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1.
Lixin Zhou 《Theoretical chemistry accounts》2000,105(1):86-92
Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,
acidity (free energy of deprotonation, ΔGo), and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-diselenone, H2C4Se2S2) are reported. The global minimum found on the potential energy surface of 1,2-diseleno-3,4-dithiosquaric acid presents a
planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE
isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference
compounds, cyclobutanediselenone, and cyclobutenedithiol. The computed aromatic stabilization energy (ASE) by homodesmotic
reaction (Eq 1) is −20.1 kcal/mol (MP2(fu)/6-311+G** //RHF/6-311+G**) and −14.9 kcal/mol (B3LYP//6-311+G**//B3LYP/6-311+G**).
The aromaticity of 1,2-diseleno-3,4-dithiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation
(Λ) −17.91 (CSGT(IGAIM)-RHF/6-311+G**//RHF/6-311+G**) and −31.01 (CSGT(IGAIM)-B3LYP/6-311+G**//B3LYP/6-311+G**). Thus, 1,2-diseleno-3,4-dithiosquaric
acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated theoretical gas-phase acidity is
ΔGo
1(298K)=302.7 kcal/mol and ΔGo
2(298K)=388.4 kcal/mol. Hence, 1,2-diseleno-3,4-dithiosquaric acid is a stronger acid than squaric acid(3,4-dihydroxy-3-cyclobutene-1,2-dione,
H2C4O4).
Received: 11 April 2000 / Accepted: 7 July 2000 / Published online: 27 September 2000 相似文献
2.
Rois Benassi 《Theoretical chemistry accounts》2001,106(4):259-263
In order to calculate more accurately the enthalpies of formation, ΔH
f°(298 K), for large molecules using the CBS-4M method, a new formulation of the empirical higher-level correction to the energy
is proposed: ΔE=a|S|2
i
i
I
i
i
+b(n
α+n
β)+cΔ<S
2>+Σn
i
d
i
. The new methodology (CBS-4MB) applied to a set of 114 molecules of different size significantly decreases the mean absolute
deviation from 3.78 to 2.06 kcal/mol.
Received: 7 February 2001 / Accepted: 5 April 2001 / Published online: 13 June 2001 相似文献
3.
Self-consistent reaction field calculation of solvent reorganization energy in electron transfer: a dipole-reaction field interaction model 总被引:1,自引:0,他引:1
Based on the continuum dielectric model, this work has established the relationship between the solvent reorganization energy
of electron transfer (ET) and the equilibrium solvation free energy. The dipole-reaction field interaction model has been
proposed to describe the electrostatic solute-solvent interaction. The self-consistent reaction field (SCRF) approach has
been applied to the calculation of the solvent reorganization energy in self-exchange reactions. A series of redox couples,
O2/O−
2, NO/NO+, O3/O−
3, N3/N−
3, NO2/NO+
2, CO2/CO−
2, SO2/SO−
2, and ClO2/ClO−
2, as well as (CH2)2C-(-CH2-)
n
-C(CH2)2 (n=1 ∼ 3) model systems have been investigated using ab initio calculation. For these ET systems, solvent reorganization energies
have been estimated. Comparisons between our single-sphere approximation and the Marcus two-sphere model have also been made.
For the inner reorganization energies of inorganic redox couples, errors are found not larger than 15% when comparing our
SCRF results with those obtained from the experimental estimation. While for the (CH2)2C–(–CH2–)
n
–C(CH2)2 (n=1 ∼ 3) systems, the results reveal that the solvent reorganization energy strongly depends on the bridge length due to the
variation of the dipole moment of the ionic solute, and that solvent reorganization energies for different systems lead to
slightly different two-sphere radii.
Received: 19 April 2000 / Accepted: 6 July 2000 / Published online: 27 September 2000 相似文献
4.
Sequential Monte Carlo/quantum mechanical calculations are performed to study the solvent effects on the electronic absorption
spectrum of formamide (FMA) in aqueous solution, varying from hydrogen bonds to the outer solvation shells. Full quantum-mechanical
intermediate neglect of differential overlap/singly excited configuration interaction calculations are performed in the supermolecular
structures generated by the Monte Carlo simulation. The largest calculation involves the ensemble average of 75 statistically
uncorrelated quantum mechanical results obtained with the FMA solute surrounded by 150 water solvent molecules. We find that
the n → π* transition suffers a blueshift of 1,600 cm−1 upon solvation and the π → π* transition undergoes a redshift of 800 cm−1. On average, 1.5 hydrogen bonds are formed between FMA and water and these contribute with about 20% and about 30% of the
total solvation shifts of the n → π* and π → π* transitions, respectively. The autocorrelation function of the energy is used
to sample configurations from the Monte Carlo simulation, and the solvation shifts are shown to be converged values.
Received: 14 March 2002 / Accepted: 3 April 2002 / Published online: 24 June 2002 相似文献
5.
The changes in Gibbs free energy (ΔG
int), enthalpy (ΔH
int) and entropy (TΔS
int) upon complexation between riboflavin (RF) and N,N-dioctadecyl-[1,3,5]triazine-2,4,6-triamine (DTT), mediated by triple hydrogen
bonds at water/carbon tetrachloride, trichloroethylene and chloroform interfaces, were determined via temperature-controlled
interfacial tension measurements. It was shown that hydrogen bonding interactions between RF and DTT were best characterized
by large and negative ΔH
int values, unlike those predicted from either the polarity in each phase or the arithmetic average of the polarities in the
two phases. Furthermore, the ΔH
int values became more positive as the dielectric constant of the oil phase was increased. These results strongly indicate that
ΔH
int is governed by the dielectric properties of the oil phase. Adsorption of RF, DTT and the RF-DTT complex at the water/oil
interface gave rise to restrictions on the translational and rotational motions of these species, as demonstrated by the ΔS
int values observed, which is another characteristic of interfacial complexation. The thermodynamic parameters evaluated in the
present study revealed the characteristic complexation behavior that occurs at a water/oil interface, as mediated by hydrogen
bonding. 相似文献
6.
Ab initio calculations were performed to investigate the structure and bonding of the phenol dimer and its cation, especially
the OH stretching frequencies. Some stable structures of the phenol dimer and its cation were obtained at the Hartree–Fock
level and were found to be in agreement with predictions based on spectroscopic investigations. In these dimers the phenol
moieties are bound by a single OH⋯O hydrogen bond. The hydrogen bond is much stronger in the dimer cation than in the neutral
dimer. The calculated binding energy of the phenol dimer in the most stable structure was 6.5–9.9 kcal/mol at various levels
of calculation, compared with the experimental value of 5 kcal/mol or greater. The binding energy of the phenol dimer cation
is more than 3 times (24.1–30.6 kcal/mol) as large as that of the neutral dimer. For the phenol dimer the OH stretching frequency
of the proton-accepting phenol (PAP) is 3652 cm−1 and that of the proton-donating phenol (PDP) is 3516 cm−1; these are in agreement with observed values of 3654 and 3530 cm−1, respectively. For the phenol dimer cation the OH stretching frequency of the PAP is 3616–3618 cm−1 in comparison with an observed value of 3620 ± 3 cm−1. That of the PDP in the dimer cation is calculated to be 2434–2447 cm−1, which is 1210–1223 cm−1 lower than that of the bare phenol. The large reduction in the OH stretching frequency of the PDP in the phenol dimer cation
is attributed to the formation of a stronger hydrogen bond in the cation than in the neutral dimer.
Received: 24 March 2000 / Accepted: 26 April 2000 / Published online: 11 September 2000 相似文献
7.
The physical properties of chemicals are strongly influenced by their protonation state, affecting, for example, solubility
or hydrogen-bonding characteristics. The ability to accurately calculate protonation states in the form of pK
as is, therefore, desirable. Calculations of pK
a changes in a series of substituted pyridines are presented. Computations were performed using both ab initio and semiempirical
approaches, including free energies of solvation via reaction-field models. The selected methods are readily accessible with
respect to both software and computational feasibility. Comparison of calculated and experimental pK
as shows the experimental trends to be reasonably reproduced by the computations with root-mean-square differences ranging
from 1.22 to 4.14 pK
a units. Of the theoretical methods applied the best agreement occurred using the second-order M?ller–Plesset/6-31G(d)/isodensity
surface polarized continuum solvation model, while the more computationally accessible Austin model 1/Solvent model 2 (SM2)
approach yielded results similar to the ab initio methods. Analysis of component contributions to the calculated pK
as indicates the largest source of error to be associated with the free energies of solvation of the protonated species followed
by the gas-phase protonation energies; while the latter may be improved via the use of higher levels of theory, enhancements
in the former require improvements in the solvation models. The inclusion of alternate minimum in the computation of pK
as is also indicated to contribute to differences between experimental and calculated pK
a values.
Received: 27 April 1999 / Accepted: 27 July 1999 / Published online: 2 November 1999 相似文献
8.
Relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart–Bonn variety and extended valence
basis sets have been used for the investigation of the dimers La2 and Lu2. It was found that the ground states for La2 and Lu2 are most likely 1∑
g
+ (σ
g
2π
u
4) and 3∑
g
− (4f
144f
14σ
g
2σ
u
2πu
2), respectively. The molecular constants including error bars were derived from multireference configuration interaction as
well as coupled-cluster calculations, taking into account corrections for atomic spin–orbit splitting as well as possible
basis set superposition errors. The theoretical values for La2 (R
e=2.70±0.03 ?, D
e=2.31±0.13 eV, ωe=186±13 cm−1) show good agreement with the experimental binding energy (D
e=2.52±0.22 eV), but the experimental vibrational constant in an Ar matrix (ωe=236±0.8 cm−1) is significantly higher. For Lu2 the theoretical values (R
e=3.07±0.03 ?, D
e=1.40±0.12 eV, ωe=123±1 cm−1) are in overall excellent agreement with experimental data (D
e=1.43±0.34 eV, ωe=122± 1 cm−1). The electronic structures of La2 and Lu2 are compared to those other lanthanide dimers and trends in the series are discussed.
Received: 25 March 2002 / Accepted: 2 June 2002 / Published online: 21 August 2002 相似文献
9.
The ground state and several low-lying excited states of the Mg2 dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS
multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and
perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has
been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A1Σ
u
+ excited state of Mg2 is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D
e, R
e and ωe deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements
between the excited singlet and triplet states of Mg2 have been evaluated as a function of internuclear separation.
Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001 相似文献
10.
M. Fujiwara T. Okano T. -H. Nakashima A. A. Nakamura G. Sugihara 《Colloid and polymer science》1997,275(5):474-479
For a sodium salt of α-sulfonatomyristic acid methyl ester (14SFNa), one of the α-SFMe series surfactants, critical micellization
concentration (CMC), solubility and degree of counterion binding (β) were determined by means of electrocon-ductivity measurements
at different temperatures (at every 5 °C) ranging from 15 to 50 °C. The phase diagram of 14SFNa in pure water was constructed
from the CMC- and solubility-temperature data, in which the Krafft temperature (critical solution temperature) was found around
0 °C. The changes in the Gibbs energy, ΔG
0
m, enthalpy, ΔH
0
m, and entropy, ΔS
0
m, upon micelle formation as a function of temperature were evaluated taking βvalues into calculation.
Received: 28 August 1996 Accepted: 5 November 1996 相似文献
11.
H. Lee Woodcock Milan Hodošček Paul Sherwood Yong S. Lee Henry F. Schaefer III Bernard R. Brooks 《Theoretical chemistry accounts》2003,109(3):140-148
A replica path method has been developed and extended for use in complex systems involving hybrid quantum/classical (quantum
mechanical/molecular mechanical) coupled potentials. This method involves the definition of a reaction path via replication
of a set of macromolecular atoms. An “important” subset of these replicated atoms is restrained with a penalty function based
on weighted root-mean-square rotation/translation best-fit distances between adjacent (i±1) and next adjacent (i±2) pathway steps. An independent subset of the replicated atoms may be treated quantum mechanically using the computational
engine Gamess-UK. This treatment can be performed in a highly parallel manner in which many dozens of processors can be efficiently
employed. Computed forces may be projected onto a reference pathway and integrated to yield a potential of mean force (PMF).
This PMF, which does not suffer from large errors associated with calculated potential-energy differences, is extremely advantageous.
As an example, the QM/MM replica path method is applied to the study of the Claisen rearrangement of chorismate to prephenate
which is catalyzed by the Bacillus subtilis isolated, chorismate mutase. Results of the QM/MM pathway minimizations yielded an activation enthalpy ΔH
†† of 14.9 kcal/mol and a reaction enthalpy of −19.5 kcal/mol at the B3LYP/6-31G(d) level of theory. The resultant pathway was
compared and contrasted with one obtained using a forced transition approach based on a reaction coordinate constrained repeated
walk procedure (ΔH
†† =20.1 kcal/mol, ΔH
rxn = −20.1 kcal/mol, RHF/4-31G). The optimized replica path results compare favorably to the experimental activation enthalpy
of 12.7±0.4 kcal/mol.
Received: 16 December 2001 / Accepted: 6 September 2002 / Published online: 8 April 2003
Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 22nd National Meeting of the American Chemical
Society, 2001.
Correspondence to: H.L. Woodcock e-mail: hlwood@ccqc.uga.edu
Acknowledgements. The authors thank Eric Billings, Xiongwu Wu, and Stephen Bogusz for helpful discussions and related work. The authors also
show grateful appreciation to The National Institutes of Health and The National Science Foundation for support of the current
research. 相似文献
12.
Addition–elimination reactions involving a nucleophile and a remote leaving group [SH
N(AE)tele] are well-known under basic conditions, especially amongst electron-poor six-membered heterocycles, but are less commonly
encountered for five-membered heterocycles and are rare under acidic conditions. Concentrated HCl converts 1-hydroxy-1H-pyrazolo[3,4-c] isoquinoline and 1-hydroxy-1H-pyrazolo[3,4-c]quinoline into 3-chloro-1H-pyrazolo[3,4-c]isoquinoline and 3-chloro-1H-pyrazolo[3,4-c]quinoline, respectively. However, apparently neither the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)-quinolines nor the parent 1-hydroxypyrazole undergo this reaction. Additionally, all these systems are refractory under
basic conditions. We present a plausible mechanism for the reaction, involving the 3-addition of Cl- to the diprotonated heterocycle, followed by the elimination of water. Calculations of the initial transition states and
intermediates, using optimisation at B3LYP/6-311+G(d,p), including thermochemistry [HF/6-31+G(d)], and single-point Poisson–Boltzmann
self-consistent reaction field determination of the free energy of solvation (Jaguar Poisson–Boltzmann self-consistent reaction
field), support this mechanism and reproduce the observed order of reactivity, the addition step being 2–4 kcal less favourable
for the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)quinolines and provide a rationalisation for the role of strong acid.
Received: 27 June 2002 / Accepted: 6 September 2002 /
Published online: 14 February 2003 相似文献
13.
The structures, properties and the bonding character for sub-carbonyl Si, SiCO and Si(CO)2, in singlet and triplet states have been investigated using complete-active-space self-consistent field (CASSCF), density
functional theory and second-order M?ller–Plesset methods with a 6-311+G* basis set. The results indicate that the SiCO species
possesses a 3∑− ground state, and the singlet 1Δ excited state is higher in energy than the 3∑− state by 17.3 kcalmol−1 at the CASSCF–MP2/6-311+G* level and by 16.4 kcalmol−1 at the CCSD(T)/6-311+G* level. The SiCO ground state may be classified as silene (carbonylsilene), and its COδ− moiety possesses CO− property. The formation of SiCO causes the weakening of CO bonds. The Si–C bond consists of a weak σ bond and two weak π
bonds. Although the Si–C bond length is similar to that of typical Si–C bonds, the bond strength is weaker than the Si–C bonds
in Si-containing alkanes; the calculated dissociation energy is 26.2 kcalmol−1 at the CCSD(T)/6-311+G* level. The corresponding bending potential-energy surface is flat; therefore, the SiCO molecule is
facile. For the bicarbonyl Si systems, Si(CO)2, there exist two V-type structures for both states. The stablest state is the singlet state (1A1), and may be referred to the ground state. The triplet state (3B1) is energetically higher in energy than the 1A1 state by about 40 kcalmol−1 at the CCSD(T)/6-311 + G* level. The bond lengths in the 1A1 state are very close to those of the SiCO species, but the SiCO moieties are bent by about 10°, and the CSiC angles are only
about 78°. The corresponding 3B1 state has a CSiC angle of about 54° and a SiCO angle of about 165°, but its Si–C and C–O bonds are longer than those in the
1A1 state by about 0.07 and 0.03 ?, respectively. This Si(CO)2 (1A1) has essentially silene character and should be referred to as a bicarbonyl silene. Comparison of the CO dissociation energies
of SiCO and Si(CO)2 in their ground states indicates that the first CO dissociation energy of Si(CO)2 is smaller by about 7 kcalmol−1 than that of SiCO; the average one over both CO groups is also smaller than that of SiCO. A detailed bonding analysis shows
that the possibility is small for the existence of polycarbonyl Si with more than three CO. This prediction may also be true
for similar carbonyl complexes containing other nonmetal and non-transition-metal atoms or clusters.
Received: 17 April 2002 / Accepted: 11 August 2002 / Published online: 4 November 2002
Acknowledgements. This work was supported by the National Natural Science Foundation of China (29973022) and the Foundation for Key Teachers
in University of the State Ministry of Education of China.
Correspondence to: Y. Bu e-mail: byx@sdu.edu.ch 相似文献
14.
Mahmoud M. Al Omari Mohammad B. Zughul J. Eric D. Davies Adnan A. Badwan 《Journal of inclusion phenomena and macrocyclic chemistry》2007,58(3-4):227-235
The complexation of terfenadine (Terf) with β-cyclodextrin (β-CD) in solution and solid state has been investigated by phase solubility diagram (PSD), differential scanning calorimetry
(DSC), powder X-ray diffractometry (PXD) and proton nuclear magnetic resonance (1H-NMR). The PSD results indicated that the salt saturation with the buffer counter ion (citrate−2, H2PO4−1 and Cl−1 ions) of Terf (pK
a = 9.5) and the hydrophobic effect play in tandem to increase the value of the complex formation constant (K11) measured at different conditions of pH, ionic strength, buffer type and buffer concentration. The correlation of the free
energy of complex formation (ΔG11) with the free energy of inherent solubility of Terf (ΔGSo) obtained by changing the pH, ionic strength and buffer concentration was used to measure the contribution of the hydrophobic
effect (desolvation) to complex formation. The hydrophobic effect was found to constitute 57.8% of the driving force for complex
stability, while other factors including specific interactions contribute −13.4 kJ/mol. 1H-NMR spectra of Terf–citrate and Terf–HCl salts gave identical chemical shift displacements (ΔΔ) upon complexation, thus
indicating that the counter anions are positioned somewhere outside of the β-CD cavity. DSC, XRPD and 1H-NMR proved the formation of solid Terf/acid/β-CD ternary complexes. 相似文献
15.
Matthew J. Bernett Adrianne K. Tipton Michael T. Huggins Jonathan H. Reeder David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2000,131(3):239-249
Summary. A crystal structure determination of 2,2′-bipyrroyl (1; 2,2′-dipyrryl-diketone, bis (2-pyrrolyl)ethanedione) and its spectroscopic properties in solution are reported. In the crystal, 1 self-assembles via hydrogen bonding into supramolecular ribbons that extend indefinitely through the crystal lattice. The observed molecular
conformation is one where each pyrrole ring and adjacent carbonyl group are co-planar (torsion angle ∼ 0.9°), with the N-H
pointing in the same direction as the C=O. The two carbonyls have a transoid but not co-planar geometry with a torsion angle
of ∼128°. Adjacent molecules in the crystal are linked by pairs of intermolecular hydrogen bonds, pyrrole NH to carbonyl oxygen,
to form a matrix of polymeric chains that lie like neatly stacked, parallel streams of ribbons. Molecular mechanics calculations
on the monomer indicate an intra-molecularly hydrogen bonded planar conformation (sp, ap, sp) at the global energy minimum. In CHCl3, 1 is monomeric according to vapor pressure osmometry (MW
obs=179±10 vsċMW
calc=188). In THF, the measured molecular weight is 340±15, which corresponds best to one molecule of 1 solvated by two THF molecules (MW=322 for C10H8N2O4ċ2 C4H8O) rather than to a dimer.
Received October 21, 1999. Accepted November 2, 1999 相似文献
16.
We determined the gas-phase acidity of methylthioacetic acid (MTA) in a triple-quadrupole mass spectrometer using the Cooks’
kinetic method with the consideration of entropy effects. The negatively charged proton-bound dimers were generated by electrospray
ionization. Collision-induced dissociation was applied to the dimer ions and the product ion ratios were measured at four
different collision energies. The gas-phase acidity (ΔH
acid) of MTA was determined to be 340.0±1.7 kcal/mol using the extended kinetic method and 339.8±1.7 kcal/mol using the standard
kinetic method. The entropy term is insignificant in this case and can be ignored. The standard kinetic method yielded a free
energy of deprotonation of MTA (ΔG
acid) of 333.0±1.7 kcal/mol. The entropy of the acid dissociation, ΔS
acid, was estimated to be 22.8 cal/mol K. Theoretical prediction at the B3LYP/6-31+G* level of theory gives a similar value for
ΔH
acid of 338. 9 kcal/mol. In the gas-phase, MTA is a stronger acid than methoxyacetic acid, although in solution, MTA is a weaker
one. 相似文献
17.
Maurizio Guerra 《Theoretical chemistry accounts》2000,104(6):455-460
Calculations with the density functional theory (DFT) method using the most popular functional, Becke's three parameter hybrid
with the Lee, Yang and Parr correlation functional, predict the π-delocalized configuration of the vinylacyl radical, CH2=CH—C(•)=O, to be more stable than the σ-localized configuration in contrast with ab initio unrestricted quadratic configuration
interaction with single and double excitations calculations as previously found for the isoelectronic vinyl radical, Y-C(•)=CH2, bearing π-type α substituents. Experimental evidence on the electronic configuration adopted by vinyl radicals is contrasting.
In the present case comparison with experiment indicates firmly that the currently available density functionals overestimate
the stability of π-delocalized versus σ-localized configurations in radicals since they favor the π configuration for the
γ-methylvinylacyl radical, CH3—CH=CH—C(•)=O, in contrast with unequivocal electron spin resonance data. This failure is mainly due to an incorrect estimate
of dynamic correlation energy with DFT functionals.
Received: 13 January 2000 / Accepted: 16 March 2000 / Published online: 21 June 2000 相似文献
18.
A novel surfactant peptide consisting of an arginine cation with laurate anion has been synthesized, purified and characterized.
The critical micellar concentration (cmc) of peptide in aqueous solutions has been determined using spectroscopic techniques
and is found to increase from 0.06 to 0.11 mM with increasing temperature (15–45 °C). Cmc is also determined in the presence
of salts like NaCl, KCl and sodium acetate and it is found that these electrolytes hinder aggregation with a significant increase
in the case of sodium acetate. The aggregation number of the surfactant peptide has been determined using fluorescence quenching
measurements and is observed to decrease from 14 to 6 with increasing temperature (15–45 °C). The standard free energy change
(ΔG
0
m) and standard enthalpy change (ΔH
0
m) of the peptide aggregate are found to be negative with a small positive value for standard entropy change (ΔS
0
m). The peptide aggregate seems to undergo phase transition above 50 °C as observed from UV–vis and fluorescence spectroscopy.
From pyrene binding studies, it is shown that the interior dielectric constant increases from 5.08 at 34 °C to 8.77 at 50 °C
and further decreases with increase in temperature indicating a phase change at 50 °C. Also, the ratio of excimer intensity
to monomer intensity, which is a measure of microviscosity of the aggregate, decreases with increase in temperature with a
change at 50 °C indicating a phase change.
Received: 14 February 1997 Accepted: 13 August 1997 相似文献
19.
T. R. Usacheva V. A. Sharnin E. Matteoli 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2011,85(11):1898-1902
Complexation of the 18-crown-6 ether (18C6) with glycine (Gly) in mixed H2O-DMSO solvents with the composition of 0.1, 0.2, and 0.25 mole fraction of DMSO (T = 298.15 K) was studied calorimetrically. Thermodynamic characteristics of the reaction of the formation of the molecular
Gly18C6 complex (Δr
G°, Δr
H°, TΔr
S°) were calculated from the calorimetric data. It was established that the change in the stability of the Gly18C6 complex
is mainly determined by the predominance of the enthalpy component of the Gibbs energy over the entropy component. It was
shown during the analysis of the enthalpy contributions of the reagents to the enthalpy of the reaction of the formation of
Gly18C6 that the change in the enthalpy of the reaction upon a change the solvent composition was due to changes in the solvation
state of 18C6. 相似文献
20.
The structures and the conformational energies of nonprotonated, monoprotonated and diprotonated 1,2-ethanediamine have been
investigated through density functional theory. The relative performance of local and gradient-corrected functionals is discussed.
The existence of hydrogen-bond formation has been determined with electron localisation function calculations. Proton affinities
for nonprotonated and monoprotonated 1,2-ethanediamine have been calculated and are in agreement with experimental data. The
influence of solvation has been accounted for through the self-consistent isodensity polarisable continuum model. The results
for the nonprotonated conformers show that solvation stabilises those conformers which have the lone pair in an antiperiplanar
conformation. Solvation of the monoprotonated conformer stabilises significantly the “anti” conformation, which is unstable
in the gas phase. For the di-protonated species, solvation stabilises slightly the gauche conformer, which is unstable in
the gas phase.
Received: 28 September 1999 / Accepted: 2 May 2000 / Published online: 27 September 2000 相似文献