Molecular stoichiometry

Platinum(II) isocyanide complexes of the general formulas [PtL4], [PtL3X], and PtL2X2 and substituted derivatives thereof (L = RNC, X = halogen, H, R, etc.), have been studied recently and will be the main subjects of discussion. It may be observed that much work on complexes of the first and third molecular stoichiometries had been reported previously. 

The most effective synthetic control of the thiolation of bismuth has been achieved using reactions of bismuth chloride with aminothiolate salts (potassium) prepared in situ. Although the reactions are not quantitative, the molecular stoichiometry can be reliably selected and a systematic series of mono-, bis-, and tris-thiolates have been isolated and comprehensively characterized. (189). 

Conversions between equilibrium constants and fractionation factors are more complicated, as it is often necessary to accoimt for molecular stoichiometry and symmetry. For a generic balanced isotopic exchange reaction,... 

Another term used to describe rate processes is molecu-larity, which can be defined as an integer indicating the molecular stoichiometry of an elementary reaction, which is a one-step reaction. Collision theory treats mo-lecularity in terms of the number of molecules (or atoms, if one or more of the reacting entities are single atoms) involved in a simple collisional process that ultimately leads to product formation. Transition-state theory considers molecularity as the number of molecules (or entities) that are used to form the activated complex. For reactions in solution, solvent molecules are counted in the molecularity, only if they enter into the overall process and not when they merely exert an environmental or solvent effect. 

UNIMOLECULAR BIMOLECULAR TRANSITION-STATE THEORY ELEMENTARY REACTION MOLECULAR MECHANICS CALCULATIONS MOLECULAR ORBITALS MOLECULAR REARRANGEMENT MOLECULAR SIMILARITY Molecular stoichiometry of an elementary reaction,... 

Rationalization of known compounds provides a level of usefulness that justifies the rule. But the rule also permits observed molecular stoichiometries of newly synthesized compounds to be translated into acluster shape. For example, [Al Bu ]2-has eve = 50 or sep =13 consistent with n = 12 and a deltahedral structure. The compound has been synthesized and an X-ray diffraction study reveals an icosahe-dral shape. The ability to suggest reasonable structures based on knowledge of a molecular formula generated by a technique like mass spectrometry accelerated the development of cluster chemistry simply because rapid spectroscopic methods can be more productively applied. Although efficient X-ray crystallographic structure determination reduces its importance for compounds that can be isolated in pure crystalline forms, transient intermediates detected in a reaction mixture can now be given reasonable structures. 

The effects of molecular structure upon impact/shock sensitivity have been analyzed and reviewed in some detail on a number of occasions [8,10,12,17]. Over the years, there have been frequent efforts to relate the experimental impact or shock sensitivities of groups of compounds of a given type (e.g. trinitroaromatics) to some molecular quantity or quantities. Molecular stoichiometry has proven to be remarkably effective in this context [10,15,16,18-20], most notably Kamlet s oxidant balance formula [15,16,18], which is essentially a measure of the oxygen that is present relative to what is needed to convert all hydrogens to H2O and carbons to CO. In general, the larger is the oxidant balance, the greater is the impact sensitivity. 

Just like a direct reaction mechanism discussed in Section II, a pathway is not merely a set of reactions, because many distinct pathways can be constructed to include the same reactions but achieve different transformations. A pathway must include a coefficient (denoted by in Section II) for each reaction, indicating the proportions in which the stoichiometries are combined. In this section, we will call these coefficients the reaction stoichiometry of the pathway. The overall transformation of net reactants to net products will be called the molecular stoichiometry of the pathway, and was denoted by in Section II. 

Usually the metal-metal distance is certain to be identified and measured within an accuracy of 0.02-0.03 even in the presence of interfering contributions, assuming the latter are of reasonable quality and quantity. The metal-metal distance imparts such high intensity to a peak that it cannot be submerged by the contributions of distances of other possible types for a given molecular stoichiometry and with a reasonably sensible model of the molecular geometry. For instance, no more than half of the area under the peak at r = 2.29 A in Fig. 3b could be accounted for solely by the contributions of all possible Re-Cl bonds. This directly confirms the need for the metal-metal bond contribution to the RDF because no other explanation of its most intense peak is possible. 

From Eq. (18), or from the matrix in Table 10.1, it is possible to immediately distinguish 2x4 limiting conditions or eight possible majority disorder types. However, it should be noted that any intrinsic disorder type (n=p in the present case) cannot occupy the near molecularity-stoichiometry region (0 0), because obviously [A n] S>, or it would otherwise not be extrinsic due to the doped acceptors. 

XPS data (Xable 2 and 3) show good agreement between molecular stoichiometry and elemental distribution in surface adlayers. Deviations between the experimental and theoretical data are attributed to contamination. 

Molecular absorption, particularly in the UV/Vis range, has been used for a variety of different characterization studies, including determining the stoichiometry of metal-ligand complexes and determining equilibrium constants. Both of these examples are examined in this section. 

MPD-1 fibers may be obtained by the polymeriza tion of isophthaloyl chloride and y -phenylenediamine in dimethyl acetamide with 5% lithium chloride. The reactants must be very carefully dried since the presence of water would upset the stoichiometry and lead to low molecular weight products. Temperatures in the range of 0 to —40° C are desirable to avoid such side reactions as transamidation by the amide solvent and acylation of y -phenylenediamine by the amide solvent. Both reactions would lead to an imbalance in the stoichiometry and result in forming low molecular weight polymer. Fibers are dry spun direcdy from solution. 

Polyetherification is similar to a polycondensation process formation of high molecular weight polymer requires precise adjustment of composition to approximately 1 1 ratio of bisphenol to dihalosulfone. Trace amounts of water gready reduce the molecular weight attainable owing to side reactions that unbalance the stoichiometry (76). The reactivity of the halosulfone is in the order expected for two-step nucleophilic aromatic displacement reactions ... 

The decrease in rate was proportional to the concentration of dioxane in the reaction mixture. An equivalent concentration of p-xylene (whose dielectric constant is similar to that of dioxane) produced a smaller decrease, consistent with simple dilution of the reactants. It was, therefore, hypothesized that dioxane forms an H-bonded molecular complex with phenol, the complexed form of the phenol being unreactive. The data could be accounted for with a 2 1 stoichiometry (phe-nokdioxane). This argument was supported by experiments with tetrahydrofuran, which also decreased the rate, but which required a 1 1 stoichiometry to describe the rate data. 

Molecularities greater than two are rarely found (and greater than three, never). When the overall stoichiometry of a reaction is greater than two (for example, as in A + B + C or 2A + B, the reaction almost always proceeds... 

ATP synthase actually consists of two principal complexes. The spheres observed in electron micrographs make up the Fj unit, which catalyzes ATP synthesis. These Fj spheres are attached to an integral membrane protein aggregate called the Fq unit. Fj consists of five polypeptide chains named a, j3, y, 8, and e, with a subunit stoichiometry ajjSaySe (Table 21.3). Fq consists of three hydrophobic subunits denoted by a, b, and c, with an apparent stoichiometry of ajbgCg.ig- Fq forms the transmembrane pore or channel through which protons move to drive ATP synthesis. The a, j3, y, 8, and e subunits of Fj contain 510, 482, 272, 146, and 50 amino acids, respectively, with a total molecular mass... 

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