Course Of Substitution Research Articles

Enhancement of thermoelectric performance in strontium titanate by praseodymium substitution

In order to identify the effects of Pr additions on thermoelectric properties of strontium titanate, crystal structure, electrical and thermal conductivity, and Seebeck coefficient of Sr1−xPrxTiO3 (x = 0.02–0.30) materials were studied at 400 < T < 1180 K under highly reducing atmosphere. The mechanism of electronic transport was found to be similar up to 10% of praseodymium content, where generation of the charge carriers upon substitution resulted in significant increase of the electrical conductivity, moderate decrease in Seebeck coefficient, and general improvement of the power factor. Formation of point defects in the course of substitution led to suppression of the lattice thermal conductivity, whilst the contribution from electronic component was increasing with carrier concentration. Possible formation of layered structures and growing distortion of the perovskite lattice resulted in relatively low thermoelectric performance for Sr0.80Pr0.20TiO3 and Sr0.70Pr0.30TiO3. The maximum dimensionless figure of merit was observed for Sr0.90Pr0.10TiO3 and amounted to ∼0.23 at 670 K and ∼0.34 at 1170 K, close to the values, obtained in similar conditions for the best bulk thermoelectrics, based on rare-earth substituted SrTiO3.

Entanglement of charge transfer, hole doping, exchange interaction, and octahedron tilting angle and their influence on the conductivity of La1−xSrxFe0.75Ni0.25O3−δ: A combination of x-ray spectroscopy and diffraction

Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3 leads to structural changes that inflect the electrical conductivity, which is caused by small polaron hopping via charge transfer and exchange interactions. The substitution forms electron holes and causes a structural transition from orthorhombic to rhombohedral symmetry, and then to cubic symmetry. The structural crossover is accompanied by a crossover from the Fe3+–O2−–Fe3+ superexchange interaction to the Fe3+–O2−–Fe4+ double exchange interaction in the course of substitution, as evidenced by a considerable increase in the conductivity at ambient temperature. The charge transfer and exchange interactions depend on the superexchange angle, which approaches 180° upon increasing Sr concentration. An increase in superexchange angle leads to an increase in overlapping between the O 2p and the Fe/Ni 3d orbitals.

Growth of LGSO: Ce crystals by the Czochralski method

Single crystals of Lu2xGd2 − 2xSiO5: Ce (0 < x < 1) compounds with different atomic ratios Lu/(Lu + Gd) have been grown by the Czochralski method. It has been shown that a change in the spatial symmetry from P21/c to C2/c in the course of substitution of lutetium for gadolinium occurs at the ratio Lu/(Lu + Gd) = 0.1. The lattice thus formed with symmetry C2/c in the structure of Lu2xGd2 − 2xSiO5: Ce crystals favors the maximum possible incorporation of Ce3+ ions into the sevenfold-coordinated position with respect to oxygen. This explains the substantial improvement of the scintillation characteristics of the grown crystals.

Silicon-Stereogenic Silanes in Asymmetric Catalysis

The exploitation of chirality at silicon in asymmetric ­catalysis is a challenging task. Silicon-stereogenic silanes were initially utilized to elucidate the stereochemical course of substitution at silicon. These mechanistic investigations are to be seen alongside a handful of synthetic transformations with covalently bound ­silicon as the stereoinducer. While in these substrate-controlled reactions the asymmetrically substituted silicon functions as a chiral auxiliary, reagent-controlled processes have remained elusive. This account summarizes the aimed design of silicon-stereogenic silanes and their introduction to stereoselective synthesis as chiral reagents.

Solid potassium-cation-conducting electrolytes in the K1−2x BaxFeO2 and K1−2x PbxFeO2 systems

Novel highly conductive potassium-cation solid electrolytes based on potassium monoferrite are synthesized via partial substitution of Ba2+ and Pb2+ for K+ and studied. Both additives lead to an abrupt increase in the conductivity of KFeO2. In the barium-containing system, the conductivity is slightly higher and the activation energy is somewhat lower. This can be explained by the effect of dimensional factor, because the Ba2+ cation is larger than the Pb2+ cation. The reasons for the conductivity increase in potassium monoferrite upon introducing the additives are the formation of potassium vacancies in the course of substitution K+ → M2+ + V’K and the extension of temperature range of existence of the high-temperature KFeO2 modification.

Base catalysed substitution reactions of octahedral cobalt(III) complexes: Mechanism switching by changing the leaving group

The rates of base catalysed hydrolysis of six asym-[Co(dmptacn)X] n+ and six asym-[Co(dmpmetacn)X] n+ complexes ( X = Br - , NO 3 - , MeSO 3 - , CF 3 SO 3 - , Me 2 SO, SO 4 2 - ) have been studied to explore the role of the leaving group and the formal charge on these complex ions in determining the effective site of deprotonation, at one of the four distinct α-CH 2 pyridyl sites or at an NH centre. The work corroborates the new pseudo-aminate mechanism established for the chloro derivatives. Deuterium exchange and proton NMR experiments have shown that the site of deprotonation is leaving group dependent, the first time this long-standing speculative idea has been proven. This work has also shown that the effect of improving the leaving group can override rate limiting deprotonation to the point where NH deprotonation, via the normal S N1CB mechanism, operates rather than the rate limiting pseudo-aminate mechanism. The steric course of substitution has been determined and the reaction via NH deprotonation, enforced by employing an exceptionally good leaving group such as the triflate ion, has uncovered a non-retentive pathway leading to the previously unknown (and unstable) sym isomeric product. The steric course of substitution is shown to be mechanism dependent. For reaction via α-CH 2 deprotonation, which of the two arms is utilised proved quite leaving group dependent, although the specific proton on a particular ‘arm’ was always the same, consistent with the requirements of this new mechanism.

Role of ventral tegmental area, periaqueductal gray and parabrachial nucleus in the discriminative stimulus effects of morphine in the rat

Previous studies have produced mixed results about the role of the ventral tegmental area, periaqueductal gray and parabrachial nucleus in morphine discriminations, perhaps owing to the considerably different methodologies used. The purpose of the present study was to compare the roles of these three brain areas using the same food-reinforced discrimination protocol, to determine whether the schedule of reinforcement influenced maximal substitution produced by site-specific morphine administration and to determine whether the time course of substitution differed by site of morphine administration. Rats were trained to discriminate 3.0 mg/kg subcutaneous morphine from saline under variable interval 15-s or fixed ratio 10 schedules of food reinforcement. Rats were then implanted with one cannula aimed at the lateral ventricle (intracerebroventricular) and one aimed at the ventral tegmental area, periaqueductal gray or parabrachial nucleus. Morphine discrimination curves were obtained by subcutaneous, intracerebroventricular and intrasite routes. When administered subcutaneously, morphine was equipotent in variable interval-trained and fixed ratio-trained rats, although it was more potent in fixed ratio-trained females than fixed ratio-trained males. When administered intracerebroventricularly, morphine (0.3-10 microg) engendered a maximum average of 63% drug-appropriate responding in both variable interval-trained and fixed ratio-trained rats; females showed significantly greater drug-appropriate responding than males, again under the fixed ratio but not under the variable interval schedule. In variable interval-trained rats, intrasite infusions of morphine (0.3-10 microg) produced maximal drug-appropriate responding of approximately 57% (ventral tegmental area), 56% (periaqueductal gray) and 41% (parabrachial nucleus); mean maximal substitution was slightly (< or = 17%) greater in fixed ratio-trained rats. When injected into the ventral tegmental area or periaqueductal gray, but not the parabrachial nucleus, naloxone methiodide (2 microg) significantly decreased drug-appropriate responding following 3.0 mg/kg subcutaneous morphine, in both variable interval-trained and fixed ratio-trained rats. The time course of the discriminative stimulus effects of morphine differed among the three brain sites: intraventral tegmental area morphine produced peak drug-appropriate responding by 15 min after injection, whereas the discriminative stimulus effects of intraperiaqueductal gray and intraparabrachial nucleus morphine peaked at approximately 60 min after injection. Taken together, these results indicate that ventral tegmental area, periaqueductal gray and parabrachial nucleus each play a role in the ability of morphine to function as a discriminative stimulus, regardless of the sex of the subject or the schedule under which the subjects are responding. Ventral tegmental area and periaqueductal gray, however, appear to be more critical than parabrachial nucleus in mediating the discriminative effects of systemic morphine in rats responding under a food reinforcement procedure. The pretreatment time and, to a lesser extent, the schedule of reinforcement are additional variables that should be considered when comparing the relative roles of different brain areas in drug discrimination.

Classic coordination complexes: The kinetics and stereochemistry of acid hydrolysis of cis- and trans-[Co(NH 3) 4Cl 2] +

The kinetics and stereochemistry of acid hydrolysis of the classic complexes cis- and trans-[Co(NH 3) 4Cl 2] + have been determined. They hydrolyse at virtually the same rate, 2.4 × 10 −3 s −1 and 2.2 × 10 −3 s −1 for the cis and trans ions, respectively, (0.02 M HClO 4, 25 °C). The ‘immeasurably fast’ hydrolysis of the cis isomer has been dismissed as a myth. The trans-[Co(NH 3) 4(OH 2)Cl] 2+ isomer has been isolated for the first time, and the trans/ cis isomerisation equilibration rate measured, again remarkably at virtually the same rate, 2.3 × 10 −3 s −1. The cis isomer is the more stable at equilibrium, 88%. The steric course of substitution has been determined as 55% cis product for the cis-dichloro reactant and 35% cis for the trans reactant. The kinetic and equilibrium data are compared to that for the well-studied [Co(en) 2Cl 2] + system, and the synthetic chemistry for the simpler tetraammine systems is elaborated.

Site—site interactions in a polymer matrix: effect of amine structure on transformations of copoly(styrene— p-nitrophenylacrylate)

Crosslinked copoly(styrene- p-nitrophenylacrylate)s ( 1a, 1b and 1c, containing 2, 4 and 10% divinylbenzene, respectively) reacted readily with secondary amines (piperidine, morpholine, piperazine, N-phenylpiperazine), the course of substitution being influenced by the crosslinking of 1, solvent polarity (dimethylformamide, methanol, toluene) and the structure of the amine. Steric interactions influenced the transformation of 1a with various primary amines (n-butylamine, i-propylamine and cyclohexylamine) giving the corresponding amides; tertbutylamine and aniline do not transform 1a under similar reaction conditions (dimethylformamide, ( T = 50°C). The swelling ability of the resins depended on the crosslinking of the starting copoly(styrene— p-nitrophenylacrylate) (bd1), the structure of the amide and solvent polarity (water, methanol, dimethylformamide, toluene, chloroform, cyclohexane).

The stereochemical course of substitution of sulfur by oxygen nucleophiles in five-membered cyclic phosphorothioates

The replacement of non-bridging sulfur in five-membered cyclic phosphorothioates by oxygen nucleophiles in the presence of bromine occurs with retention of configuration and has allowed the 31P NMR signals of the diastereoisomeric tert-butyldiphenyl silyl esters of 2′,3′-cyclic adenosine monophosphate to be assigned.

Activation and thermodynamic parameters of nucleophilic substitution. Reaction of alkyl halides with solid salts under phase‐transfer catalysis conditions

AbstractThe paper is concerned with the effect of temperature and the nature of a solid ionophore (MCl, where M = Li, Na, K, Rb or Cs) on the rate and equilibrium constants in the process of substitution of chlorine for bromine in hexyl bromide. The non‐linearity of the log k vs 1/T, dependence is shown to be linked with a change in the physical meaning of the constants with temperature variations. This is attributed to the occurrence of a strong exothermic process consisting in the formation of a kinetically independent intermediate, that precedes the limiting stage. A mechanism including the formation of two stable ternary complexes coordinated on the solid‐phase surface, one of which is structurally close to the starting reagents and the other to the final reaction products, is proposed. The observed activation energies, enthalpies of formation of the ternary complex and activation energies of the limiting stages for the forward and reverse substitution processes were calculated. Thermal effects of the substitution in the presence of various solid MCl species were determined from the thermodynamic data and observed activation energies for both reaction types. The substitution is shown to involve a limiting stage represented by a transition between the ternary complexes. A linear relationship between the activation energy for the limiting stage and the solid ionophore crystalline lattice was established, suggesting incorporation of the solid salt molecule into the transition reaction state. Potential energy changes in the course of substitution in the presence of the various MCl solid salts are discussed. The Bell—Evans—Polyani principle is shown to hold for these studies.

The stereochemical course of substitution at sulphur in a sulphite diester

A single diastereoisomer of phenyl epiandrosterone sulphite and the single diastereoisomer of benzyl epiandrosterone sulphite derived from it by nucleophilic substitution with benzyl alcohol have both been shown by X-ray crystallographic analysis to have the Rs configuration and hence the reaction proceeds with inversion at sulphur.

ChemInform Abstract: Geometric Equivalents of Enantiomers in Studies of the Stereochemical Course of Substitution at Carbon. Electronic Effects in Nucleophilic Addition to Carbonyl Groups and to Carbocations. Virtual Proof of the Existence of σ Participation by Unstrained Carbon ‐ Carbon Bonds.

AbstractAdamantones (1) with electron‐withdrawing S‐substitucnts favor syn‐addition of nucleophiles while donating groups in this position lead to favored anti‐approach.

Geometric equivalents of enantiomers in studies of the stereochemical course of substitution at carbon. Electronic effects in nucleophilic addition to carbonyl groups and to carbocations. Virtual proof of the existence of .sigma. participation by unstrained carbon-carbon bonds

Geometric equivalents of enantiomers in studies of the stereochemical course of substitution at carbon. Electronic effects in nucleophilic addition to carbonyl groups and to carbocations. Virtual proof of the existence of .sigma. participation by unstrained carbon-carbon bonds

Synthesis and Substitution Stereochemistry of trans-Chlorobis(Ethylenediamine)-(Trifluoromethanesulfonato)Cobalt(III) Trifluoromethanesulfonate

The achiral green salt trans-[Co(en)2(O3SCF3) Cl ] CF3SO3 has been synthesized by controlled solvolysis of Λ- cis -or trans-[Co(en)2Cl2] Cl in CF3SO3H.Kinetic studies and proton n.m.r . and electronic absorption spectral data indicate that cis /trans isomerization of Co(en)2(O3SCF3) Cl+ in triflic acid is extremely rapid, and that the equilibrium strongly favours the trans geometry. The loss of the first Cl - from Co(en)2Cl2+ was too fast to measure (t1/2 &lt; 1 s, -20°C, CF3SO3H), but the second Cl - was released at least 105-fold more slowly: k(3.5�0.1)x10-4 s-1, 25°C; ΔH 73.1�1.7kJ mol-1, ΔS - 66�5 J mol-1 K-1 in the range 30-60°C. Reasons for this enormous rate difference are discussed. Solvolysis of CF3SO3- from trans- Co(en)2(O3SCF3) Cl + was immeasurably rapid in H2O, dimethyl sulfoxide (Me2SO) and dimethylformamide (Me2NCHO) (t1/2 &lt; 1 s, 25°C), but it was detectable in MeCN : k 5.0×10-2 s-1, 25°C. The steric course of substitution for these solvents has been determined spectrophotometrically , and by n.m.r . spectroscopy: 0.1 M HClO4, 40.5% cis - and 59.5% trans-Co(en)2(OH2)Cl2+; Me2SO, 77% cis - and 23% trans- Co(en)2(OSMe2)Cl2+; Me2NCHO, 73% cis - and 27% trans-Co(en)2(OCHNMe2)Cl2+; MeCN , 53% cis - and 47% trans-Co(en)2(NCMe2)Cl2+ (25°C). These results are discussed in the context of a wider body of data for the stereochemistry of substitution of trans- and cis -Co(en)2ClXn+ in H2O and non-aqueous solution, where a clear leaving group dependence emerges for the trans but not the as isomers. Finally, trans- [Co(en)2(O3SCF3) Cl ] CF3SO3 provides a useful synthetic route to cis- and trans-Co(en)2ClXn+ complexes.

An analysis of the cost-of-selection concept.

It is shown for a continuous haploid model that the common standard assumptions used in calculating the cost of gene substitution, namely, large constant population size and small constant selective value, are unnecessary. Population size may fluctuate during the course of substitution without affecting the calculated total cost. The selective intensity does not need to be small and constant to give the standard result for substitution cost. Diploid models with multiple alleles are analyzed and contrasted with standard two-allele models in respect to calculation of substitution cost. The influence of population structure on the probability of occurrence of complete gene substitution is discussed on the basis of a numerical example. The robust nature of the cost-of-selection concept is examined in the light of a conservation principle.

Stereochemistry of sulfur compounds. VII. Course of substitution at sulfur attached to four different ligands

Stereochemistry of sulfur compounds. VII. Course of substitution at sulfur attached to four different ligands

ChemInform Abstract: RINGERWEITERUNG IN ORGANISCHEN MOLEKELN UNTER ELEKTRONENBESCHUSS, DIE MASSENSPEKTROMETRISCHE FRAGMENTIERUNG VON 2,3‐DICARBALKOXY‐SPIRO‐CYCLOPROPAN‐1,9′‐FLUORENEN

Chemischer InformationsdienstVolume 4, Issue 20 Physical Organic Chemistry ChemInform Abstract: RINGERWEITERUNG IN ORGANISCHEN MOLEKELN UNTER ELEKTRONENBESCHUSS, DIE MASSENSPEKTROMETRISCHE FRAGMENTIERUNG VON 2,3-DICARBALKOXY-SPIRO-CYCLOPROPAN-1,9′-FLUORENEN WILHELM J. RICHTER, WILHELM J. RICHTERSearch for more papers by this authorANDRE M. BRAUN, ANDRE M. BRAUNSearch for more papers by this author WILHELM J. RICHTER, WILHELM J. RICHTERSearch for more papers by this authorANDRE M. BRAUN, ANDRE M. BRAUNSearch for more papers by this author First published: May 15, 1973 https://doi.org/10.1002/chin.197320085AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume4, Issue20May 15, 1973 RelatedInformation

Ringerweiterung in organischen Molekeln unter Elektronenbeschuss: Die massenspektrometrische Fragmentierung von 2,3‐dicarbalkoxy‐spiro‐cyclopropan‐1, 9′‐fluorenen

AbstractRing Expansion of a three‐ to a five‐membered cyclic structure in conjunction with loss of a carbalkoxy radical from the molecular ions is postulated for a series of 2,3‐dicarbal‐koxy‐spiro‐cyclopropane‐1,9′‐fluorenes. Initial rupture of the strained spiro‐cyclopropane system with destruction of the distinct molecular geometry is inferred from virtually identical spectral patterns of, cis‐ and trans‐isomers, whereas reclosure of the ‘open’ intermediate to a cyclopentadiene ring can be deduced from the secondary decomposition of the resulting (M–COOR) fragments. These latter ions exhibit loss of the second carbalkoxy group together with a hydrogen atom of unusual positional origin (1′ or its equivalent 8′) to form a pair of neutral species, ROH and CO. Since the ejection of hydrogen from aromatic positions can be taken as evidence of rearomatisation in the course of substitution, this origin is liable to mark the site of the new ring junction and thus to reflect the size of the enlarged cyclic moiety.

Kinetics of nucleophilic substitution by chloride ion in 1,1-diaryl-2-halogenoethylenes

The reactivity of cis- and trans-2-chloro(bromo)-1-p-methoxyphenyl-1-phenylethylenes with 36Cl-labelled lithium chloride has been tested in dimethylformamide at 170–186°. The stereochemical course of substitution has been followed by infrared analysis, which has revealed a partial inversion of configuration. Chloride–chloride exchange, isomerisation, and bromide–chloride displacement rate constants have been measured. The suggested mechanism for the substitution accounts for the experimental facts by assuming an intermediate carbanion.