Historical Aspects of the Chemical Bond. Chemical Relationality <Emphasis Type="Italic">versus</Emphasis> Physical Objectivity

Summary. Chemists have always defined the properties of materials on the basis of the changes observed when they reacted with other substances. Whereas this approach led chemists to relational concepts such as affinity, acid, and oxidant, physicists made measurements of objects they considered unchanged, determining their mass, charge, dipole moment, etc. In the middle of the 18th century, the Jesuit Josip Ruder Bocovi started thinking about the way in which atoms might be present in crystals, introducing a new concept according to which atoms in condensed phases represented punctual centres of attracting and repelling forces. Josef Loschmidt became aware of these ideas through the philosopher J. F. Herbart, and it is to Loschmidt that we owe the first representation of bonds as atomic spheres penetrating one another. The term quantum chemistry was first used by the physicist Arthur Erich Haas, who was born in Brno. However, Hans Hellmann (1903–1938) was the true founder of quantum chemistry. Hellmann, who was shot in Moscow in 1938, was the first person to realize the quantum-physical effect that leads to the chemical bond. In the 1960s K. Ruedenberg and others took theory a stage further when – thanks to the concept of the localized electron pair – they realized that the different approaches to the phenomenon of the chemical bond taken by chemists and physicists were largely comparable. This made it possible to bring the differing standpoints largely into line with one another.     

The Dissociation Constants of 1,1'-Bis (pyridinium-4-aldoxime)trimethylene dibromide

The dissociation constants of the two oxime groups of 1,1-bis(pyridinium-4-aldoxime)trimethylene dibromide (TMB-4) were determined using spectrophotometric data. Two numerical methods were applied to treat the overlapping equilibria. The results obtained by both agreed with each other and their mean values at 25°C corrected for the ionic strength of 0.05moldm^–3 are pK_a1=7.49±0.11 and pK_a2=8.96±0.09. These values were discussed in terms of the pK_as of 1,1-bis(pyridinium-4-aldoxime)oxydimethylene dichloride (Toxogonin), a similar dioxime, which were derived by extrapolation of literature data.     

TPD of ammonia from H-ZSM-5 zeolites deactivated during coupled methanol hydrocarbon cracking (CMHC)

The deactivation of H-ZSM-5 zeolite samples during coupled methanol/hydrocarbon cracking (CMHC) with n-butane as feed has been investigated in two different reaction modes (with intermediate regeneration and without regeneration). TPDA was used to characterize strong Brönsted acidity of fresh, regenerated and steamed H-ZSM-5 zeolite. The prevention of dealumination by blocking the acid centers with carbonaceous products could be proved, but the conversion rate of n-butane and the olefin selectivity declined drastically with increasing coking rate. H-ZSM-5 - / - - ( -). , - H-ZSM-5. - , , - - .     

Phase equilibrium diagram for the tin(II) iodide-copper(I) iodide system

The phase diagram of the SnI₂-CuI system was determined by the method of thermal analysis. No intermediates were found. It is suggested that limiting solid solutions of SnI₂ in-CuI,-CuI and-CuI, respectively, are formed. The eutectic point parameters (32.7 mol% CuI, 270.4 ±0.1 °C) and those of two invariant points (64.0 mol% CuI, 385.3 ±0.5 °C and 61.5 mol% CuI, 366.6±0.2 °C), corresponding to the equilibria+liquid and+liquid , respectively, were determined.Mittels Thermoanalyse wurde das Phasendiagramm SnI₂-CuI bestimmt. Es konnten keine Intermediäre gefunden werden. Es scheint, daß Grenzmischkristalle von Shl2 und-CuI,-CuI,-CuI gebildet werden. Es wurden ein Eutektikum (32,7 mol% CuI, 270,4 ± 0,1 °C) und zwei den Gleichgewichten+Flüssigkeit und+Flüssigkeit entsprechende invariante Punkte (64,0 mol% CuI, 385,3±0,5 °C und 61,5 mol% CuI, 366,6±0,2 °C) gefunden. SnI₂-CuI, . , , -CuI,-CuI Cul. (32,7 % CuI, 270,4±0,1°) (64,0 % Cul, 385,3 ±0,5° 61,5 % CuI, 366,6±0,2°), + + .

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The present work was carried out within the framework of contract CPBP 01.15.     

Abnormally high fractal dimension of silica surfaces measured by adsorption of aliphatic alcohols

Factors leading to abnormally high fractal dimension D of silica surfaces measured by adsorption of aliphatic alcohols have been analyzed. , D .     

Kinetics and deuterium isotope effects of proton transfer reactions of di (4-nitrophenyl) methane with alkoxide bases in appropriate alcohols

The second-order rate constants, activation parameters and primary deuterium isotope effects are reported for the proton-transfer reactions from di (4-nitrophenyl)-methane to sodium ethoxide, iso-propoxide and t-butoxide in appropriate alcohols. The mechanism for the reaction is discussed. , -(4-) , - . .     

Studies of the interaction of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) with imidazole in acetonitrile

The reaction of DDQ with imidazole in acetonitrile is a two-step process. The formation of an anion radical is followed by slow substitution of Cl atoms to form diaminobenzoquinone. Kinetics of the second step is reported. . - Cl CN- . .     

Thermodynamic studies of clathrate hydrates

Thermodynamic studies of clathrate hydrates, mainly of structures I and II, are considered in this review which is based on 147 references. There are two main subjects. The first is the host lattice energy and the guest-host interaction energy, both of these quantities being related to the enthalpy of dissociation and composition of the hydrates. The second subject concerns orientational ordering phenomena occurring in both host and guest, as reflected in the low temperature heat capacity. The classical theoretical treatment of clathrate formation has been reconsidered on the basis of recent experimental results. Particular emphasis has been given to orientational ordering since this topic is undoubtedly central to clarifying the nature of clathrate hydrates.Ausgehend von 147 Literaturangaben wurden in diesem Review thermodynamische Untersuchungen von Klathrathydraten hauptsächlich der Struktur I und II betrachtet. Es gibt zwei Hauptaugenmerke. Als erstes die Wirtsgitterenergie und die Gast-Wirt-Wechselwirkungsenergie, beide bezogen auf die Dissoziationsenthalpie und die Bildungsenthalpie der Hydrate. Das zweite Hauptaugenmerk betrifft Orientierungs-Konditionierungserscheinungen sowohl in Wirt als auch Gast, wie in den Wärmekapazitäten bei niedrigen Temperaturen widergespiegelt wird. Auf der Basis jüngster experimenteller Ergebnisse wurde die klassische theoretische Betrachtung über die Bildung von Klathraten überprüft. Der Orientierung-Konditionierung wurde besonderer Nachdruck verliehen, da dies zweifellos eine entscheidende Rolle bei der Klärung der Natur der Klathrathydrate spielt. 147 I II. . «» « — », . «» « », . . , .

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Contribution No. 155 from the Chemical Thermodynamics Laboratory.     

Some observations of HZSM-5 zeolite dehydroxylation

The dehydroxylation of 3610 cm^–1 OH groups in HZSM-5 zeolite and in two Ni and Mo modified catalysts containing the same zeolite has been studied at 673 K. The presence of water vapor is a necessary condition of dehydroxylation, the extent of which depends on the number and duration of breaks consisting in cooling to room temperature and reheating to 673 K with evacuation. OH HZSM-5, Ni Mo- , 3610 cm^–1, 673 . , 673 .     

Chemical algebra. II: Discriminating pairing products

The algebra of stereogenic pairing equlibria is presented in a very general context. Starting from the notions of fuzzy subgroup and conjugacy link, chemical pairing constants between molecular speciesu andv having a skeletal symmetry groupG are formulated as pairing products on aG-Hilbert space. Discriminating pairing productsK are defined by the conditions: K 1 and K = 1 the representative vectors of the paired species areG-equivalent. WhenG has only two elements, the pairing product is always discriminating. For several skeletal symmetries, if the vectors are enantiomorphic (v = u, ² =e, G), thenK is greater than 1 and reaches 1 only ifu is achiral: chirality indexes and general permutational indexes are then defined fromK(u u). The general model is illustrated by some examples.     

Salt effects in the reaction between IrCl62? and MnEDTA2?

The oxidation of MnEDTA^2–. (EDTA=ethylenediaminetetra-acetate) by hexachloroiridate(IV) has been studied in concentrated electrolyte solutions at 298 K. The estimation of the activity coefficients of the species from the Stokes-Robinson hydration theory indicates that the observed salt effects have their origin in a greater destabilization of the initial state with respect to the transition state when increasing salt concentration. MnEDTA^2– (EDTA= ) (IV) . -, , .     

Exciton Chirality. (A) Origins of and (B) Applications from Strongly Fluorescent Dipyrrinone Chromophores

(A) The origin of exciton interaction and examples of its application to organic stereochemistry are presented. (B) N,N-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration. N,N-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield (_F) 0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester of an analog with benzoic acid replacing propionic, N,N-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (_F=0.68, _em=493nm, _ex=422nm in CHCl₃) and UV-Vis absorption (21000 at 424nm) in organic solvents. Its diester with (1S,2S)-cyclohexanediol is fluorescent and exhibits exciton circular dichroism (=+15dm³·mol^–1·cm^–1, =432nm; =–4dm³·mol^–1cm^–1, =380nm) that correlates with the Exciton Chirality Rule.     

On the mechanism of water dissociation on the surface of Al2O3. Quantum-chemical calculations

Quantum-chemical calculations of clusters modelling adsorption complexes on the -Al₂O₃ surface have been used to evaluate two points of view on the mechanism of surface dissociation of water molecules on this oxide. The calculations show that this process involves interaction of water molecules with basic surface oxygen atoms rather than with Lewis acid sites.- , -Al₂O₃, H₂O . , .     

The feedback controlled chemical reactor, I. General theoretical description

Some general dependences of the injection volumes of reaction components on time have been derived for the feedback controlled chemical reactor. .