共查询到20条相似文献,搜索用时 15 毫秒
1.
Shijun Zheng Lingpeng Meng Xinhua Cai Zhenfeng Xu Xiaoyuan Fu 《Journal of computational chemistry》1997,18(9):1167-1174
Topological properties of potential energy and electronic density distribution on five reaction paths X+H2→XH+H (X=H, N, HN, H2C, NC) are investigated at the level of UMP2/6–311G(d,p). It has been found that in the region of the reaction paths studied, where B(rc)|s>0 [B(rc)|s is the product of ρ(rc) and ∇2ρ(rc) at the point of reaction process, i.e., B(rc)|s=ρ(rc)∇2 ρ(rc)] is basically the same as the region of V′(s)<0[V′(s) is the second derivative of potential energy with respect to the reaction coordinate, i.e., V′(s)=d2V/ds2], and the point with maximum B(rc)|s is almost coincident with the point of minimum V′(s). It can be concluded from the calculated results that there is a good correlation between the topological properties of potential energy and electronic density distribution along the reaction path. The structure transition state of such collinear reactions may be determined by topological analysis of electronic density. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1167–1174 相似文献
2.
The reaction heats of epychlorhydrin (ECH) with diethylene-triamine (DETA) and triethylene tetramine (TETA) solutions were measured for the first polycondensation stage. The polycondensation takes place by a chain mechanism, evolving a large quantity of heat. For this reason, a method was developed to measure the reaction heat under dynamic conditions, by circulating cooling water from an ultrathermostat through the reaction vessel. It was possible to measure the reaction heat summing the quantities of heat taken up by the cooling water, at various stages in the polycondensation: (ΔHor) = ΣQp(i).This method is suitable for strongly exothermic reactions, which develop for a few hours (1–5 h). There were also studies of the influence of the amine concentration and the rate of feed of ECH as well as the amine chain length. 相似文献
3.
Thermal
and structural properties of model silicate-phosphate glasses containing the
different amounts of the glass network modifiers, i.e. Mg2+
and Ca2+ were studied. To explain the changes of
the parameters characterizing the glass transition effect (Tg, Δcp) and the crystallization
process (Tc, ΔH) depending on the cations modifiers additions,
analysis of the bonds and chemical interactions of atoms in the structure
of glasses was used.
31P
MAS-NMR spectra of SiO2–P2O5–MgO(CaO)–K2O glasses show that the phosphate complexes are mono- and diphosphate. It
has been found that increasing amounts of Mg2+
or Ca2+ cations in the structure of glasses causes
the reduction of the degree of polymerization of the phosphate framework (Q1→Q0).
The influence of increasing of modifiers in the structure of silicate- phosphate
glasses on the number of non-bridging oxygens per SiO4
tetrahedron and density of glasses was presented. 相似文献
4.
Edwin H. Battley 《Journal of Thermal Analysis and Calorimetry》2011,104(1):13-22
Calculations are made using the equations Δr
G = Δr
H − TΔr
S and Δr
X = Δr
H − Δr
Q where Δr
X represents the free energy change when the exchange of absorbed thermal energy with the environment is represented by Δr
Q. The symbol Q has traditionally represented absorbed heat. However, here it is used specifically to represent the enthalpy listed in tabulations
of thermodynamic properties as (H
T
− H
0) at T = 298.15 K, the reason being that for a given substance TS equals 2.0 Q for solid substances, with the difference being greater for liquids, and especially gases. Since Δr
H can be measured, and is tangibly the same no matter what thermodynamics are used to describe a reaction equation, a change
in the absorbed heat of a biochemical growth process system as represented by either Δr
Q or TΔr
S would be expected to result in a different calculated value for the free energy change. Calculations of changes in thermodynamic
properties are made which accompany anabolism; the formation of anabolic, organic by-products; catabolism; metabolism; and
their respective non-conservative reactions; for the growth of Saccharomyces cerevisiae using four growth process systems. The result is that there is only about a 1% difference in the average quantity of free
energy conserved during growth using either Eq. 1 or 2. This is because although values of TΔr
S and Δr
Q can be markedly different when compared to one another, these differences are small when compared to the value for Δr
G or Δr
X. 相似文献
5.
Ya. P. Yarmolyuk and P. I. Kripyakevich (Kristallographiya 19, 539 (1974)) showed that all tetrahedrally close packed (t.c.p.) structures have coordination formulae PpQqRrXx → (PX2)i(Q2R2X3)j (R3X)k, where P, Q, R, and X represent coordination numbers (CN) 16, 15, 14, and 12 polyhedra respectively: p, q, r, and x indicate the numbers of such polyhedra in the unit cells of t.c.p. structures and i, j, and k are positive integers. We propose and demonstrate a limitation to the above formulation: if i ≥ 1 and k ≥ 1, then j ≥ 1 (or if both p> 0 and r> 0, then q> 0). We give reasons for this and discuss the Aufbauprinzip of t.c.p. structures and the results of C. B. Shoemaker and D. P. Shoemaker (Acta Crystallogr. B 42, 3 (1986)). 相似文献
6.
Using a Monte Carlo simulation in three dimensions, we studied the variation of the root-meansquare (rms) displacement (Rrms) of polymer chains with time and the rates of their mass transfer (j) as a function of biased field (B), polymer concentration (p), chain length (Lc), porosity (ps), and temperature (T). In homogeneous/annealed system, the rms displacement of the chains shows a drift-like behavior, Rrms ∼ t, in the asymptotic time regime preceded by a subdiffusive power-law (Rrms ∼ tk, with k < 1/2) at high p. The subdiffusive regime expands on increasing Lc and p but reduces on increasing T or B. In quenched porous media, the drift-like behavior of Rrms persists at low barrier concentration (pb) and high T. However, at high pb and/or low T, chains relax into a subdrift and/or subdiffusive behavior especially with high p or long Lc. Flow of chains is measured via an effective permeability (σ) using a linear response assumption. In annealed system, σ increases monotonically with B at high T and low p but varies nonmonotonically at low T, high p and high Lc. We find that σ decays with Lc, σ ∼ L, where α depends on B, p and T with a typical value a α ∼ 0.43−0.64 for p = 0.1-0.3 at B = 0.5. Further, σ decays with p, σ ∼ − Cp with a decay rate C sensitive to T and B. In quenched porous media, even at low pb and high T, σ varies nonmonotonically with bias, i.e., the increase of σ is followed by decay on increasing the bias beyond a characteristic value (Bc). This characteristic bias seems to decrease logarithmically with barrier concentration, Bc ∼ −klnpb. The prefactor k depends on the chain length, k ≈ 0.35 for shorter chains (Lc = 20, 40) and ≈ 0.15 for longer chains (Lc = 60). Scaling dependence of σ on Lc similar to that in annealed system is also observed in porous media with different values of exponent α. The current density shows a nonlinear power-law response, j ∼ Bσ, with a nonuniversal exponent δ ≈ 1.10−1.39 at high temperatures and low barrier concentrations. 相似文献
7.
V. P. Korolev 《Journal of Structural Chemistry》2009,50(6):1188-1192
Relations for the apparent molar heat capacity ϕc of urea in an aqueous solution depending on the molality m and temperature were obtained. A transition to the relations ϕc(m,T) for D2O-(ND2)2CO and T2O-(NT2)2CO systems was effected by temperature scaling. At low temperatures, the isotherms of the molar heat capacity C
p(m) of the protium and deuterium systems have minima shifted to more dilute solutions at elevated temperatures. At m = 1, C
p of a solution does not depend on temperature in both systems. The dependences C
p(T) also have minima at constant concentrations. The temperature of the minimum heat capacity is most effectively lowered by
small additions of urea. For m = 0.25, T
min is 7.5 K lower than T
min of pure water, and its heat capacity is 0.08 J/(mol K) higher. A transition from m = 1.5 to m = 2 lowers the temperature of the minimum heat capacity by 3.6 K; thus, the heat capacity of solutions differs by 0.02 J/(mol
K) only. 相似文献
8.
Zhiyuan Qian Luke Galuska William W. McNutt Michael U. Ocheje Youjun He Zhiqiang Cao Song Zhang Jie Xu Kunlun Hong Rene B. Goodman Simon Rondeau‐Gagn Jianguo Mei Xiaodan Gu 《Journal of Polymer Science.Polymer Physics》2019,57(23):1635-1644
Thermomechanical properties of polymers highly depend on their glass transition temperature (T g). Differential scanning calorimetry (DSC) is commonly used to measure T g of polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuring T g in such type of polymers. The changes in specific heat capacity (Δc p) at T g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc p from flexible polymer (0.28 J g?1 K?1 for polystyrene) to rigid CPs (10?3 J g?1 K?1 for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc p at T g by a factor of 10, confirming that backbone rigidity reduces the Δc p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc p. We conclude that the difficulties of determining T g for CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1635–1644 相似文献
9.
《The Journal of chemical thermodynamics》2003,35(3):529-553
Apparent molar volumes Vφ and apparent molar heat capacities Cp,φ were determined for aqueous solutions of l-proline, l-proline with equimolal HCl, and l-proline with equimolal NaOH at the pressure p=0.35 MPa. Density measurements obtained with a vibrating-tube densimeter at temperatures (278.15⩽T/K⩽368.15) were used to calculate Vφ values, and heat capacity measurements obtained with a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter at temperatures (278.15⩽T/K⩽393.15) were used to calculate Cp,φ values. Speciation arising from equilibrium was accounted for using Young’s Rule, and semi-empirical equations describing (Vφ, m, T) and (Cp,φ, m, T) for each aqueous equilibrium species were fitted by regression to the experimental results. From these equations, the volume change ΔrVm and heat capacity change ΔrCp,m for the protonation and deprotonation reactions were calculated. Additionally, the ΔrCp,m expression was integrated symbolically to yield values of the reaction enthalpy change ΔrHm, reaction entropy change ΔrSm, and equilibrium molality reaction quotient Q for both reactions. The results provide a much-improved thermodynamic characterization of aqueous l-proline and of its protonation and deprotonation equilibria. 相似文献
10.
Yang PK 《Journal of computational chemistry》2011,32(13):2783-2799
The electric dipole moment p ( r ) was computed as the integral of the permanent dipole moment of the solvent molecule μ( r ) weighted by the orientational probability distribution Ω( r ; O ) over all orientations, where O is the orientation of the solvent molecule at r . The relationship between Ω( r ; O ) and the potential of the mean torque was derived; p ( r ) is proportional to the electric field E ( r ) under the following assumptions: (1) the van der Waals (vdW) interaction is independent of the orientation of the solvent molecule at r ; (2) the solvent molecule and its electrical effect are modeled as a point dipole moment; (3) the solvent molecule at r is in a region far from the solute; and (4) μE( r ) ? kBT, where kB is Boltzmann's constant and T is absolute temperature. The errors caused by calculating near‐solute Ω( r ) and p ( r ) from E ( r ) are unclear. The results show that Ω( r ) is inconsistent with the value calculated from E ( r ) for water molecules in the first and second shells of solute with charge state Q = ±1 e, and a large variation in solvent molecular polarizability γmol(r), which appeared in the first valley of 4πr2E(r) for |Q| < 1 e. Nonetheless, p (r) is consistent with the values calculated from E (r) for |Q| ≤ 1 e. The implication is that the assumptions for calculating p ( r ) can be ignored in the calculation of the solvation free energy of biomolecules, as they pertain to protein folding and protein–protein/ligand interactions. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011 相似文献
11.
Mitsuru Ueda Toshihisa Kumakura Yoshio Imai Charles U. Pittman 《Journal of polymer science. Part A, Polymer chemistry》1982,20(10):2829-2838
The kinetics of methacryloyl fluoride (MAF) homopolymerization was investigated in methyl ethyl ketone (MEK) with azobis(isobutyronitrile) as initiator. The rate of polymerization (Rp) followed the expression Rp = k[AIBN]0.55[MAF]1.18. The overall activation energy was calculated as 74.4 kJ/mol. The relative reactivity ratios of MAF(M2) copolymerization with styrene (r1 = 0.083, r2 = 0.14), and methyl methacrylate (r1 = 0.48, r2 = 0.81) in methyl ethyl ketone were obtained. Application of the Q–e scheme (in styrene copolymerization) led to Q = 2.22 and e = 1.31. The glass transition temperature (Tg) of poly(MAF) was 90°C by thermomechanical analysis. Thermogravimetry of poly(MAF) showed a 10% weight loss of 228°C in air. 相似文献
12.
M. M. Hafiz N. Afify M. A. Osman M. Dongol 《Journal of Thermal Analysis and Calorimetry》1990,36(7-8):2417-2426
The crystallization kinetics of the chalcogenide glass Se0.8Te0.2 was studied by means of differential scanning calorimetry. The variation in partial area (X) with temperature (T) revealed that the transition from the amorphous to the crystalline phase occurs in two dimensions.Activation energies were determined for both the glass transition (E
t) and the crystallization (E
c).E
t was calculated from the variation inT
g with the heating rate (a).E
c was determined by three different methods: (i) variation inX withT, (ii) variation inT
p
witha, and (iii) variation inT
c witha.E
t andE
c have values of 161.01±2.75 and 84.75 ±8.21 kJ/mol, respectively.
This work was partly supported by a Grant-in-Aid for Scientific Research from the GTZ GmbH and DAAD, W. Germany. 相似文献
Zusammenfassung Mittels DSC wurde die Kristallisierungskinetik des Chalkogenidglases Se0.8Te0.2 untersucht. Eine Änderung partieller Gebiete (X) mit der Temperatur (T) zeigte, daß der Übergang von der amorphen zur kristallinen Phase zweidimensional verläuft.Es wurde die Aktivierungsenergie sowohl für den Glasübergang (E t) als auch für die Kristallisierung (E c) bestimmt.E t wurde mittels der Abhängigkeit vonT g von der Aufheizgeschwindigkeit (a) ermittelt.E c wurde auf drei verschiedene Wege bestimmt: (i) Änderung vonX in Abhängigkeit vonT, (ii) Änderung vonT p in Abhängigkeit vona und (iii) Änderung vonT c in Abhängigkeit vona. Die Werte vonE t undE c betragen 161.01±2.75 bzw. 84.75±8.21 kJ/mol.
This work was partly supported by a Grant-in-Aid for Scientific Research from the GTZ GmbH and DAAD, W. Germany. 相似文献
13.
Maria Laura Di Lorenzo Maria Cristina Righetti 《Journal of Polymer Science.Polymer Physics》2007,45(23):3148-3155
Isothermal crystallization of poly(butylene terephthalate) (PBT) blended with oligomeric poly(ε‐caprolactone) (PCL) is investigated by polarized optical microscopy and differential scanning calorimetry at various temperatures (Tc). The growth rate of PBT spherulites is found to depend on time (t), as the spherulite radius (r) linearly increases with t at the early stages of crystallization (r ∝ t), then, with the progress of phase transition, the spherulite radius becomes dependent on the square root of the time (r ∝ t1/2) until termination of crystal growth. The nonlinear advance of the crystal growth front is caused by a varied composition of the melt phase in contact with the growing crystals, due to diffusion of mobile PCL chains away from the spherulite surface. The melt phase becomes spatially inhomogeneous, causing self‐deceleration of PBT crystallization until a limit composition that prevents further crystallization is reached in the melt. The maximum crystallinity achievable during isothermal crystallization decreases with Tc. The lowering of the temperature after termination of the isothermal crystallization allows to complete the crystal growth, but the final developed crystallinity still depends on Tc, being lower at higher Tcs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3148–3155, 2007 相似文献
14.
N. Kato 《Journal of Chemical Sciences》1983,92(4-5):429-435
Recently, a thermodynamically stable phase between a (low) andΒ (high) phases of quartz has teen found. This paper explains the three phases by a statistical theory of Bragg-William’s type
by assuming three states of SiO4 tetrahedron along a-axis. Adjusting the interaction parameters one can predict the successive transitions of the first-order
(Tc) and the nearly second-order (TQ. Also with an assignment of Tc = 573‡C, one obtains TQ = Tc+ 1.2 degree which fits to the experiment. The difference in energy between Dauphiné twin bonding and the normal bonding is
estimated to be 0.525 eV. 相似文献
15.
C. H. Cheng E. M. Pearce 《Journal of polymer science. Part A, Polymer chemistry》1980,18(6):1883-1888
Copolymers of styrene/p-vinyltrifluoroacetophenone were prepared by two different reaction routes: (1) modification of polystyrene with trifluoroacetyl chloride and (2) copolymerization of styrene and p-vinyltrifluoroacetophenone (VTFA). There appears to be a limit to the modification method because only a maximum content of 14.5 mole % trifluoroacetyl functionality could be attached to the polymer before the onset of crosslinking. Differential scanning calorimetry (DSC) was used to determine their Tg's. In addition, the reactivity ratios of styrene and VTFA were investigated. The reactivity ratios and Q and e values were r1 = 0.30 ± 0.09 (styrene) and r2 = 1.3 ± 0.3 (VTFA); Q1 = 1.0 and e1 = ?0.8 (styrene); Q2 = 0.44 and e2 = 1.93 (VTFA). 相似文献
16.
A. M. Chibiryaev A. Ermakova I. V. Kozhevnikov 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2011,85(8):1347-1357
Revision of the experimental data on α-pinene thermal isomerization attained in supercritical ethanol allowed us to expand
the reaction scheme, which includes now six main products and eleven reversible reactions. The equilibrium constants of every
reaction (K
T, j
and K
Φ, j) were calculated to allow for reversibility of reactions. The thermochemical data of the pure compounds required to calculate
constants K
T, j
and K
Φ, j
(standard enthalpy and entropy of formation Δf
H° (298.15 K), Δf
S° (298.15 K), heat capacity C
p
(T), critical parameters T
cr and p
cr, boiling point T
b, and the acentric factor ω) were preliminary estimated using the empirical Joback and Benson methods. A kinetic model based
on the new expanded scheme of reversible reactions was successfully identified and its kinetic parameters k
j
(600 K) and E
j
were determined. Detailed examination of the new kinetic model allowed us to refine the generally accepted mechanism of α-pinene
thermal isomerization and to distinguish additional features of the multistep process. 相似文献
17.
《International journal of quantum chemistry》2018,118(11)
Perturbed structures for QTAIM dual functional analysis (QTAIM‐DFA) are proposed to generate using the coordinates corresponding to the compliance force constants in internal vibrations (CIV). In QTAIM‐DFA, total electron energy densities Hb( r c) are plotted versus Hb( r c) – Vb( r c)/2 at bond critical points (BCPs) of interactions in question, where Vb( r c) are potential energy densities at BCPs. Each plot of an interaction based on the data from both perturbed structures and fully optimized one takes the form (θp, κp), where θp corresponds to the tangent line of the plot and κp is the curvature. The θp values evaluated with CIV are equal to those obtained by partial optimizations with the interaction distance in question being fixed suitably, within the calculation errors. Very high applicability of CIV is demonstrated to generate the perturbed structures for QTAIM‐DFA. Dynamic nature of interactions based on (θp, κp) with CIV is called “intrinsic dynamic nature of interactions.” 相似文献
18.
S. Y. Tawfik N. N. Messiha S. H. El-Hamouly 《Journal of polymer science. Part A, Polymer chemistry》1993,31(2):427-433
Three new monomers of p-phenylacrylamide derivatives were prepared by either the reaction of p-methyl-, p-nitro-, and p-chloroaniline with acryloyl chloride or with acrylic acid in the presence of dicyclohexyl carbodiimide (DCCI). The prepared monomers were copolymerized with each of tri-n-butyltinacrylate and tri-n-butyltinmethacrylate. Copolymerization reactions were carried out in dioxane at 70°C using 1 mol % azobisisobutyronitrile as a free radical initiator. The structure of the new monomers and the prepared copolymers were investigated by IR and 1H-NMR spectroscopy. The monomer reactivity ratios for the copolymerization of p-chlorophenylacrylamide (M1) with each of tri-n-butyltinacrylate (TBTA) and tri-n-butyltinmethacrylate (TBTMA) (M2) were found to be r1 = 2.6; r2 = 0.83 and r1 = 1.3; r2 = 1.71, respectively. In case of p-tolyacrylamide (M1) with TBTA and TBTMA (M2) r1 = 0.35, r2 = 1.03 and r1 = 1.38, r2 = 0.366 respectively. The Q and e values for the prepared p-tolyl- and p-chlorophenylacrylamide were calculated © 1993 John Wiley & Sons, Inc. 相似文献
19.
Acrolein was copolymerized by radical initiation in aqueous solutions with sodium p-styrenesulfonate and acrylic acid, respectively, in the pH range of 3–7. The reactivities were shown to be pH-dependent. For the acrolein (M1)–sodium p-styrenesulfonate (M2) pair, r1 = 0.33 ± 0.15 and r2 = 0.32 ± 0.05 at pH 3; r1 = 0.23 ± 0.12 and r2 = 0.05 ± 0.03 at pH 5; r1 = 0.26 ± 0.03 and r2 = 0.025 ± 0.025 at pH 7. For the acrolein (M1)–acrylic acid (M2) pair, r1 = 0.50 ± 0.30 and r2 = 1.15 ± 0.2 at pH 3; r1 = 2.40 ± 0.50 and r2 = 0.05 ± 0.05 at pH 5; r1 = 6.70 ± 3.00 and r2 = 0.00 at pH 7. For acrolein, the new values of Q = 1.6 and e = 1.2 have been calculated. For sodium p-styrenesulfonate, the values Q = 0.76 and e = ?0.26 at pH 3, Q = 0.51 and e = ?0.87 at pH 5, Q = 0.39 and e = ?1.00 at pH 7 were obtained; and for acrylic acid, the values Q = 1.27 and e = 0.50 at pH 3, Q = 0.11 and e = ?0.22 at pH 5 were derived. The changes in reactivity are explained on the basis of inductive and resonance effects. 相似文献
20.
The peak temperature (T
p) and different temperature (ΔT) are the basic information in the differential thermal analysis (DTA). Considering the kinetic relation and the heat equilibrium
in DTA, a correctional differential kinetic equation (containing T
p and ΔT parameter) is proposed. In the dehydration reaction of CaC2O4·H2O, the activation energy calculated from the new equation showed some smaller than that from Kissinger equation, but some
bigger than that from Piloyan equation. 相似文献