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1.
The 96 pKa values of 85 carboxylic acids in aqueous solution were calculated with the density functional theory method at the level of B3LYP/6‐31+G(d,p) and the polarizable continuum model (PCM) was used to describe the solvent. In the calculations of pKa values, the dissociation Gibbs free energies were directly calculated using carboxylic acid dissociation reactions in aqueous solution, i. e., no thermodynamic cycle was employed, which is different from the previous literatures. A highly significant correlation of R2=0.95 with a standard deviation (SD) of 0.36 between the experimental pKa values and the calculated dissociation Gibbs free energies [ΔG(calc.)] was found. The slope of pKa vs. (G(calc.)/(20303RT) is only 47.6% of the theoretically expected value, which implies that the ΔG(calc.) value from the theoretical calculation is larger than the actual one for all 85 carboxylic acids studied. Thus, by adding the 0.476 scaling‐factor into the slope, we can derive a reliably procedure that can reproduce the experimental pKa values of carboxylic acids. The pKa values furnished by this procedure are in good agreement with the experimental results for carboxylic acids in aqueous solution. 相似文献
2.
3.
In order to assess radionuclide diffusion and transport properties in compacted bentonite, the “in-diffusion” method based
on bentonite filled capillaries is used. The effect of 99TcO4
- concentration and pH value of the solution, the contact time and the dry density of compacted bentonite on the apparent diffusion
coefficient (D
a) and on the distribution coefficient (K
d) values obtained from the capillary test was studied. The D
a and K
d values decrease with increasing of the bulk dry density of compacted bentonite. Ion exclusion influences the diffusion of
99TcO4
- in the same substance. As compared to literature data, the K
d values obtained from capillary tests are in most cases lower than those from batch tests, the difference between the two
K
d values is a strong function of dry density of the compacted bentonite.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Partition coefficients K
av
for the partition of sugars between a polystyrene gel and aqueous solvents were determined. The results show that monosaccharides have different affinities for polystyrene. Their K
av
values increase markedly in NaCl solution but reduce to nearly the same value in solutions of NaSCN and LiSCN, both highly chaotropic salts. The K
av
of maltodextrins increase with their molecular weight. These partition peculiarities are attributed to the hydrophobic nature of sugars presumably due to their CH surface. The order of increasing hydrophobicity based on K
av
is: D-galactose < D-glucose < D-mannose < (D-arabinose, D-xylose) < D-ribose. This order is consistent with that based on the free energy change pertaining to the transfer of sugar molecules from water to 1-butanol. 相似文献
5.
《Physics and Chemistry of Liquids》2012,50(2):234-250
Charge transfer (CT) complex formation between 4-dimethylaminopyridine (4-DMAP) as the electron donor and 2,5-dihydroxy-p-benzoquinone (DHBQ) as the π-electron acceptor has been investigated spectrophotometrically in methanol (MeOH), ethanol (EtOH) and acetonitrile (AN). The stoichiometry of the complex has been identified by Job’s and photometric titration methods to be 1:1. The Benesi–Hildebrand equation has been applied to estimate the formation constant (KCT) and molecular extinction coefficient (ε). It was found that the value of KCT is larger in AN than in MeOH and EtOH. The thermodynamic parameters are in agreement with the KCT values in that the enthalpy of formation (?ΔH) has a larger value both in EtOH and MeOH than in AN, suggesting higher stability of the complex in EtOH. The complex formed between 4-DMAP and DHBQ has been isolated as a solid and characterised using elemental analysis, FTIR and 1H NMR measurements. Moreover, it has been found that the formed complex involves proton transfer in addition to CT. 相似文献
6.
P. C. Carman 《Journal of solution chemistry》1978,7(11):845-858
In a recent derivation of relaxation effects in the Debye-Hückel-Onsager theory of electrolyte conductance, with a length parameter a, terms are included which have been omitted in earlier treatments (see Appendix). The new expression was applied earlier in a reanalysis of conductance data for aqueous solutions and is applied here to solutions in acetonitrile and in formamide, representing respectively dielectric constants considerably lower and higher than water. As in aqueous solutions, a minimum standard deviation is found over a wide range of (K
A,a) pairs without much effect on A
0
, so that only approximate determinations ofK
A are possible. On the whole, the most appropriate length parametera is the physical contact distance between counterions, not a fixed radius, independent of ionic size, such as the Bjerrum value, nor a much larger radiusR serving as a boudary between free and associated ions in the ionic atmosphere about a central ion. Relaxation effects calculated by the new analysis are smaller than those from previous expressions for equal values ofa, and this leads to considerably larger values ofK
A than in the original papers. As a consequence, specific short-range ion-ion and ion-solvent forces in most solutions predominate over electrostatic attraction between counterions in their contribution toK
A. A table of limiting equivalent conductance based on the A
0
values obtained is presented; this differs little from previous tables since A
0
values obtained by the new analysis are similar to those obtained originally. 相似文献
7.
The ratio between the relaxed enthalpy and volume (so-called aging modulus, Ka) was expressed in frame of the Tool-Narayanaswamy-Moynihan theory. The common case where various experimental arrangements are used for measuring these quantities was analyzed. It was found that relatively small differences between the conditions of enthalpy and volume relaxation experiments may cause a significant shift of observed Ka value. The sensitivity of Ka modulus to the difference between the enthalpy and volume relaxation conditions is significantly higher in the case of organic polymeric glasses in comparison with silicate and chalcogenide glasses. The reason for such grouping resides in higher values of glass transition temperature and lower values of activation enthalpy of inorganic glasses. 相似文献
8.
The N‐acetylglucosamine‐related 1,2,4‐triazole 14 and 1,2,3‐triazole 16 have been prepared by N‐acetylation of the known amines 19 and 20 , and their Ki values determined against bovine kidney β‐N‐acetylglucosaminidase, a mammalian hexosaminidase. The 1,2,3‐triazole 16 (Ki=4 μM ) is a markedly weaker inhibitor than the isosteric azoles 13 – 15 . The Ki value of the 1,2,4‐triazole 14 (0.034 μM ) is smaller than that of the tetrazole 13 (0.2 μM ), but larger than that of the imidazole 15 (0.0035 μM ), confirming the correlation between inhibitory strength and basicity of the azole, as expected on the basis of an anti‐protonation mechanism of mammalian hexosaminidases. 相似文献
9.
Rui Zuo Yanguo Teng Jinsheng Wang 《Journal of Radioanalytical and Nuclear Chemistry》2009,281(3):653-662
Study of fine-particle media, because of their high sorption capacities, is of particular importance for the use as backfill
materials in waste repository design, and because argillaceous formations are particularly suitable as host rock formations.
In this study, sorption and retardation characteristics of strontium in fine-particle media were studied to evaluate the distribution
coefficient (K
d
) and retardation factor (R
d
) of this radioactive element in fine-particle media, which was comprised of selected particles with a diameter less than
1 mm from a candidate site to dispose very low level waste (VLLW). The results indicated that K
d
values of strontium under different initial concentrations ranged between 20 and 110. Values of strontium R
d
measured from column experiments ranged between 36 and 102, with the corresponding K
d
values, determined from solving the inverse problem of R
d
calculating formula, ranging between 5 and 20. In conclusion, the K
d
value of Sr from the batch tests was found to be higher than these from the column experiments. 相似文献
10.
11.
Yoshihisa Kurasawa Tomoyoshi Hosaka Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1995,32(2):445-450
The p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-i and 2a-d exhibited tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The substituent and solvent effects on the tautomer ratios of A to B in a series of mixed media were studied for compounds 1a-i and 2a-d by the nmr spectroscopy. The linear correlation of the Hammett σp and σm values with the tautomeric equilibrium constants KT ([A]/[B]) was found in the dimethyl sulfoxide media of compounds 1a-i and 2b-d . On the other hand, the linear correlation of the Hammett σp and σm values with the log C'(A:B = 1:1) was also observed in a series of mixed media of compounds 1a-h and 2a-c , wherein C'(A:B = 1:1) indicated the concentrations of trifluoroacetic acid (mol/l) giving 1:1 tautomer ratios in a series of mixed media. The increase in the Hammett σp or σm values decreased the KT values in dimethyl sulfoxide media and augmented the C'(A:B = 1:1) values in a series of mixed media. The Hammett σp or σm values controlled the electron density of the side chain nitrogen atom, which influenced the C'(A:B = 1:1) values. In the KT value temperature dependence, the higher temperature provided the larger KT values in dimethyl sulfoxide media regardless of the Hammett σp or σm values. 相似文献
12.
Knowledge on pKA values is an eminent factor to understand the function of proteins in living systems. We present a novel approach demonstrating that the finite element (FE) method of solving the linearized Poisson–Boltzmann equation (lPBE) can successfully be used to compute pKA values in proteins with high accuracy as a possible replacement to finite difference (FD) method. For this purpose, we implemented the software molecular Finite Element Solver (mFES) in the framework of the Karlsberg+ program to compute pKA values. This work focuses on a comparison between pKA computations obtained with the well‐established FD method and with the new developed FE method mFES, solving the lPBE using protein crystal structures without conformational changes. Accurate and coarse model systems are set up with mFES using a similar number of unknowns compared with the FD method. Our FE method delivers results for computations of pKA values and interaction energies of titratable groups, which are comparable in accuracy. We introduce different thermodynamic cycles to evaluate pKA values and we show for the FE method how different parameters influence the accuracy of computed pKA values. © 2015 Wiley Periodicals, Inc. 相似文献
13.
Phanibhusan Roy-Chowdhury 《Journal of Polymer Science.Polymer Physics》1969,7(9):1451-1459
The adiabatic compressibility for two samples (F-1 with DP-3748 and F-2 with DP-2114) of poly(4-vinyl-N-n-butylpyridinium bromide) in aqueous solution has been determined from ultrasonic velocity and density data. The sample (F-1) with the higher degree of polymerization shows comparatively higher velocity and density in solution. However, the evidence for the difference in compressibility is not very decisive. The apparent molal volume ΦV2 and apparent molal compressibility ΦK2 for F-1 are found to be slightly higher than for F-2. In aqueous solution, the decrement of adiabatic compressibility per unit concentration, (β1 ? β)/c, is found to be almost constant throughout the entire concentration range, whereas in the presence of excess added electrolyte (1.0M KBr solution), the compressibility decrement shows a decrease with dilution. The latter values are lower than those found in water, since the molecules, in the presence of excess electrolyte, are coiled up more and are less compressible. The ΦV2 and ΦK2 values in water are constant throughout the entire concentration range, as the free counterions formed on dissociation in the dilute region are not solvated and hence contribute little to the compressibility. On the other hand, in the presence of excess KBr (1.0M), the ΦV2 and ΦK2 values show a sharp decrease with increase of polyelectrolyte concentration and finally attain a constant value. This is explained by the fact that because of the formation of a charge-transfer complex between the bromide ion and the polycation, more than the equivalent number of bromide ions is bound, leaving free an equal amount of K+ ions which are solvated and cause the lowering of apparent volumes and compressibilities. Condensation of charges begins at a certain polyelectrolyte concentration, and no further increase of K+ ions is observed. A special situation arises in 0.1M KBr solution. The ΦV2 and ΦK2 values at first increase sharply with increase of polyelectrolyte concentration, but then level off to attain a constant value, at comparatively high concentration. In 2.0% poly(4-vinyl-N-n-butylpyridinium bromide) solution, the concentration of polymer repeat unit (0.08M) is almost equal to the concentration of the added electrolyte (0.1M KBr) used to suppress dissociation. As the polyelectrolyte concentration in 0.1M KBr solution is progressively decreased, more bromide ions are made available for forming the charge-transfer complex with the polycation, leaving the K+ ions free to contribute to the compressibility. 相似文献
14.
Ion-pair formation between Ca2+ and -isosaccharinate, Ca2+ + ISA-CaISA+, was studied by two independent methods: an ion-exchange and a potentiometric method (Ca-selective electrode). The two methods gave similar values for the complexation constant, log KCaISA+o at I = 0, (22 ± 1)°C. The ion-exchange method gave a value of log KoCaISA+ = (1.8 ± 0.1) and the potentiometric method resulted in logKCaISA+o = (1.78 ± 0.04). These values are in good agreement with the estimated value, log KCaISA+o = 1.7, based on the formation of a Ca-gluconate ion pair. 相似文献
15.
Heba A. Gaballa Luke M. Geever John A. Killion Clement L. Higginbotham 《Journal of Polymer Science.Polymer Physics》2013,51(21):1555-1564
Physically crosslinked hydrogels based on N‐vinylcaprolactam/acrylic acid and N‐vinylcaprolactam/methacrylic acid were prepared via free radical polymerization. These temperature responsive hydrogels were characterized in terms of glass transition, phase separation temperature, potentiometric titration and swelling properties. Results showed that phase transition temperature was dependent on the pH value of the solution; increasing pH led to higher lower critical solution temperature (LCST) values which was related to the dissociative behaviors of the carboxylic group of MAc in the buffered solutions. Additionally, with the incorporation of N,N‐dimethylacrylamide into the system, cloud point measurements and MDSC showed an increased in the LCST. This increase was based on hydrophilicity, the hydrophilic–hydrophobic balance was disturbed, and consequently, the LCST behavior was shifted. The pKa of the copolymers ranged between 5.6 and 6.5, while for the terpolymers pKa ranged between 5.3 and 6. At high pH (>10), the ? COOH group is deprotonated and negatively charged (? COO?), while at low pH (1–3) the carboxylic group remains protonated which results in hydrogen bonding between the hydroxyl groups (from NaOH) and the excess of HCl. These results correlate with swelling studies where above the pKa value the hydrogels dissolved rapidly compared to below pKa they did not dissolve at all. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1555–1564 相似文献
16.
Mariana Peñuela Vásquez Maurício Bezerra De Souza Jr. Nei Pereira Jr. 《Applied biochemistry and biotechnology》2006,129(1-3):256-264
Biotechnology production of xylitol is an excellent alternative to the industrial chemical process for the production of this
polyalcohol. In this work the behavior of Candida guilliermondii yeast was studied when crucial process variables were modified. The K
La (between 18 and 40/h) and the initial cell mass (between 4 and 10 g) were considered as control variables. A response surface
methodology was applied to the experimental design to study the resulting effect when the control variables were modified.
A regression model was developed and used to determine an optimal value that was further validated experimentally. The optimal
values determined for K
La and X
0 were 32.85/h and 9.86 g, respectively, leading to maximum values for productivity (1.628 g/h) and xylitol yield (0.708 g/g). 相似文献
17.
Yasuyuki Takeda Aiko Yasui Shoichi Katsuta 《Journal of inclusion phenomena and macrocyclic chemistry》2004,50(3-4):157-164
The constants for overall extraction into various diluents of low dielectric constants (Kex) and aqueous ion-pair formation (KMLA) of dibenzo-18-crown-6 (DB18C6)–sodium and potassium perchlorate 1:1:1 complexes (MLA) were determined at 25°C. The Kex value was analyzed by the four underlying equilibrium constants. The KMLA values were determined by applying our established method to this DB18C6/alkali metal perchlorate extraction system. The KM(DB18C6)A value of the perchlorate is much greater for K+ than for Na+, and is much smaller than that of the picrate. The KMLA value makes a negative contribution to the extractability of DB18C6 for MClO4, whereas the value of the MLA distribution-constant does a major one. The partition behavior of M(DB18C6)ClO4 obeys the regular solution theory. However, the M(DB18C6)ClO4 complexes in the diluent of high dipole moment somewhat undergo the dipole–dipole interaction. DB18C6 always shows high extraction selectivity for KClO4 over NaClO4, which is governed largely by the much greater KMLA value for K+ than for Na+. The K+ extraction-selectivity of DB18C6 over Na+ for perchlorate ions is comparable to that for picrate ions. By comparing this perchlorate system with the picrate one, the anion effects on the extraction-efficiency and -selectivity of DB18C6 for Na+ and K+ was discussed in terms of the fundamental equilibrium constants. 相似文献
18.
Shuming Zhang 《Journal of computational chemistry》2012,33(31):2469-2482
The ionization (dissociation) constant (pKa) is one of the most important properties of a drug molecule. It is reported that almost 68% of ionized drugs are weak bases. To be able to predict accurately the pKa value(s) for a drug candidate is very important, especially in the early stages of drug discovery, as calculations are much cheaper than determining pKa values experimentally. In this study, we derive two linear fitting equations (pKa = a × ΔE + b; where a and b are constants and ΔE is the energy difference between the cationic and neutral forms, i.e., ΔE = Eneutral?Ecationic) for predicting pKas for organic bases in aqueous solution based on a training/test set of almost 500 compounds using our previously developed protocol (OLYP/6‐311+G**//3‐21G(d) with the the conductor‐like screening model solvation model, water as solvent; see Zhang, Baker, Pulay, J. Phys. Chem. A 2010 , 114, 432). One equation is for saturated bases such as aliphatic and cyclic amines, anilines, guanidines, imines, and amidines; the other is for unsaturated bases such as heterocyclic aromatic bases and their derivatives. The mean absolute deviations for saturated and unsaturated bases were 0.45 and 0.52 pKa units, respectively. Over 60% and 86% of the computed pKa values lie within ±0.5 and ±1.0 pKa units, respectively, of the corresponding experimental values. The results further demonstrate that our protocol is reliable and can accurately predict pKa values for organic bases. © 2012 Wiley Periodicals, Inc. 相似文献
19.
New Approach to Determine the Activation and Reaction Volumes of Low Polar Molecular Processes
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For low polar molecular processes, we found the reliable relationship between the logarithm changes of the rate or equilibrium constants in the pressure range 1–1000 bar, [Ln(KP = 1000/KP = 1)/1000], and tangent modulus at P = 1 bar, ?Ln(KP)/?P: ?Ln(KP)/?P = 1.15·[Ln(KP = 1000/KP = 1)/1000], R = 0.995, which allows to predict the value of activation and reaction volume at ambient pressure. Therefore, it is sufficient to determine the values of the rate or equilibrium constants only at ambient pressures and at 1000 bar for the reliable estimation of the values of activation or reaction volume. 相似文献
20.
Motoi Yamashita Akitaka Hoshino Minoru Kato 《Journal of Polymer Science.Polymer Physics》2007,45(6):684-697
Crystal growth of the trigonal form of isotactic poly(butene‐1) (it‐PB1) was successfully observed in the melt at atmospheric pressure. The growth rate of trigonal crystals was obtained by in situ optical microscopy. It is one hundredth that of it‐PB1 tetragonal crystals. The growth rate of trigonal crystals, as well as that of tetragonal crystals, shows supercooling dependence derived from the nucleation theory. The value of the kinetic constant K of trigonal crystals is about 3.3 times larger than that of tetragonal crystals. The value of the pre‐exponential factor G0 of trigonal crystals was found to be 41 times as large as that of tetragonal crystals. The difference between these K values can be attributed to the conformational entropy of the ethyl side groups in a nucleating stem. The discrepancy found in the values of G0 could be explained by introducing pinning and nucleation barriers, which originate from the crystal thickness δlc, which does not depend on the crystallization temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 684–697, 2007 相似文献