a previously studied heptapeptide (2), which was shown to adopt a predominantly 3(10)-helical conformation in CDCl(3) solution and an alpha-helical conformation in the solid state. Circular dichroism, IR, and solution-phase (1)H NMR studies strongly suggested that acyclic precursors 3 and 4 and the fully saturated macrocyclic products 7 and 8 also adopted helical conformations in apolar organic solvents. Single-crystal X-ray diffraction of cyclic peptide 8 showed it to exist as a right-handed 3(10)-helix up to the fifth residue. Solution-phase NMR structures of both acyclic peptide 4 and cyclic peptide 8 in CD(2)Cl(2) indicated that the acyclic diene assumes a loosely 3(10)-helical conformation, which is considerably rigidified upon macrocyclization. The relative ease of introducing carbon-carbon bonds into peptide secondary structures by RCM and the predicted metabolic stability of these bonds renders olefin metathesis an exceptional methodology for the synthesis of rigidified peptide architectures. 相似文献
77.
The solubility of cellulose triacetate in a range of solvents was measured, and the results for tetrachloroethane, chloroform, and acetic acid were compared with those from initial phase separation in solvent–nonsolvent mixtures and viscosity–concentration studies. The correlation found between solubilities, precipitation values, and values of the Huggins viscosity constant is discussed with reference to the type of polymer–solvent interaction proposed previously to explain fractionation behavior. A qualitative comparison of solubility–swelling behaviour was also made for a very low molecular weight cellulose triacetate sample in a wide range of solvents. Results are compared with those for higher molecular weight samples and discussed with regard to the cohesiveenergy densities of solvent and polymer. Some attempt has been made to predict suitable solvents for cellulose triacetate, based on consideration of their molecular structures. 相似文献
78.
Nabih S. Girgis Howard B. Cottam Roland K. Robins 《Journal of heterocyclic chemistry》1988,25(2):361-366
The 2′-deoxyribofuranose analog of the naturally occurring antibiotics SF-2140 and neosidomycin were prepared by the direct glycosylation of the sodium salts of the appropriate indole derivatives, with 1-chloro-2- deoxy-3,5-di-O-p-toluoyl-α-D-erythropentofuranose ( 5 ). Thus, treatment of the sodium salt of 4-methoxy-1H- indol-3-ylacetonitrile ( 4a ) with 5 provided the blocked nucleoside, 4-methoxy-1-(2-deoxy-3,5-di-O-p-toluoyl-β- D-erythropentofuranosyl)-1H-indol-3-ylacetonitrile ( 6a ), which was treated with sodium methoxide to yield the SF-2140 analog, 4-methoxy-1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indol-3- ylacetonitrile ( 7a ). The neosidomycin analog ( 8 ) was prepared by treatment of the sodium salt of 1H-indol-3-ylacetonitrile ( 4b ) with 5 to obtain the blocked intermediate 1-(2-deoxy-3,5-di-O-p-toluoyl-β-D-erythropentofuranosyl) ?1H-indol-3-ylace-tonitrile ( 6b ) followed by sodium methoxide treatment to give 1-(2-deoxy-β-D-erythropentofuranosyl)-1H- indol-3-ylacetonitrile ( 7b ) and finally conversion of the nitrile function of 7b to provide 1-(2-deoxy-β-D- erythropentofuranosyl)-1H-indol-3-ylacetamide ( 8 ). In a similar manner, indole ( 9a ) and several other substituted indoles including 1H-indole-4-carbonitrile ( 9b ), 4-nitro-1H-indole ( 9c ), 4-chloro-1H-indole-2-carboxamide ( 9d ) and 4-chloro-1H-indole-2-carbonitrile ( 9e ) were each glycosylated and deprotected to provide 1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole ( 11a ), 1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole-4- carbonitrile ( 11b ), 4-nitro-1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole ( 11c ), 4-chloro-1-(2-deoxy-β-D- erythropentofuranosyl)-1H-indole-2-carboxamide ( 11d ) and 4-chloro-1-(2-deoxy-β-D-erythropentofuranosyl)- 1H-indole-2-carbonitrile ( 11e ), respectively. The 2′-deoxyadenosine analog in the indole ring system was prepared for the first time by reduction of the nitro group of 11c using palladium on carbon thus providing 4-amino-1-(2-deoxy-β-D-erythropentofuranosyl)- 1H-indole ( 16 , 1,3,7-trideaza-2′-deoxyadenosine). 相似文献
79.
July 1969, 528 pages, illustrated, $25.50 相似文献
80.
Direct observations of infrared absorption in the weakly bound dimer of carbonyl sulphide, produced in pulsed molecular beams, have been made. Diode laser spectra in the 5 micron, carbonyl stretching, wavelength region were recorded with FWHM linewidths of approximately 100 MHz. All of the observations are consistent with OCS dimer having a C 2h , centrosymmetric geometry, with the centres of mass of the monomers nearly opposite one another. This essentially perpendicular structure places the S atoms close to the centre of the complex. The perpendicular distance between the monomer axes is 3·64 Å. Intermolecular potential functions containing dispersion and electrostatic contributions are discussed. 相似文献