Crystal two-component

Solid-state photoreactions of heterocycles in two-component crystals 98S1. 

Solid-state photoreactions in two-component crystals with formation of heterocycles 98S1. 

Solid-state C NMR techniques have been applied to the characterization of the different phases of several polybibenzoates [25,30], including P7MB, PDTMB and PTEB. The last two polymers offer the advantage of the stability of the mesophase at room temperature. The spectra corresponding to the pure mesophase of these samples only exhibited a broad component, while the spectra of the annealed samples were separated into two components crystal and noncrystal. The shapes of the mesophase and the noncrystal components are very similar, and only modest variations in the relaxation times were observed between these two components. The degree of crystallinity of these samples was determined... 

On the other hand, a very large number of two-component crystals are known in which the components are present in definite proportions, and whose structures are different from those of the separate materials in their crystals. These are the complexes, or compounds in the Phase Rule sense. 

We now consider various examples of two-component crystals, stressing the ways in which the chemical environment of a molecule may be varied from that in its own crystalline phase and, of course, from that in any fluid phase, and the chemical consequences of this variation. 

The Gibbs free energy of phase y is represented by a straight line connecting the standard state potentials of the two end-members in the mixture. Because we use the term mixture, it is evident that the standard state of both end-members is the same and is that of pure component. The two components are totally immiscible in any proportion and the aggregate is a mechanical mixture of the two components crystallized in form y ... 

Keywords tetracyanobenzene, benzyl cyanide, two-component crystal, photo-reaction... 

Chiral two-component crystal (1 2, 800 mg) in preparative scale prepared by seeding was pulverized, placed between two Pyrex plates, and irradiated with a... 

In a solid solution, the two components crystallize together in a homogeneous one-phase system. The particle size of the drug in the solid solution is reduced to its molecular size. Thus, a solid solution can achieve a faster dissolution rate than the corresponding eutectic mixture. 

When Se and Sio are dissolved together in CS2 and the solution is cooled, then, under special concentration conditions, a stoichiometric well ordered solid solution of the two components crystallizes as orange-yellow opaque crystals of m.p. 92 [34]. The structure of Se-Sio consists of alternating lay-... 

When the structure of one of the two-component crystals was obtained and analyzed (15), it was concluded that the ground state orientation of the components would predict essentially no face discrimination and that approximately equimolar amounts of the enantiomeric dimers 4a and 4b should have been obtained. As a result, it was concluded that the thiophene compound must deform in the excited state to an arrangement that favors the formation of one of the products. This hypothesis was supported by theoretical calculations based on the assumption that the reaction proceeds through an intermediate exciplex (16) spectroscopic evidence for such a species was found in the case of diene 1 (17). 

The two-component crystal may be divided into three categories 1.) mixed crystal (solid solution), 2.) crystalline molecular compounds, 3.) a simple mechanical mixture of component crystals. I propose the term mixed molecular crystal to represent both mixed crystal and crystalline molecular compounds and have used it as such in the title of this chapter. 

A solid mixture is any mixture of crystals of different substances, regardless of its solid-state structure. It may be a simple mechanical mixture of component crystals, a mixed crystal, a molecular complex, or an ionic salt. In this chapter, a simple mechanical mixture as well as all uncharacterized two-component crystals are grouped under solid mixtures. 

Two-component crystals are usually prepared either 1.) by traditional cocrystallization or slow evaporation of solvent from a solution containing the components,... 

Rate constants have been determined for these processes, and it is concluded that in the case of the benziloxy radicals real-time monitoring provides a direct observation of the transition state for C-C bond scission. A highly selective photodecarboxylation of aralkyl carboxylic acids such as 3-indolepro-pionic acid and 1-naphthylacetic acid has been achieved by irradiating the two-component crystals formed with acridine or phenanthridine as electron acceptor at -70 Electron transfer followed by proton transfer produces a carboxylate radical and either a hydroacridine or hydrophenanthridine radical which decays by decarboxylation. 

Solid-state photoreactions of heterocycles in two-component crystals 98S1. Stereoselectivity of chiral homoenolate equivalents in reactions of heterocycles ... 

The category of two-component crystals was added to the fourth blind test. 

A simple eutectic mixture consists of two compounds that are completely miscible in the liquid state (melt) but only show limited miscibility in the solid state. At a specific composition (E in Fig. 2.1), the two components crystallize simultaneously when the temperature is reduced (Fig. 2.1). If mixtures with different compositions to the eutectic composition of A and B are cooled, one component will start to crystallize before the other, which initially leads to a mixture of pure solid compound and liquid. Therefore, a true eutectic only exists for a defined composition of A and B. The microstructure of a eutectic mixture is different from the microstructure of either components, and this property may be used to differentiate the eutectic mixture from other forms of crystalline mixtures. A theoretical method to determine the eutectic composition of a binary mixture and the temperature at which it crystallizes has been suggested by Karunakaran (1981). 

Medium-density polyethylene is a blend of high-density and low-density polyethylenes. The two components crystallize in separate crystal lamellae with different melting points. 

Two absolute asymmetric [7, 8] syntheses have been described, both by these laboratories one involves reaction of a gas with a chiral solid, while the second is a (2+2)-photo-addition reaction in a chiral two-component crystal. Such syntheses have a number of strictly defined requirements [9]. Here we describe the planning and execution of such a synthesis of a polymer which is chiral non-racemic by virtue of its backbone configuration. Such a polymerization process must involve production of new chiral centers during propagation. 

In a recent paper. Bond listed a number of interesting controversial examples of co-crystals [81]. For example, dioxane, C4H8O2, forms isostructural 1 1 two-component crystals with I2, Br2 or CI2, which at room temperature and 1 bar pressure are solid, liquid, and gas, respectively [82-84]. Clearly a distinction based on the aggregation state of one component has little significance. 

The crystal structures of 13 co-crystals formed by -alkylcarboxylic acids and pyrazine in 2 1 stoichiometry have been reported [85,86]. The acids include formic acid, which is liquid at room temperature, up to the tridecandioic acid which is solid. Pyrazine is also a solid. Clearly, the early members of this series cannot be called solvates only because the co-former is liquid, while from decanoic to tridecandioic the acid/pyrazine systems are co-crystals. Another example is the two-component crystals formed by picric acid forms and benzene, naphthalene and anthracene (amongst others), which contain stacks of alternating molecular components that are clearly comparable in all three structures [87-89]. All of these crystals are stable at room temperature. 

---