Molecular complexes stoichiometry

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. 

Interactions between tertiary aliphatic amines or N,N-dialkylanilines and substituted phenols are generally reported as models of O—H- N hydrogen bonds affording molecular complexes (equilibrium 4). A number of complexes between primary, secondary and tertiary aliphatic amines and dihydroxy benzenes (or dihydroxynaphthalenes) were isolated55 to investigate the stoichiometry of these complexes. The phenol/amine ratios observed included values of 1 1, 2 1, 3 1 and 3 2. 

Complexation with caffeine and theophylline-7-acetate depresses the rate of alkaline hydrolysis of substituted phenyl benzoates and is consistent with the formation of molecular complexes with 1 1 stoichiometry between the hosts and esters stacking of the xanthines is excluded as an explanation in the range of concentrations studied. Inhibition of hydrolysis is attributed to repulsion of the hydroxide ion from the host-ester complex by the extra hydrophobicity engendered by the xanthine host, as well as by the weaker binding of the transition state to the host compared with that in the host-ester complex. ... 

The intermolecular (interionic) distances must be regular. This "mixed valency" requires that there be only one crystallographically unique molecular site, which must share its partial valency with the nearest neighbor sites along the stack. The many "complex stoichiometry" TCNQ salts—for example, Cs2(TCNQ)32 or triethylammonium(TCNQ)2-, which exhibit "trimeric" or "tetrameric" units of several crystallographically distinct TCNQ molecules and TCNQ- anions held at van der Waals separations—do not conduct well. 

A molecular complex may be dissociated in the ground state, and yet be associated in an excited electronic state. An excited molecular complex of defined stoichiometry, which dissociates in the ground state, is known as an exciplex [110,111], the term being derived from exci (ted comp) plex by analogy to excimer (= exci (ted di) mer ). 

The term exciplex (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes which fall into this broad classification include ... 

Phosphine oxides. Few molecular complexes of trivalent transplutonium elements have been reported. Several studies examine the extraction chemistry of Am, Cm, and Bk with a combination of /3-diketones and tri-n-alkyl phosphine oxides and tiialkylphosphates. From these, compounds reported to be of the formula AnF3(R3PO) c (An = Am, Cm R = n-octyl, Bu"0) were isolated, where L = CF3COCHCOR (R = Me, CF3, Bu ). The stoichiometry of the complexes (An P=0) was not always reported. The complex Am(CF3COCHCOCF3)3[OP(OBu )3]2 is reported to be volatile at 175 °C. ... 

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

Relevant studies concern proton donor/proton acceptor molecular complexes with different stoichiometries of 2 1, 1 1 and 1 2, as depicted in Scheme 14. The 1 2 complexes of pentafluoroaniline, 4-nitrotetrafluoroaniline and 4-aminotetrafluoropyridine in 1 2 complexes with THF and HMPA were investigated. The 1 1 complexes display a different stability129,137 from that of 1 2 complexes. As expected, the AH values for the 1 1 complexes were higher137,138 than for the 1 2 complexes, when calculated for a single ArNH2-PA bond134. 

When the intermolecular forces are relatively strong and directional, crystalline molecular compounds (crystalline molecular complexes) are formed. They have fixed stoichiometries and ordered structures. These two-component molecular crystals are also called cocrystals or maybe adduct crystals. Hydrogen-bonded cocrystals, donor-acceptor crystals (charge transfer crystals), and inclusion crystals (host-guest crystals) are examples of crystalline molecular complexes. Crystalline organic salt is a special case of hydrogen-bonded cocrystal or donor-acceptor crystal, i.e., proton (or electron) transfer from the acid (or donor) to base (or acceptor) occurred. 

For example, each batch of crystals from cocrystallization can be examined under a polarizing microscope to determine whether the crystals have homogeneous morphologies or whether mixtures of crystals are present. Solution NMR results and elemental analyses can give the stoichiometry of components. Melting points of complexes are usually sharper than those of mixtures of components. A clearer distinction can be made from measurements of PXD, solid-state C NMR, IR and UV, and DSC spectra. Because the spectral patterns for a simple mechanical mixture are composites of those for each component, observation of new peaks in these spectra indicates the formation of a molecular complex. When a molecular complex is available as a single crystal of sufficient size and quality, its crystal structure and miolecular conformation can be unequivocally determined by single-crystal X-ray diffraction [4-11,13],... 

The formation of molecular complexes of defined stoichiometry in solution is usually accompanied by isosbestic points in titration spectra and by changes of... 

Chromatography is one of the most important methods for direct studies of molecular and chiral recognition by CyDs. Today it has split into several branches, e.g. gas chromatography, GC, high-performance liquid chromatography, HPLC, and capillary electrophoresis and other electromigration techniques, that enable us not only to detect the recognition but also to estimate the complex stoichiometry and formation constant and, consequently, the enthalpies and entropies of complex for-... 

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful and versatile methods for the elucidation of molecular structure and dynamics. It is also very well suited to study molecular complexes and their properties [1]. Therefore, it has been widely used for studying inclusion complexes formed by cyclodextrins (CyD) [2-4]. Some examples of the applications of NMR in conjunction with other techniques are presented in other chapters, in particular in Chapter 6. The success of NMR spectroscopy in this field is due to its ability to study complex chemical systems and to determine stoichiometry, association constants, and conformations of molecular complexes, as well as to provide information on their symmetry and dynamics. Furthermore, compared to other techniques, NMR spectroscopy provides a superior method to study complexation phenomena, because guest and host molecules are simultaneously observed at the atomic level. 

R. Sahia, V. Singh, and R. K. Verma, Role of Dielectric Constant on Stoichiometry of Some Iodine Molecular Complexes and Their Transformation into Ion Pairs, Indian J. Chem. Sec. A 20A(10), 1017 (1981). 

The first requirement, somewhat difficult with coal because of its molecular complexity, is that the overall stoichiometry of the reaction must always be established. For these purposes, coal is usually represented by carbon, which can react with oxygen in two ways, producing either carbon monoxide or carbon dioxide ... 

AFM images provide structural information on proteins and their interactions with other proteins or other molecules such asDNA. From visual inspection of the data, conformational properties of proteins can often be directly derived, as shown for example in Fig. 5a [9]. Quantitative statistical analyses allow for the extraction of further information on protein states and interactions. From the measured volume of particles in the images, their molecular mass can be calculated [10]. This calculation is based on an empirically derived linear relationship obtained with calibratirm proteins with a range of known molecular weights. From the molecular mass of the protein molecules and complexes, information can be derived on protein oligomeric states, protein-protein interactions, and complex stoichiometries. Single molecule resolution coupled with the analysis of statistical numbers of molecules or molecular assemblies supplies... 

Solubility Studies allow molecular interactions to be studied by means of solubility measurements. Increasing amounts of CD are added to a constant amount (in excess of its normal solubility) of ACT in pH 5 acetate buffer. After equilibration, the solvent may be analysed for ACT and the equilibrium constant of the reaction may be calculated from the slope of the graph of concentration of act against concentration of CD (presuming complex stoichiometry is... 

---