Larger Oligomers

As in the case of linear, open oligomers, the stretching frequencies of the HF bonds undergo progressive red shift as the number of monomers increases. These shifts are illustrated in Fig. 5.13 where they may be seen to surpass 400 cm in the tetramer. The reader should understand that in a cyclic structure (i.e., n 2), each molecule acts as both donor and acceptor, and the symmetry is such that vibrations caimot be distinguished as belonging to one particular molecule. It is also worth pointing out that a third v(FH) stretch is present in the tetramer, which at 3571 cm , amounts to a red shift of 611 cm from the monomer. 

The behavior of these three frequencies as the ring is enlarged is illustrated in Fig. 5.14. V, undergoes the characteristic sharp drop as n is increased, like the other HF stretches in Fig. 5.13. The enhancement in this sensitivity at the correlated level is evident by a com- 

The cooperativity in aggregates of water is particularly important for attempts to understand the behavior of the liquid and aqueous solvation. It is in part responsible for the ability of water to maintain H-bonds up to very high temperatures, probably even above 800 K in supercritical water . This importance has motivated numerous attempts to incorporate nonadditivity into water-water potentials - and, more recently, related liquids such as al-cohols .  

The question of optimal orientation in oligomers of water differs from the (HX) case since each water molecule contains two lone pairs and two protons. Based upon the general principles of cooperativity, it is reasonable to presume that if each water molecule has two neighbors, it will prefer to act as donor to one and acceptor to the other, as compared to serving as double donor or double acceptor. A full complement of four neighbors would permit the central water to serve as donor in two H-bonds and acceptor in two others. 

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The self-assembly of cyclic dimers is entropically favored over the assembly of higher cyclic oligomers, but the assembly of larger oligomers is possible if the geometry of the monomer is incompatible with dimerization. 

Aggregation of proteins is a microscopic process of protein molecule association. The aggregates may be dimers or larger oligomers that remain in solution, yet may alfect the observed biological activity. Precipitation refers to the formation of visible proteinaceous particles, which may reduce potency in addition to altering the appearance of a formulation and its performance in various infusion devices. 

Although it would be logical to assume that larger oligomers could be made in improved yield by starting with suitable smaller oligomers, this strategy has seldom been used preparatively. One of the few and successful examples is the preparation of the tetranuclear aqua chromium(III) species either by condensation of two dinuclear species or by condensation of Cr3+ with the trinuclear species (31). 

Unfortunately, this reaction is a gross oversimplification of the series of reaction steps that occur during the hydrolysis reaction. Hydrolysis has been shown to proceed via the formation of an alkylaluminum-water complex, which subsequently eliminates methane to form a dimethylaluminum hydroxide complex. This rapidly associates to give dimers or larger oligomers in solution. In the case of f-butylalumoxane, some of the intermediate species have been isolated and characterized structurally (73-76). 

A-type bound anthocyanin-flavanols were detected in wine as dimers and as larger oligomers (anthocyanin-(epi)cat , with n > 1) (Remy et al. 2000 Salas et al. 2005). The presence of such oligomers (n = 1 through 7) was confirmed by mass spectrometry (Hayasaka and Kennedy 2003). 

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