178,321 research outputs found
Out-of-plane deformation pathways of the R(X=)C-NR2 fragment present in amides, thioamides, amidines, enamines, and anilines. A concerted study making use of structural data, molecular mechanics, and ab initio calculations
No full textThe R(X=)C-NR(1)R(2) (R(1), R(2) = alkyl groups) fragment is present in many classes of molecules and assumes, usually, a planar conformation owing to the C-N partial double-bond character. It can undergo,however, a cis-trans isomerization process by rotation around the C-N bond and concomitant nitrogen pyramidalization. In a previous paper (Gilli, G.; Bertolasi, V.; Bellucci, F.; Ferretti, V. J. Am. Chem. Sec. 1986, 108, 2420) the isomerization pathway was mapped by the use of some 90 crystal structures containing the fragment of interest, and a semiempirical potential giving the total energy of the fragment during its deformation was proposed. In the present work the previous sample of crystal structures is updated to the current state of the crystallographic databases; the observed geometries are compared with the out-of-plane deformation energy maps obtained by ab initio SCF calculations at the 4-31G level for sample molecules (thioformamide, formamide, formamidine, vinylamine, and aniline) representative of the five chemical classes investigated. It is shown that the originally proposed potential is validated by this analysis and that the values of the energetic barriers involved in the reaction, evaluated from the ab initio energy maps, are in good agreement with the available experimental data. It is found, moreover, that there is a specific class of compounds (o- and p-nitroanilines and polyconjugated enamines and amidines) which deviate from the general behavior. The crystal structures of two of these compounds are reported, and possible reasons for the discrepancies discussed
Analysis of stability and bifurcations of limit cycles in Chua's circuit through the harmonic balance approach
No full textIEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS: PART
The high-redshift Universe with the International X-ray Observatory
No full textWe discuss some of the main open issues related to the light-up and evolution of the first accreting sources powering high redshift luminous quasars. We discuss the perspectives of future deep X-ray surveys with the International X-ray Observatory and possible synergies with the Wide Field X-ray Telescope
Identification of (high-redshift) AGN with WFXT: lessons from COSMOS and CDFS
No full textThe Wide Field X-ray Telescope (WFXT) will provide tens of millions of AGN, with more than 4x10^5 expected at z>3. Here we review the issues present in the identification of (large) samples of faint and high-redshift X-ray sources, and describe a statistical, powerful tool that can be applied to WFXT catalogs. The depth of associated optical and near infrared catalogs, needed for a reliable and as much complete as possible identification, are also discussed, along with the combined synergies with existing or planned facilitie
Presupuesto integral y beyond budgeting : Una propuesta superadora
No full textFil: Smolje, Alejandro R.. Universidad de Buenos Aires. Facultad de Ciencias Económicas. Buenos Aires, Argentina.Fil: Lavolpe, Antonio. Universidad de Buenos Aires. Facultad de Ciencias Económicas. Buenos Aires, Argentina.Fil: Gilli, Juan José. Universidad de Buenos Aires. Facultad de Ciencias Económicas. Buenos Aires, Argentina.Fil: Giménez, Carlos Manuel. Universidad de Buenos Aires. Facultad de Ciencias Económicas. Buenos Aires, Argentina.Fil: Meléndez, Horacio R.. Universidad de Buenos Aires. Facultad de Ciencias Económicas. Buenos Aires, Argentina
Structure of 3-(p-chlorophenyl)-1-phenyl-1,3-propanedione enol
No full textAbstract: 1-(4-Chlorophenyl)-3-hydroxy-3-phenyl-2-propen-1-one, C15H11ClO2, M(r) = 258.7, C2/c, a = 24.324 (4), b = 6.537 (2), c = 15.598 (2) angstrom, beta = 93.52 (1)-degrees, V = 2475.5 (9) angstrom3, Z = 8, D(x) = 1.382 g cm-3, lambda(Mo Kalpha) = 0.71069 angstrom, mu = 2.95 cm-1, F(000) = 1072, T = 298 K, final R = 0.043 for 1192 observed reflections. The compound displays a strong intramolecular asymmetric hydrogen bond [O...O = 2.471 (4), O-H = 1.09 angstrom, IR nu(OH) stretching frequency = 2577 cm-1, H-1 NMR chemical shift of the enolic proton = 16.8 p.p.m.] which can be interpreted in terms of resonance-assisted hydrogen bonding. The proton localization, that is, the preference displayed by the proton for settling on an O atom of the beta-diketone fragment rather than on the other O atom, is discussed and related to the different environments of the two O atoms in the crystal packing and, in particular, to the asymmetry of their C-H...O short contacts
Crystal structures of five policyclic compounds related to the natural product forskolin
No full text(3alphabeta(Z),4beta,6aalpha,9abeta,9bbeta)-(+/-)-3a,4,6a,7,8,9,9a,9b-octahydro-4,7,7,9b-tetramethyl-3a-[3-(methoxymethylocy)-3-methyl-l-butenyl]-5H-naphto[1,8-de]-1,3-dioxin-6-one (I), C22H36O5, M(r) = 378.51, monoclinic, P2(1)/n, a = 6.330(1), b = 14.576(2), c = 22.837(2) angstrom, beta = 93.04(1)-degrees, V = 2104.1(2) angstrom3, Z = 4, D(c) = 1.19 Mg/m3, lambda(MoKalpha) = 0.71069 angstrom, mu = 0.8 cm-1, F(000) = 832, T = 298 K, R = 0.054 for 1971 observed reflections; (7abeta, 10aalpha, 10bbeta, 12a)-(+/-)-7a9,10,10a,10b,11,12,12a-octahydro-2,2,2,10,10,10b,12a-hexamethyl-2H,8H-1-benzopy-rano[4a,5,6,-de][1,32]-benzodioxin-11-one (II), C20H29O4, M(r) = 334.5, triclinic, P-1, a = 10.595(2), b = 12.152(1), c = 8.073(1) angstrom, alpha = 106.52(1), beta = 105.65(1), gamma = 66.29(1)-degrees, V = 897.9(2) angstrom3, Z = 2, D(c) = 1.24 Mg/m3, lambda(MoKalpha) = 0.71069 angstrom, mu = 0.8 cm-1, F(000) = 362, T = 298 K, R = 0.046 for 2848 observed reflections; (7abeta, 10aalpha, 10bbeta, 12alpha, 12aalpha)-(+/-)-7a,9,10,10a,10b,11,12,12a-octahydro-2,2,10,10,10b,12a-hexamethyl-2H,8H-1-benzopy-rano[4a,5,6-de][1,3,2]-benzodioxin-11,12-diol (III), C20H32O5 (two molecules in the asymmetric unit), M(r) = 352.2, triclinic, P-1, a = 12.948(3), b = 13.615(3), c = 12.197(4) angstrom, alpha = 101.16(2), beta = 111.88(2), gamma = 69.48(2)-degrees, V = 1863.8(9) angstrom3, Z = 2, D(c) = 1.26 Mg/m3, lambda(MoKalpha) = 0.71069 angstrom, mu = 0.8 cm-1, F(000) = 768, T = 298 K, R = 0.060 for 4570 observed reflections; 4-acetoxy-4-[[(4abeta,5alpha,8abeta)-(+/-)-hexahydro-4a,6,6-trimethyl-4H-1,3-benzodioxin-4-one]-5-yl]butan-2-one (IV), C17H26O6, M(r) = 326.4, monoclinic, P2(1)/c, a = 10.495(2), b = 12.050(2), c = 14.216(2) angstrom, beta = 108.51(1)-degrees, V = 1704.8(5) angstrom3, Z = 4, D(c) = 1.27 Mg/m3, lambda(MoKalpha) = 0.71069 angstrom, mu = 0.9 cm-1, F(000) = 704, T =298 K, R = 0.049 for 2455 observed reflections; (3aalpha,4beta,5beta,6beta,6aalpha,9abeta,9bbeta)-(+/-)-4,5-epoxy-decahydro-3,3a-dihydroxy-2-ethoxy-4,7,7,9b-tetramethyl-naphto-[1,8-bc]-pyran-6-ol-acetate (V), C20H32O7, M(r) = 383.5, monoclinic, C2/c, a = 10.353(2), b = 17.975(3), c = 21.188(3) angstrom, beta = 91.29(1)-degrees, V = 3942(1) angstrom3, Z = 8, D(c) = 1.29 Mg/m3, lambda(MoKalpha) = 0.71069 angstrom, mu = 0.8 cm-1, F(000) = 1664, T =298 K, R = 0.051 for 2120 observed reflections. We report here the complete structures of four decalin derivatives (compounds I, II, III, V) and one related compound (compound IV) synthetized in order to find an efficient synthetic approach for the natural product forskolin
Intramolecular N–H⋯O hydrogen bonding assisted by resonance. III. Structural studies of 1-ketone-2-arylhydrazone derivatives
No full textThe crystal and molecular structures of the following 1-ketone-2-arylhydrazones have been determined: (1) 4-(phenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H9N3O2, M(r) = 203.2, monoclinic, C2/c, a = 11.100 (3), b = 7.960 (2), c = 21.744 (4) Angstrom, beta = 95.82 (2)degrees, V = 1911.3 (8) Angstrom(3), Z = 8, D-x = 1.41 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.096 mm(-1), F(000) = 848, T = 295 K, R = 0.040 for 1737 observed reflections; (2) 4-(4-nitrophenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H8N4O4, M(r) = 248.2, monoclinic, P2(1)/c, a = 7.046 (2), b = 7.774 (1), c = 20.917 (4) Angstrom, beta = 98.09 (2)degrees, V = 1134.3 (4) Angstrom(3), Z = 4, D-x = 1.45 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.108 mm(-1), F(000) = 512, T = 295 K, R = 0.048 for 1599 observed reflections; (3) 4-(2-chlorophenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H8ClN3O2, M(r) = 237.6, orthorhombic, Pca2(1), a = 18.324 (2), b = 5.771 (1), c = 20.016 (2) Angstrom, V = 2116.6 (6) Angstrom(3), Z = 8, D-x = 1.49 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.345 mm(-1), F(000) = 976, T = 295 K, R = 0.044 for 1359 observed reflections: (4) 4-(4-nitrophenylhydrazono)-3-methyl-1-phenyl-4H-pyrazol-5-one, C16H13N5O3, M(r) = 323.3, monoclinic, P2(1)/n, a = 12.817 (1), b = 7.595 (5), c = 15.652 (3) Angstrom,beta = 95.74 (1)degrees, V = 1516 (1) Angstrom(3), Z = 4, D-x = 1.42 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.096 mm(-1), F(000) = 672, T = 295 K, R = 0.045 for 1672 observed reflections. All molecules are planar and present pi-delocalization within the ketohydrazone moiety. These conditions should favour the formation of a strong intramolecular N-H...O hydrogen bond assisted by resonance, but in the present case all compounds display hydrogen bonds which can be considered weak or very weak. An analysis of the bond angles suggests that the discrepancy is to be imputed to the fact that the heterocyclic five-membered rings introduce a geometrical constraint which hinders the strengthening of the hydrogen bond. Spectroscopic data, nu(NH) IR frequencies and delta(NH) NMR chemical shifts are in agreement with the hydrogen-bonding magnitude measured by N...O distances. The steric impossibility of obtaining short N...O distances gives rise to the formation of other intermolecular interactions, relevant to the crystal packing, such as bifurcated N-H...O hydrogen bonds, C-H...O short contacts and pi-pi interactions
Un percorso di avvicinamento all'arte per bambini: la comprensione dell'opera pittorica in età evolutiva
No full textQualitative analysis of the dynamics of the time delayed Chua's circuit
No full textIEEE TRANS. CIRCUITS SYST.
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