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41.
Richard M. Kettler Donald A. Palmer David J. Wesolowski 《Journal of solution chemistry》1995,24(1):65-87
The first and second molal dissociation quotients of succinic acid were measured potentiometrically with a hydrogen-electrode, concentration cell. These measurements were carried out from 0 to 225°C over 25° intervals at five ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The dissociation quotients from this and two other studies were combined and treated with empirical equations to yield the following thermodynamic quantities for the first acid dissociation equilibrium at 25°C: log K1a=–4.210±0.003; H
1a
0
=2.9±0.2 kJ-mol–1; S
1a
0
=–71±1 J-mol–1-K–1; and C
p1a
0
=–98±3 J-mol–1-K–1; and for the second acid dissociation equilibrium at 25°C: log K2a=–5.638±0.001; H
2a
0
= –0.5±0.1 kJ-mol–1; S
2a
0
=–109.7±0.4 J-mol–1-K–1; and C
p2a
0
= –215±8 J-mol–1-K–1. 相似文献
42.
C. Kourkoumelis L. K. Resvanis T. A. Filippas E. Fokitis A. M. Chops S. Iwata R. B. Palmer D. C. Rahm P. Rehak I. Stumer C. W. Fabjan T. Fields D. Lissauer I. Mannelli P. Mouzourakis A. Nappi W. J. Willis 《Zeitschrift fur Physik C Particles and Fields》1980,5(2):95-104
The inclusive cross-section for π0 production near 90° inpp collisions at the CERN Intersecting Storage Rings has been studied for thep T range 3<p T <16GeV/c at four different centre-of-mass energies ( \(\sqrt s = 30.6\) , 44.8, 52.7, and 62.8 GeV). In this experiment the two photons from the π0→yy decay were resolved and measured separately forp T values up to 10 GeV/c. Results indicate an agreement with thep T ?8 behaviour for the lower values ofp T and a slower decrease of the cross-section for the higher values ofp T . The high-p T data deviate from the scaling expressionp T ?n F(x T ), which holds for the lowerp T values (p T <8GeV/c). 相似文献
43.
Amphiphilic copolymers of 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) with dihydrocholesteryl acrylate (pDHCHAt) or tert-octyl acrylamide (ptOAm) as comonomers were synthesized, characterized and used as new polymeric pseudostationary phases in electrokinetic chromatography. High-efficiency separations with theoretical plates around 500,000 to 870,000/m were achieved. Linear solvation energy relationships analysis of 20 solutes was performed to investigate the retention mechanism. Polycyclic aromatic hydrocarbons (PAHs) were separated under 30% v/v acetonitrile. The separation of nine aromatic model solutes by pDHCHAt showed significantly different selectivity from that of sodium dodecyl sulfate. Although only minor difference in selectivity was found for small aromatic compounds on pDHCHAt and poly(AMPS-lauryl acrylate), significantly different retention mechanism was found for PAHs and n-dodecanophenone comparing both copolymers. Significant chemical selectivity difference was found for selected small aromatic compounds on ptOAm and poly(AMPS-stearyl acrylamide). 相似文献
44.
The effect of the conformation of a polymeric pseudostationary phase on performance and selectivity in electrokinetic chromatography was studied using an amphiphilic pH-responsive polymer that forms compact intramolecular aggregates (unimer micelles) at low pH and a more open conformation at high pH. The change in conformation was found to affect the electrophoretic mobility, retention, selectivity, and separation efficiency. The low-pH conformer has higher electrophoretic mobility and greater affinity for most solutes. The unimer micelle conformation was also found to provide a solvation environment more like that of micelles and other amphiphilic self-associative polymers studied previously. It was not possible to fully characterize the effect of conformation on efficiency, but very hydrophobic solutes with long alkyl chains appeared to migrate with better efficiency when the unimer micelle conformation was employed. The results imply that polymers with a carefully optimized lipophilic-hydrophilic balance that allow self-association will perform better as pseudostationary phases. In addition, the results show that electrokinetic chromatography is a useful method for determining the changes in solvation environment provided by stimuli-responsive polymers with changes in the conditions. 相似文献
45.
The pressure and/or temperature dependencies of the rates of substitution of thiourea in cis- and/or trans-[PtBr(2, 4, 6-Me3Ph)(Et3P)2] were studied in ethanol, DMSO, and acetone. The rate constants for both the nucleophile independent and nucleophile dependent reactions were measured. The rate and activation parameters are discussed in reference to data reported earlier for these reactions in methanol. 相似文献
46.
Palmer FN Lach F Poriel C Pepper AG Bagley MC Slawin AM Moody CJ 《Organic & biomolecular chemistry》2005,3(20):3805-3811
Various approaches to the tyrosine-derived fragment of the marine secondary metabolite diazonamide A are described. Initial efforts were focused on the originally proposed structure of the natural product, and a feasibility study established that a model 4-aryltryptamine could be readily prepared. Protected 4-bromotryptamine underwent Pd0-catalyzed coupling with the boronic acid derived from 2-bromophenyl allyl ether by Claisen rearrangement, O-methylation and lithiation-boration. The resulting biaryl was elaborated into an alpha-diazo-beta-ketoester, dirhodium(II)-catalyzed reaction of which with N-Z-valinamide gave the desired tryptamine-oxazole following cyclodehydration of the intermediate ketoamide. A potential precursor to the benzofuran ring of the original structure of diazonamide A was prepared in eight steps from N-Z-tyrosine tert-butyl ester. Iodination, O-protection and Stille coupling gave the cinnamyl alcohol 25, converted via the bromide into the allyl aryl ether 27. Subsequent Claisen rearrangement and oxidative cleavage of the alkene gave the lactol 29, converted into the desired benzofuranone 31. The revision in the structure of diazonamide A to 2 resulted in the targeting of an alternative tyrosine-derived model benzofuranone 41 synthesized in four steps from N-Z-tyrosine methyl ester 36 by a route involving Claisen rearrangement of cinnamyl ether 37. Poor yields in this sequence prompted an investigation into the intramolecular Heck reaction as a route to benzofuranone 50. Coupling of 3-iodotyrosine 44 with 2-phenylbutenoic acid 48 gave ester 49 that readily underwent intramolecular Heck reaction to give benzofuranone 50, albeit with poor stereocontrol. 相似文献
47.
New spectioscopic and vibronic population data support the essential correctness of BaO2* as the nascent polyatomic emitter and as the precursor to BaO (A 1Σ+ → X1Σ+) and (A' 1Π → X1Σ+) visible chemiluminescence from metal-rich Ba(g) + O2 (+ Ar) diffusion flames at 2–350 mTorr- Absolute visible photon yields are reported over this pressure regime. 相似文献
48.
Proton transfer reaction ion trap mass spectrometer 总被引:1,自引:0,他引:1
Prazeller P Palmer PT Boscaini E Jobson T Alexander M 《Rapid communications in mass spectrometry : RCM》2003,17(14):1593-1599
Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H(3)O(+) as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is "soft" and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results. 相似文献
49.
Approximate formulas for the transverse-relaxation rate constant and resonance frequency have been derived for nuclear spins subject to two-site chemical exchange. The new results are more accurate than previous approximations when chemical exchange does not approach the fast-exchange limit or when site populations are not highly asymmetric. Results are derived from an effective average evolution operator that contains contributions from both exact eigenvalues of the Bloch-McConnell equations. These expressions are useful for interpretation of chemical exchange processes in NMR spectroscopy. 相似文献
50.
The volumes of activation in cm3 mol?1 for the aquation of Co(CN)5X3? were determined at 40°C and μ = 1 M (NaClO4) to be + 7.8 ± 0.5 for X = Cl?, + 7.6 ± 0.6 for X = Br?, + 14.0 ± 0.7 for X = I?, and + 16.8 ± 0.5 for X = N3? (0.1 M HClO4), respectively. The volumes of activation for the aquation of Co(CN)5Cl3? at μ = 0.1 M are + 10.0 ± 0.6 cm3 mol?1 and ± 9.1 ± 0.3 cm3 mol?1 at 40°C and 25°C, respectively. The corresponding values for the anation of Co(CN)5OH22? (at 40°C) and μ = 1 M by Br?, I?, and NCS? are +8.4 ± 1.0, +9.4 ± 1.6, and +8.2 ± 0.9 cm3 mol?1, respectively. These data are discussed in terms of a dissociative (D) mechanism. 相似文献