Tetramers, determination

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

The immunoglobulin structure in Figure 6.45 represents the confluence of all the details of protein structure that have been thus far discussed. As for all proteins, the primary structure determines other aspects of structure. There are numerous elements of secondary structure, including /3-sheets and tight turns. The tertiary structure consists of 12 distinct domains, and the protein adopts a heterotetrameric quaternary structure. To make matters more interesting, both intrasubunit and intersubunit disulfide linkages act to stabilize the discrete domains and to stabilize the tetramer itself. 

In all cases where the structure has been determined, the Au attains planar four-fold coordination and polymerizes as appropriate to achieve tliis. The halides for instance are dimeric but with the cyanide, which forms linear rather than bent bridges, tetramers are produced ... 

Lifnbach et al. [92JA9657 97BBPG889] made an exhaustive study of proton transfer in solid pyrazoles. For instance, the activation barriers, isotope and tunneling effects of the dimer 67, the trimer 68, and the tetramer 69 were determined. Catemers, like pyrazole itself, do not show dynamic behavior. 

The reactions depicted in Eq. (1) are suitable for calorimetric investigations since they proceed rapidly and quantitatively as monitored by NMR. spectroscopy. The. solution calorimetric protocol has been described elsewhere." The enthalpy values were determined by anaerobic solution calorimetry in THF at 30 C by reacting 4 equivalents of each carbene with one equivalent of tetramer. The results of this study are presented in Table I. 

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxane have been determined. In this case (unlike the cyclotetrasiloxane in Fig. 26), the cis isomer does contain an intramolecular hydrogen bond, and intermolecular hydrogen bonds link the molecules into cyclic pairs 57. The trans isomer cannot form intramolecular hydrogen bonds, but forms cyclic tetramers which are further hydrogen-bonded to form infinite sheets 58 (280). 

To remove volatile impurities, the naterial was heated in vacuo at 135 °C. This treatment resulted in further condensation. Thus, the distillation residue was composed of a mixture of doubly-bridged dimers and tetramers, x = 1 and 2 as determined from the molecular weight, boron and nitrogen analyses and the volatile condensibles produced. 

As documented in detail for organolithium species, ligand and donor play a key role in determining the degree of aggregation. Methyllithium adopts a hexameric structure in hydrocarbon solvents.13,15 In the presence of monodentate, donors such as THF or diethyl ether tetramers are observed, while the increase in donor denticity to 2 (1,1-Dimethoxyethane (DME), N,N,N, N -Tetramethylethylenediamine (TMEDA)) affords monomeric structures. Further documenting the differences between solution and solid states, [CH3Li]4 adopts a tetrameric structure in the latter.15,15a-15c... 

A DFT study was performed in order to determine the structures of phenylcalcium hydride and its magnesium analog in donor solvents.391 A dimeric phenylcalcium hydride was found to be the most stable species in solution, but monomers or tetramers cannot be excluded at very low or very high concentrations. Hydride bridging is favored over phenyl bridging, and a coordination number of six is predicted to be dominant in solution. 

A great deal of effort has been directed to determining the structures of lithium alkyls. It has been determined that in hydrocarbon solutions the dominant species is a hexamer when the alkyl groups are small. In the solid phase, the structure is body-centered cubic with the (LiCH3)4 units at each lattice site. Each unit is a tetramer in which the four lithium atoms reside at the comers of a tetrahedron and the methyl groups are located above the centers of the triangular faces. The carbon atoms of... 

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