Cage molecules, structure

The hydrophobic effect. Water molecules around a non-polar solute form a cage-like structure, which ices the entropy. When two non-polar groups associate, water molecules are liberated, increasing the entropy. 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

We showed the possible existence of various forms of helically coiled and toroidal structures based on energetic and thermodynamic stability considerations. Though the formation process of these structures is not the subject of this work, the variety of patterns in the outer and inner surface of the structures indicates that there exist many different forms of stable cage carbon structures[10-19]. The molecules in a onedimensional chain, or a two-dimensional plane, or a three-dimensional supermolecule are possible extended structures of tori with rich applications. 

The size of these macrocycles and their cage-like structure renders them suitable to serve as host molecules or artificial receptors for small molecules, in addition to having possible biological activity. 

The availability of the trace metals is easily determined without any of the above risks, and the results used to assess both deficiencies and toxici-ties. The metals need to be removed from the sites where they are bound to the soil particles by use of an even stronger binding agent than the soil. This is achieved with two possible complexing reagents EDTA and DTPA. They are a class of chemicals known as complexones, which form complex molecules with metals in a cage-like structure called a chelate. 

In this chapter, the chemistry and the structures of Zintl ions and cage molecules are summarized and put in the context of intermetalloid clusters. The emphasis is put on the observation that irrespective of the starting material (Zintl ions or small low-valent organometallic compounds) the same types of intermetalloid clusters are formed. 

All other homoatomic Pn cluster anions are cage molecules with localized two-center-two-electron bonds. The cyclic [PnJ anion has been characterized by NMR spectroscopy for Pn = P [210], and structures are known for Pn = P, As, Sb, Bi (Table 2). A planar [Ps] anion has only been detected in solution by P NMR spectroscopy [210]. Envelop-shaped five- and six-membered rings are found for [Pns] and [Pe]", whereas [Asg]" deviates only slightly from planarity and has a chair conformation. The aromaticity of the planar anions [P4] and [Pe]" is distinct from that of regular 671-aromatic hydrocarbons and has been described... 

The heavier congeners of phosphorus resemble it in a tendency to form cages. Both arsenic and antimony form unstable tetramenc molecules which readily revert to polymeric structures. Cage molecules as well as polymeric forms arc also known for As4Oft und Sb40,. In addition there are a number of sulfides, some of which arc known to exist as cages (big. I6.3H). 

The systematic treatment of such structures is beyond the scope of the present review. The majority of examples selected belongs to bonding type 12, constructed of two or more three-membered rings and other structural elements. Propellanes are of the 11 type. A monograph on cage molecules has appeared recently308. 

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