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1.
Stefan E. Boiadjiev David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2002,1(1):1469-1480
Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted rotation about an sp3–sp2 C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and thermodynamic parameters associated with the atropisomerism: ΔG ‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C2D2Cl4 solvent. 相似文献
2.
Stefan E. Boiadjiev David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2002,133(11):1469-1480
Summary. Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted
rotation about an sp3–sp2 C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution
at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration
of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and
thermodynamic parameters associated with the atropisomerism: ΔG
‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C2D2Cl4 solvent.
Corresponding author. E-mail: lightner@scs.unr.edu
Received July 1, 2002; accepted July 8, 2002 相似文献
3.
The reentrant behavior of Poly(vinyl alcohol) (PVA)–borax aqueous semidilute solutions with a PVA concentration of 20 g/l
and borax concentrations varies from 0.0 to 0.20 M was investigated using dynamic light scattering (DLS) and dynamic viscoelastic
measurements. Two (fast and slow modes) and three (fast, middle, and slow) relaxation modes of PVA semidilute aqueous solutions
without and with the presence of borax, respectively, were observed from DLS measurements. The fast and middle relaxation
modes were q
2-dependent (q is the scattering vector) characteristic of diffusive behavior; however, the slow modes were q
3-dependent, characteristic of intraparticle dynamics. The experimental results showed that the slow relaxation mode dominates
the DLS relaxation. The DLS slow mode relaxation time, τs, and the viscoelastic modulus G′(ω) and G′′(ω) data had a similar trend and demonstrated reentrant behavior as the borax concentration was increased from 0.0 to 0.20 M,
i.e. τs, G′(ω), and G′′(ω) fluctuated with increasing borax concentration. The excluded-volume effect of polymers, charge repulsion among borate
ions bound on PVA molecules, and intermolecular cross-linking didiol–borate complexation caused an expansion of the polymer
chain; however, the screening effect of free Na+ ions on the negative charge of the borate ions bound on PVA and intramolecular cross-linking didiol–borate complexation led
to a shrinkage of the polymer chain. The reentrant behavior was the consequence of the balance between expansion and shrinkage
of the PVA–borate complex.
Received: 26 March 1999/Accepted in revised form: 3 September 1999 相似文献
4.
Lutz Preu Wolfgang Kliegel Steven J. Rettig James Trotter 《Monatshefte für Chemie / Chemical Monthly》2001,132(10):1213-1224
Summary. The structure of the dehydrogenation product 1′,3a′-dihydro-3′-((1,3-dioxoindan-2-ylidene)-phenyl-methyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′-(5′H, 6a′H)-tetrone derived from the cycloadducts (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((R)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone and/or (±)-(3a′S,6a′R)-1′,3a′-dihydro-3′-((S)-α-(1,3-dioxoindanyl)-benzyl)-5′-phenyl-spiro-(indan-2,1′-pyrrolo[3,4-c]pyrrole)-1,3,4′,6′(5H,6a′H)-tetrone, which were synthesized by 1,3-dipolar cycloaddition of N-phenylmaleimide to 2-((2-(1,3-dioxoindan-2-yl)-2-phenyl-ethenyl)-imino)-indan-1,3-dione,
was determined by X-ray analysis. Crystal data (CCD, 180 K): rhombohedral, R&3macr;;, a = 34.0871(7), c = 13.9358(5) ?, Z = 18; the structure was solved by direct methods and refined by full-matrix least-squares procedures to R(F, I ≥ 3σ(I)) = 0.053. The molecule contains a central folded ring system of two cis-fused 5-membered heterocyclic rings; each ring is nearly planar, and the angle between the rings amounts to 59.0°. Dynamic
1H NMR spectroscopy of the product revealed an exchange process caused by restricted rotation of the double bonded 1,3-indandione
moiety and the phenyl group about the Csp2-Csp2 single-bonds. Molecular modeling and complete lineshape analysis indicated a four site exchange process for which free energies
of activation and free energies could be established. ΔG
‡ values for the barriers of rotation are in the range of 57–59 kJ · mol − 1 at 273 K, which is unusually high for an unsubstituted phenyl group.
Received May 3, 2001. Accepted (revised) June 8, 2001 相似文献
5.
The conformational behavior of 1,2-difluoroethane (1), 1,2-dichloroethane (2), 1,2-dibromoethane (3), and 1,2-diiodoethane (4) have been analyzed by means of complete basis set CBS-QB3, hybrid-density functional theory (B3LYP/Def2-TZVPP) based methods
and natural bond orbital (NBO) interpretation. Both methods showed the expected greater stability of the gauche conformation
of compound 1 compared to its anti conformation. Contrary to compound 1, the anti conformations of compounds 2–4 are more stable than their gauche conformation. The stability of the anti conformation compared to the gauche conformation
increases from compound 1 to compound 4. The NBO analysis of donor–acceptor (σ → σ*) interactions showed that the generalized anomeric effect (GAE) is in favor of
the gauche conformation of compound 1. Contrary to compound 1, GAE is in favor of the anti conformations of compounds 2–4. The GAE values calculated (i.e., GAEanti − GAEgauche) increase from compound 1 to compound 4. On the other hand, the calculated dipole moment values for the gauche conformations decrease from compound 1 to compound 4. In the conflict between the GAE and dipole moments, the former succeeded in accounting for the increase of the anti conformation
stability from compound 1 to compound 4. There is a direct correlation between the calculated GAE, ∆[r
c–c(G) − r
c–c(A)] and ∆[r
c–x(A) − r
c–x(G)] parameters. The correlations between the GAE, bond orders, total steric exchange energies (TSEEs), ΔG
Anti–Gauche, ΔG
‡(Gauche → Gauche′, C
2v), ΔG
‡(Anti → Gauche, C
2), dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–4 have been investigated. 相似文献
6.
Muhammad A. Motin Tapan K. Biswas Entazul M. Huque 《Monatshefte für Chemie / Chemical Monthly》2003,134(4):475-487
Summary. Density and viscosity of NaNO3 and KNO3 in aqueous and in H2O-urea solutions were determined as a function of electrolyte concentrations at 308, 313, 318, 323, and 328 K, respectively.
The apparent molal volume (φ
v
) of the electrolytes were found to be linear functions of the square root of the solute molality (b). The φ
v
and data were fitted to the Masson equation [1] by the least square method to obtain the apparent molar volume at infinite dilution (φ
v
^), which is practically equal to the partial molar volume . The viscosity coefficients A and B were calculated on the basis of the viscosity of the solutions and the solvent concerned using the Jones–Dole [2] equation. The activation parameters for viscous flow (ΔG
≠, ΔS
≠, and ΔH
≠) were calculated according to Eyring [3]. The values of for the two systems were also calculated from B-coefficient data. The results were found to be of opposite nature in the two electrolyte systems. Where sodium nitrate showed
structure making behaviour both in aqueous and in H2O-urea solutions, KNO3 showed structure breaking behaviour in aqueous solutions and structure making behaviour in 5 molal H2O-urea solutions in the studied temperature range. The behaviour of these two electrolytes in aqueous binary and in aqueous-urea
ternary systems are discussed in terms of charge, size, and hydrogen bonding effects.
Corresponding author. E-mail: chemistry_ru@yahoo.com
Received January 24, 2002; accepted (revised) April 5, 2002 相似文献
7.
W. Earle Waghorne 《Monatshefte für Chemie / Chemical Monthly》2003,29(2):655-667
The solubilities of the silver halides in three non-aqueous solvents: methanol, acetonitrile, and dimethylsulfoxide, and
in their aqueous mixtures, are reviewed. Values for the solubility product, K
SO
, the enthalpies of solution, Δ
sol
H
o
, and the equilibrium products for AgX
(i−1)−
i
silver halide complexes, β
i
, are listed and, where possible, compared. The solvent systems provide examples for three types of mixed aqueous solvent
system: aqueous alcohol mixtures and aqueous mixtures with dipolar aprotic solvents that are weakly or strongly basic. The
experimental data are discussed in terms of the solvation of the silver and halide ions in the mixed solvents. 相似文献
8.
Clemente Bretti Francesco Crea Ottavia Giuffrè Silvio Sammartano 《Journal of solution chemistry》2008,37(2):183-201
Acid-base properties of some open-chain polyamines (ethylenediamine, diethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine
and pentaethylenehexamine) were studied at different ionic strengths in different aqueous ionic media at 25 °C. Measured were:
(i) the protonation constants of triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine from potentiometric
measurements [0 ≤I≤2.5 mol⋅L−1 in NaCl and (CH3)4NCl)]; and (ii) protonation enthalpies of ethylenediamine, diethylenetriamine, and spermine from calorimetric measurements
[NaCl: 0≤I≤1 mol⋅kg−1 for ethylenediamine, diethylenetriamine, 0 ≤I≤2 mol⋅kg−1 for spermine; (C2H5)4NI: 0≤I≤1 mol⋅kg−1; (CH3)4NCl: 0 ≤I≤2.5 mol⋅kg−1 only for diethylenetriamine]. Previously published protonation data for these polyamines in aqueous NaCl, (CH3)4NCl and (C2H5)4NI, were also examined. The general trends for the Gibbs energy and entropic contributions are, for ΔG: NaCl>(CH3)4NCl>(C2H5)4NI, and for TΔS: (C2H5)4NI>(CH3)4NCl>NaCl. This trend is more pronounced for the first protonation step. The dependences of these quantities on ionic strength
were modeled with the SIT (Specific ion Interaction Theory) equations, and differences found among the different media were
interpreted in terms of weak complex formation. 相似文献
9.
1. Results of thermodynamic and kinetic investigations for the different crystalline calcium carbonate phases and their phase
transition data are reported and summarized (vaterite: V; aragonite: A; calcite: C). A→C: T
tr=455±10°C, Δtr
H=403±8 J mol–1 at T
tr, V→C: T
tr=320–460°C, depending on the way of preparation,Δtr
H=–3.2±0.1 kJ mol–1 at T
tr,Δtr
H=–3.4±0.9 kJ mol–1 at 40°C, S
V
Θ= 93.6±0.5 J (K mol)–1, A→C: E
A=370±10 kJ mol–1; XRD only, V→C: E
A=250±10 kJ mol–1; thermally activated, iso- and non-isothermal, XRD
2. Preliminary results on the preparation and investigation of inhibitor-free non-crystalline calcium carbonate (NCC) are
presented. NCC→C: T
tr=276±10°C,Δtr
H=–15.0±3 kJ mol–1 at T
tr, T
tr – transition temperature, Δtr
H – transition enthalpy, S
Θ – standard entropy, E
A – activation energy.
3. Biologically formed internal shell of Sepia officinalis seems to be composed of ca 96% aragonite and 4% non-crystalline calcium carbonate.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Boris N. Afanasyev Yulia P. Akulova Maria M. Kotlyar 《Journal of Solid State Electrochemistry》1997,1(1):68-76
The function Δ(ΔG
A
0), which is the difference of Gibbs energies characterizing surface-active substance (surfactant, SAS) adsorption at metal/solution
and air/solution surfaces, has been introduced. The equation connecting the function Δ(ΔG
A
0) with SAS ionization potential has been obtained using the elementary theory of donor-acceptor interactions. Published experimental
data on SAS adsorption at mercury, bismuth and gold have been used for Δ(ΔG
A
0) calculation. The dependence of Δ(ΔG
A
0) on ionization potentials can be described by an equation derived in this work. It has been demonstrated that the value of
the hydrophilicity of gold is much higher than the values for mercury and bismuth. The lifetime of SAS molecules at a metal
surface has been estimated. The question of the possibility of theoretica
l estimation of standard energies ΔG
A
0 characterizing SAS adsorption at a metal/solution surface has been discussed.
Received: 9 December 1996 / Accepted: 13 January 1997 相似文献
11.
W. Earle Waghorne 《Monatshefte für Chemie / Chemical Monthly》2003,134(5):655-667
Summary. The solubilities of the silver halides in three non-aqueous solvents: methanol, acetonitrile, and dimethylsulfoxide, and
in their aqueous mixtures, are reviewed. Values for the solubility product, K
SO
, the enthalpies of solution, Δ
sol
H
o
, and the equilibrium products for AgX
(i−1)−
i
silver halide complexes, β
i
, are listed and, where possible, compared. The solvent systems provide examples for three types of mixed aqueous solvent
system: aqueous alcohol mixtures and aqueous mixtures with dipolar aprotic solvents that are weakly or strongly basic. The
experimental data are discussed in terms of the solvation of the silver and halide ions in the mixed solvents.
E-mail: Earle.Waghorne@ucd.ie
Received September 30, 2002; accepted October 29, 2002
Published online April 7, 2003
RID="a"
ID="a" Dedicated to Prof. Heinz Gamsj?ger on the occasion of his seventieth birthday 相似文献
12.
The yellow parts of the flowers of Asystasia gangetica were extracted with 85% methanol under reflux. The alcoholic extract was concentrated in vacuo and the aqueous extract was fractionated with solvents of increasing polarity. The ethyl acetate fraction yielded a yellow
solid, which was characterized as apigenin 7-O-glucosyl(3′ → 6″)luteolin 7″-O-glucoside using UV, 1H, 1H–1H COSY, 13C NMR, and mass spectral studies. 相似文献
13.
Seyed Majid Hashemianzadeh Shirin Faraji Amir Hossein Amin Sepideh Ketabi 《Monatshefte für Chemie / Chemical Monthly》2008,139(2):89-100
Summary. Interactions of the DNA bases adenine (A), guanine (G), cytosine (C), and thymine (T) with various metal ions (M) of groups IA and IIA of the periodic table of the elements were studied at the HF, MP2, and DFT levels of theory. The structures and thermodynamic stabilities of these species were studied at the gas phase. The calculations
uphold that there exist two active sites in G and one in A, C, and T. The calculations also show that the O2 atom in T is a more active site for metal ion bindings than that in C. The stability energies for G … M complexes are larger than those for A … M complexes and the stability energies for T … M complexes are larger than those for C … M complexes. As z/r ratio for the metal ion increases, the interaction energy for the complex increases systematically. Thermodynamic quantities
such as ΔH, ΔG, ΔS, and ln K were determined for each complexation reaction, [Base+M
n+ →(Base … M)
n+]. A, G, and C complexation reactions except for C … Rb+ are exothermic. The situation is quite different for T complexation reactions and all except for T … Be2+ and T … Mg2+ are endothermic. 相似文献
14.
Enthalpies (Δisom
H
(g)o), Gibbs free energies (Δisom
G
(g)o), and equilibrium constants (log K
isom) for the trans → cis isomerization of various 3,3′-, 4,4′-, and 5,5′- disubstituted 2,2′-diphenoquinones with a range of electron withdrawing
and releasing moieties (methyl, fluoro, chloro, bromo, trifluoromethyl, and amino) were calculated in the gas phase and in
the solvent phase (n-hexane, benzene, n-octanol, acetonitrile, and water). In the gas phase, the trans isomer of the parent and all substituted 2,2′-diphenoquinones is predicted to be more thermodynamically stable than the cis configuration, with log K
isom ranging from −2.8 to −7.0. For all compounds, increasing solvent polarity/proticity progressively favors shifting the cis/trans equilibrium towards greater contributions of the cis configuration and substantially increases the log K
isom by up to 5.1 units relative to the gas phase. In polar protic and polar aprotic solvents, the estimated log K
isom ranges as low as −0.4, indicating significant populations of the cis isomers should be present. The findings support the polar solvent phase mechanistic predictions for a cis configuration of 2,2′-diphenoquinones participating in the thermal transformation of trans-2,2′-diphenoquinones to oxepino[2,3-b]benzofurans. With limited exceptions for some amino derivatives, the cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran isomerization is expected to be thermodynamically favorable for all substituents/phases
under consideration. The cis-2,2′-diphenoquinone to oxepino[2,3-b]benzofuran rearrangement is predicted to become less thermodynamically favored with
increasing solvent polarity/proticity. 相似文献
15.
S. P. Chen X. X. Meng Q. Shuai B. J. Jiao S. L. Gao Q. Z. Shi 《Journal of Thermal Analysis and Calorimetry》2006,86(3):767-774
A
solid complex Eu(C5H8NS2)3(C12H8N2) has been obtained from reaction of
hydrous europium chloride with ammonium pyrrolidinedithiocarbamate (APDC)
and 1,10-phenanthroline (o-phen⋅H2O)
in absolute ethanol. IR spectrum of the complex indicated that Eu3+
in the complex coordinated with sulfur atoms from the APDC and nitrogen atoms
from the o-phen. TG-DTG investigation provided
the evidence that the title complex was decomposed into EuS.
The
enthalpy change of the reaction of formation of the complex in ethanol, Δr
H
m
θ(l), as –22.214±0.081 kJ mol–1,
and the molar heat capacity of the complex, c
m,
as 61.676±0.651 J mol–1 K–1,
at 298.15 K were determined by an RD-496 III type microcalorimeter. The enthalpy
change of the reaction of formation of the complex in solid, Δr
H
m
θ(s), was calculated as 54.527±0.314 kJ mol–1
through a thermochemistry cycle. Based on the thermodynamics and kinetics
on the reaction of formation of the complex in ethanol at different temperatures,
fundamental parameters, including the activation enthalpy (ΔH
≠
θ),
the activation entropy (ΔS
≠
θ),
the activation free energy (ΔG
≠
θ),
the apparent reaction rate constant (k),
the apparent activation energy (E), the
pre-exponential constant (A) and the reaction
order (n), were obtained. The constant-volume
combustion energy of the complex, Δc
U,
was determined as –16937.88±9.79 kJ mol–1
by an RBC-II type rotating-bomb calorimeter at 298.15 K. Its standard enthalpy
of combustion, Δc
H
m
θ,
and standard enthalpy of formation, Δf
H
m
θ,
were calculated to be –16953.37±9.79 and –1708.23±10.69
kJ mol–1, respectively. 相似文献
16.
Mercury-mercury (II) sulphide electrode has been prepared and its electrochemical and thermodynamic behaviour has been studied
in different media. The electrode is found to show Nernstian response to pS (− log [S2−]) over the range 5.19–10.38. In the pH range 7.96–11.98, at constant [S2−]v, its response is also Nernstian. The values of thermodynamic functions, viz., ΔG0. ΔH0, and ΔS0 for the electrode reaction: Hg(3)+S2−
aμ
⇌HgS(s)+2e, have been determined. Further, the standard free energy of formation (ΔG
f
0
) and solubility product constant (K
vp
) of HgS in aqueous medium at 25±0.1°C have also been determined. 相似文献
17.
Deprotonation constants of phthalic (H2A) and biphthalic (HA−) acids and of mono-protonated (BH+) and di-protonated (BH22+) piperazine acids have been determined at 25 °C by measuring the Emf of galvanic cells comprising H+-sensitive glass GE(H+) and Ag,AgCl electrodes in non-aqueous isodielectric mixtures of protic ethylene glycol (EG) and dipolar aprotic N,N-dimethylformamide
(DMF). Solvent effects on deprotonation of the acids: ∂(ΔG
disso)=2.303RT[p(s
K
a)−p(R
K
a)], have been dissected into transfer Gibbs energies, ΔG
to
, of the species involved by evaluating ΔG
to of the uncharged phthalic acid and base piperazine (B) from the measured solubilities of the acid and base, respectively,
and using ΔG
to of H+ based on the TATB reference electrolyte assumptions, as evaluated earlier. The contributions of the different species involved
in the protolytic equilibria i.e., H+,H2A,HA−,BH22+ and BH+ and their respective conjugate bases HA−,A2−,BH+ and B have been discussed in terms of their solvation behavior as guided by the ‘acid-base’, dispersion, structural and electronic
characteristics of the acid-base species and of the co-solvent molecules and binary mixtures, ignoring the Born-type electrostatic
interactions on the ionic species as the solvent system is quasi isodielectric. 相似文献
18.
Wenshen C. Yi L. Chuanpei Z. Songsheng Q. 《Journal of Thermal Analysis and Calorimetry》2001,66(2):463-468
The solid-state coordination reaction: Nd(NO3)3·6H2O(s)+4Ala(s) → Nd(Ala)4(NO3)3·H2O(s)+5H2O(l) and Er(NO3)3·6H2O(s)+4Ala(s) → Er(Ala)4(NO3)3·H2O(s)+5H2O(l) have been studied by classical solution calorimetry. The molar dissolution enthalpies of the reactants and the products
in 2 mol L–1 HCl solvent of
these two solid-solid coordination reactions have been measured using a calorimeter. From the results and other auxiliary
quantities, the standard molar formation enthalpies of [Nd(Ala)4(NO3)3·H2O, s, 298.2 K] and[Er(Ala)4(NO3)3·H2O, s,298.2 K] at 298.2 K have been determined to be Δf
H
m
0 [Nd(Ala)4(NO3)3·H2O,s, 298.2 K]=–3867.2 kJ mol–1, and Δf
H
m
0 [Er(Ala)4(NO3)3·H2O, s, 298.2 K]=–3821.5 kJ mol–1.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
19.
Yue Zhang Natsumi Watanabe Yuuko Miyawaki Yutaka Mune Kenta Fujii Yasuhiro Umebayashi Shin-ichi Ishiguro 《Journal of solution chemistry》2005,34(12):1429-1443
Aprotic N,N-dimethylpropionamide (DMPA) and N,N,N′,N′-tetramethylurea (TMU) are both strong donor solvents and coordinate to metal ions through the carbonyl oxygen atom. These
solvents show a different conformational aspect in the bulk phase, i.e., DMPA exists as either a planar cis or a nonplanar staggered conformer, while TMU exists in a single planar cis conformer. It has been established that the manganese(II) ion is solvated by five molecules in both solvents. Interestingly,
although the planar cis conformer of DMPA is more favorable than the nonplanar staggered one in the bulk phase, the reverse is the case in the coordination
sphere of the metal ion, i.e., a conformational change occurs upon solvation. To reveal the thermodynamic aspect of this conformational change, the complexation
of Mn(II) with bromide ions in DMPA and TMU has been studied by titration calorimetry at 298 K. It was found that the Mn(II)
ion forms mono-, di- and tri-bromo complexes in both solvents, and their formation constants, enthalpies and entropies were
obtained. The Δ H∘1 value for MnBr+ strongly depends on the solvent, i.e., it is positive (19.4 kJ-mol−1) in DMPA and negative (−8.7 kJ-mol−1) in TMU, whereas the Δ H^∘2 and Δ H∘3 values for the stepwise formation of MnBr2 and MnBr3− are both small and negative. The enthalpy of transfer ΔtH∘ from DMPA to TMU, which is evaluated on the basis of the extrathermodynamic TATB assumption, is 25.5 kJ-mol−1 for Mn2+ and −3.6 kJ-mol−1 for MnBr+. These values indicate that the difference between the formation enthalpy of MnBr+ in the two solvents, Δ H^∘1 (DMPA) – Δ H∘1 (TMU), is mainly ascribed to the value of ΔtH∘(Mn2+). It is found that the metal ion is also five-coordinated in the monobromo complex, MnBr(DMPA)4+ . The enthalpy for the conformational change of DMPA from its planar cis to the nonplanar staggered form is evaluated to be −11 and −5.5 kJ-mol−1 for Mn(DMPA)52 + and MnBr(DMPA)4+, respectively. Note that these values are significantly smaller than the corresponding value (5.0 kJ-mol−1) in the bulk phase. We thus conclude that, although steric hindrance among solvent molecules is reduced by replacing one
DMPA of Mn(DMPA)52 + with the relatively small bromide ion, DMPA molecules are still sterically hindered in the MnBr(DMPA)4+ complex. 相似文献
20.
The kinetics of the anation reaction of cis-diaquo-bis-oxalatochromate(III) ion by DL-alanine has been studied spectrophotometrically
in the pH range 3.8 to 7.3, where DL-alanine remains in zwitterionic form. A second-order rate law has been established. Reaction
rates in three different ethanol-water mixtures were measured. In each solvent medium the anation rate is higher as compared
to water exchange reaction at a particular temperature. The activation parameters (gDH# and ΔS#) in different ethanol-water mixtures were obtained from Eyring plots. ΔG#(ΔH# −TΔS
#) values were calculated in each solvent medium and compared with that of the isotopic water exchange process. A reaction
mechanism involving theS
N2 path has been suggested. 相似文献