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排序方式: 共有771条查询结果,搜索用时 15 毫秒
701.
702.
M. Carme Rovira Juan J. Novoa Myung-Hwan Whangbo Jack M. Williams 《Chemical physics》1995,200(3):319-335
On the basis of ab initio MP2/6–31 + + G(2d,2p) calculations, we examined the potential energy surfaces of the water·hydrocarbon complexes H2O·CH4, H2O·C2H2 and H2O·C2H2 to locate all the minimum energy structures and estimate the hydrogen bond energies and vibrational frequencies associated with the C(spn)---H·O and the O---H·C(spn) bonds (n = 1−3). Our calculations show that H2O·C2H2, H2O·C2H4 and H2O·CH4 have two minimum energy structures (i.e., the C---H·O and O---H·C hydrogen bond forms), but H2O·C2H4 has only one when the vibrational motion is taken into account, the O---H·C hydrogen bond form. We have also computed the barrier for the interconversion from one minimum to the other. The fully optimized geometries of H2O·CH4, H2O·C2H4 and H2O·C2H2 as well as the vibrational shifts of the C---H stretching frequencies in their C---H·O hydrogen-bonded forms are in good agreement with the available experimental data. The calculated hydrogen bond energies show that the C(spn---H·O bond strengths decrease in the order C(sp)---H·O>C(sp2)---H·O>C(sp3)---O>C(sp3---H·O, which is also consistent with the available experimental data. 相似文献
703.
Motos Gallardo A Hernández JA Hernández JM Rovira JM Fluvià L Bosch A Pastor MC 《Journal of mass spectrometry : JMS》2001,36(8):943-949
This work illustrates the practical use of combined microbore reversed-phase high-performance liquid chromatography (RP-HPLC) with electrospray ionization mass spectrometry (ESI-MS) in protein identification. The approach consisted of the detection of the abnormal beta-globin chain by ESI-MS analysis of mixtures of intact globins, which simultaneously provided their molecular masses. Separation of the polypeptide globin chains was carried out using microbore C4 RP-HPLC on-line with ESI-MS. Direct peptide-mapping ESI-MS without previous chromatographic separation was performed in order to identify tryptic peptides from whole blood. For the sequence confirmation of the abnormal peptide containing the mutation point, C18 RP-HPLC tryptic separation of the globin mixture on-line with collision-induced dissociation (CID) fragmentation was done. The y series ions allowed the identification of the hemoglobin (Hb) variant as [beta134(H12) Val > Ala]. This new Hb was named Hb Mataró, after the city where it was detected. 相似文献
704.
J. L. Mateo P. Bosch F. Catalina R. Sastre 《Journal of polymer science. Part A, Polymer chemistry》1991,29(13):1955-1961
The behavior of the nonconjugated aminated benzophenones—4-[2′-N,N-(diethylamino)ethoxy]benzophenone (E4), 2-[2′-N,N-(diethylamino)ethoxy]-4-methoxybenzophenone (E2), and 4-N,N-dimethylaminomethylbenzophenone (DM)—as photoinitiators of MMA polymerization has been studied and the results compared with those obtained with the conjugated aminobenzophenone 4-N,N-dimethylamino-4′-isopropyl-benzophenone (CU—MI). Photoreduction behavior of these compounds in various solvents in the presence and absence of MMA has been also examined. The order of the polymerization reaction with respect to monomer and initiator concentrations has been investigated; values of initiation quantum yield (Φi), Kp/K1/2t and efficiencies of the different radicals have also been determined. Similar polymerization rates (Rp) of methyl methacrylate (MMA) were found when E4 and CU-MI were used as photoinitiators under the same range of absorbed irradiation intensity. This fact results from a compensation between the higher rate of E4 radical production (n-π* transition type) and the greater reactivity of the radicals generated from CU-MI. 相似文献
705.
J. J. Ten Bosch W. F. Verhelst K. H. Chadwick 《Journal of polymer science. Part A, Polymer chemistry》1972,10(6):1679-1685
A correlation between the radiation-induced nine-line electron spin resonance signal and the unstable component of the optical absorption in the near ultraviolet in poly(methyl methacrylate) is made on the basis of the radiation dose relationship, the dependence of production on irradiation temperature at a constant radiation dose, and on the fading of both signals on oxygen diffusion into the poly(methyl methacrylate) after irradiation. It is concluded that this correlation is applicable only immediately after irradiation and the stability of the initial radical is discussed with respect to the fading results. A process of radical decay and conversion is suggested to account for differences found in the fading characteristics of the two signals. 相似文献
706.
Micellar electrokinetic chromatography estimation of size and composition of procyanidins after thiolysis with cysteine 总被引:1,自引:0,他引:1
Herrero-Martínez JM Ràfols C Rosés M Bosch E Lozano C Torres JL 《Electrophoresis》2003,24(9):1404-1410
This paper describes the characterization of procyanidin mixtures by acid depolymerization in the presence of cysteine (thiolysis with cysteine) and micellar electrokinetic chromatography (MEKC). Reversed-phase liquid chromatography (RP-HPLC) and MEKC were investigated for the separation of the major components of the depolymerized mixtures (catechins and their cysteinyl derivatives). The solutes could only be effectively separated using MEKC. Two background electrolytes (BGEs) are recommended: (i) 50 mM phosphate at pH 7, containing 40 mM sodium cholate (SC) and 10 mM sodium dodecyl sulfate (SDS); (ii) a BGE with the same composition but containing only 50 mM SDS. The MEKC procedures here reported, are cheap, reliable and fast, and their potential in the determination of the size and composition in procyanidin mixtures has been shown. The proposed MEKC methods were validated by comparison with our intralaboratory reference RP-HPLC method using cysteamine as thiol donor. 相似文献
707.
708.
709.
710.
The dissociation constants of a series of indicators inN,N-dimethylformamide medium have been determined, and compared with those for the indicators in other aprotic solvents (dimethylsulphoxide and acetonitrile) and in some amphiprotic solvents (water, methanol, ethanol, propan-2-ol and tertbutanol). The effect of the solvent characteristics on the pK values have been evaluated, and the pK + p
ws
t
(H+) values shown to be linearly correlated with the Dimroth and ReichardtE
T
N
and acceptor number solvatochromic parameters. 相似文献