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101.
Balandina A Kalinin A Mamedov V Figadère B Latypov S 《Magnetic resonance in chemistry : MRC》2005,43(10):816-828
(1)H, (13)C and (15)N NMR chemical shifts for a variety of novel quinoxalines were determined by different 2D methods and were calculated using the GIAO DFT approach. Comparison with experimental data shows good correlations in the case of (1)H, (13)C and (15)N chemical shifts. Different combinations of basis sets were tested. In non-polar solvents quinoxalines exist as dimers owing to strong hydrogen bonding. Calculations for dimers improve the correlation between experiment and theory. Additive empirical methods for estimating chemical shifts have drawbacks and have to be used with a great care for this type of compound. 相似文献
102.
Photochemistry of ε,ζ-Methano-α,γ-dienones and 7,8-Methano-1,3,5-trienes Irradiation of the δ-cyclopropyl-dienone (E)- 6 (λ ≥ 347 nm) gives (Z)- 6, 10 (1,5-sigmatropic H-shift), (E/Z)- 9 (electrocyclic process involving C(ε), C(ζ)-cleavage) and 11 (ring opening). The corresponding 6-cyclopropyl-triene (E)- 7 gives on singlet excitation (δ > 280 nm) 14 (1,5-sigmatropic H-shift) and, to a smaller extent, the bicyclo [3.2.0] heptenyl-dienes (E/Z)- 13 . However, on triplet excitation (λ ≥ 347 nm, benzophenone) (E)- 7 gives (E/Z)- 13 as the main products. On both 1π,π*- and 3π,π*-excitation, (Z)- 7 and 15 are formed in small amounts. 相似文献
103.
Armando Navarro-Huerta Marcus J. Jellen Jessica Arcudia Simon J. Teat Rubn A. Toscano Gabriel Merino Braulio Rodríguez-Molina 《Chemical science》2021,12(6):2181
This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I. The crystal structures of these rotors, determined by synchrotron radiation experiments at different temperatures, show the presence of such C–H⋯F–C contacts between extended carbazole stators featuring fluorinated phenyl rings and the 1,4-diazabicyclo[2.2.2]octane (DABCO) rotator. According to the 2H NMR results, using deuterated samples, and periodic density functional theory computations, the rotators experience fast angular displacements (preferentially 120° jumps) due to their low rotational activation energies (Ea = 0.8–2.0 kcal mol−1). The higher rotational barrier for 1 (2.0 kcal mol−1) is associated with a larger number of weak C–H⋯F–C contacts generated by the stators. This strategy offers the possibility to explore the correlation among weak intermolecular forces, cavity shape, and internal dynamics, which has strong implications in the design of future fine-tuned amphidynamic crystals.This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I. 相似文献
104.
Bruno Therrien Ludovic Vieille-Petit Petr Štěpni?ka Georg Süss-Fink 《Journal of organometallic chemistry》2004,689(15):2456-2463
A series of arene-ruthenium complexes of the general formula [RuCl2{η6-C6H5(CH2)2R}L] with R=OH, CH2OH, OC(O)Fc, CH2OC(O)Fc (Fc=ferrocenyl) and L=PPh3, (diphenylphosphino)ferrocene, or bridging 1,1′-bis(diphenylphosphino)ferrocene, have been synthesized. Two synthetic pathways have been used for these ferrocene-modified arene-ruthenium complexes: (a) esterification of ferrocene carboxylic acid with 2-(cyclohexa-1,4-dienyl)ethanol, followed by condensation with RuCl3 · nH2O to afford [RuCl2{η6-C6H5(CH2)2OC(O)Fc}]2, and (b) esterification between ferrocene carboxylic acid and [RuCl2{η6-C6H5(CH2)3OH}L] to give [RuCl2{η6-C6H5(CH2)3OC(O)Fc}L]. All new compounds have been characterized by NMR and IR spectroscopy as well as by mass spectrometry. The single-crystal X-ray structure analysis of [RuCl2{η6-C6H5(CH2)3OH}(PPh3)] shows that the presence of a CH2CH2CH2OH side-arm allows [RuCl2{η6-C6H5(CH2)3OH}(PPh3)] to form an intramolecular hydrogen bond with a chlorine atom. The electrochemical behavior of selected representative compounds has been studied. Complexes with ferrocenylated side arms display the expected cyclic voltammograms, two independent reversible one-electron waves of the Ru(II)/Ru(III) and Fe(II)/Fe(III) redox couples. Introduction of a ferrocenylphosphine onto the ruthenium is reflected by an additonal reversible, one-electron wave due to ferrocene/ferrocenium system which is, however, coupled with the Ru(II)/Ru(III) redox system. 相似文献
105.
Castagnola M Rossetti DV Inzitari R Vitali A Lupi A Zuppi C Cabras T Fadda MB Petruzzelli R Giardina B Messana I 《Electrophoresis》2003,24(5):801-807
Measurements by capillary electrophoresis (CE) of bacitracin A(1) effective mobility at different pH values permitted to estimate the five acidic dissociation constants and the Stokes radii at different protonation stages of the macrocyclic dodecapeptide. The pK(a) values were 3.6 and 4.4 for the two carboxylic groups of the lateral chains of D-Asp-11 and D-Glu-4, respectively, 6.4 for the aza-atom of the imidazole ring of His-10, 7.6 for the amino group of N-terminal Ile-1 and 9.7 for the delta-amino group of D-Orn-7, very close to the values obtained by other researchers by titration experiments. In agreement with a rigid macrocyclic structure the Stokes radii of different protonated forms ranged only between 14.3 and 14.8 A. Best fitting procedures performed on experimental mobility measured at two different pH values (5.50 and 6.72) in the presence of increasing Zn(+2) concentration allowed confirming the model that assumes the binding of Zn(+2) to P(0) peptide form with a 1.5 x 10(3) M(-1) intrinsic association constant. Following to Zn(+2) binding, the pK(a) of the amino group of N-terminal Ile-1 is shifted from 7.6 to 5.9 and the Stokes radius is reduced of about 3 A. The mean charge of the bacitracin A(1)-Zn(+2) complex resulted +1.67 and +1.12 at pH 5.50 and 6.72, respectively. These results suggest that the amino group of N-terminal Ile-1 is not essential for Zn(+2) binding. 相似文献
106.
Jesús M. Ruiz Gonzalez Marcos Loroo Tania Crdova Gabriel Chuchani 《Journal of Molecular Structure》2005,732(1-3):55-61
Møller-Plesset MP2/6-31G method was used to examine the gas-phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamates. The results of these calculations support a concerted non-synchronous six-membered cyclic transition state mechanism for carbamates containing a Cβ–H bond at the alkyl side of the ester. These substrates produce the N,N-dimethylcarbamic acid and the corresponding olefin. The unstable intermediate, N,N-dimethylcarbamic acid, rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO2 gas. Correlation of the logarithm of theoretical rate coefficients against original Taft's σ* values gave an approximate straight line (ρ*=−1.39, r=0.9558 at 360 °C). In addition to this fact, when log krel is plotted against the theoretical log krel for 2-substituted ethyl N,N-dimethylcarbamates a reasonable straight line (r=0.9919 at 360 °C) is obtained, suggesting similar mechanism. 相似文献
107.
A recently characterized oviposition-deterring pheromone (ODP, structure 1) of the European cherry fruit fly was used as a test case for probing the potential of tandem mass spectrometry (MS/MS) in structure elucidation as a stand-alone technique. The glycolipid-taurinate 1 was subjected to MS/MS analyses under a variety of conditions with and without preceding chemical degradation. Acidic methanolysis of 1 and subsequent in-batch derivatization (trideuterioacetylation) yielded methyl 2,3,4,6-tetrakis-O-trideuterioacetyl-glucopyranoside (2), methyl 8,15-bis-trideuterioacetoxy-palmitate (3), and taurine (4) as suitable target compounds for direct mixture analysis.Low energy collision induced dissociation (CID) on selected precursor ions (MS/MS on [M + H – CH3OH]+ and [M + H]+ produced by fast atom bombardment (FAB)) allowed direct identification of 2 and 4, respectively, by comparison with appropriate reference ions. In the case of 3, low energy CID (desorption chemical ionization (DCI) instead of FAB, MS/MS on [M + H]+) permitted deduction of gross molecular structure, but failed to provide positional detail. In sharp contrast,high energy CID of trideuterioacetylated intact 1 (FAB-MS/MS on [M – H]– ions of la) clearly revealed a linear 8,15-hydroxylated palmitic acid backbone. Less certain was assignment of 15-O-glucosylation by this approach. 相似文献
108.
On singlet excitation (λ = 254 nm, THF, pentane or hexane), the diastereoisomeric methano-epoxydienes (E)- 6 and (E)- 7 undergo interconversion and yield the products 8 – 11 . The main process is the cleavage of the oxirane ring to the vinyl carbene intermediate e which undergoes addition to the adjacent double bond furnishing the cyclopropene 8 . The alternative carbene intermediate f is evidenced by the formation of the cyclobutene 10 . For the fragmentation leading to 11 , the carbene f as well as the dipolar species h are considered as possible intermediates. On triplet sensitization (acetone, λ > 280 nm), (E)- 7 shows behavior typical of epoxydienes, undergoing fission of the C? O bond of the oxirane ring and isomerization to the compounds 13 , 14 and (E/Z)- 15 . 相似文献
109.
Employing tin and lead solution calorimetry, the formation enthalpies of those phases having platinum and palladium as major component have been determined in the PdGa, PdIn, PdTl, PtGa und PtIn systems. These enthalpies are highly exothermic and suggest strong atomic interaction betweens palladium and platinum on the one hand and the third subgroup elements on the other. The experimental values have been compared with those obtained from the Miedema model. The thermodynamic data are discussed and related to the crystal structures found with these alloys. 相似文献
110.
P. Bruno M. Caselli M. L. Curri P. Favia R. Lamendola A. Mangone A. Traini C. Laganara 《Fresenius' Journal of Analytical Chemistry》1994,350(3):168-177
Medieval ceramic sherds have been studied by inductively coupled plasma (ICP) emission spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to acquire knowledge about technological achievements in pottery production in Apulia during the Middle Ages. The XPS results allow to characterize the surface glazes, which are transparent owing to the presence of PbO or opaque-white in case of Sn addition. The study of coloured glossy surfaces, after an optimized chemical etching, has also shed light on the pigment nature of the painted decorations, whose red colour is to be attributed to the presence of Pb3O4 and not of Fe oxides. Quantitative determinations were performed by ICP measurements on samples of differently coloured glazes, clayey slips and ceramic bulks. Pb and Cu, present in one green glaze, were also determined by differential pulsed anodic stripping voltammetry (DPASV). The analytical results were treated by different statistical techniques of multivariate analysis. 相似文献