Composite molecules

Fig. 56. Partial ORTEP drawings for [TpB l]Zn(CN)09Io 1 with disordered site refined as (CN)0.906(4)Io.094(4> Center superposition of CN and I groups at the disordered site. Left and right disordered site separated into composite molecules. Reprinted with permission from Ref. (213). Copyright 1992 American Chemical Society.

The result is that, depending upon the relative magnitude of the two interactions, the ethylenic it MO can be either raised or lowered in energy. On the other hand, the ethylenic it MO will interact principally with it and the LUMO of the composite molecule will necessarily have an energy which is lower than the ethylenic it MO and will be mostly localized on the substituent carbonyl group. These considerations can be understood by reference to Fig. 1. 

Figure 25. Diagrammatic representation for a system with two chromophores (A and B) held together by covalent bonding or weak intermolecular forces. Local excitations are shown (left and right) for the chromophores in their locally excited (A or B ) monomer states. In the composite molecule or system (center), excitation is delocalized between the two chromophores and the excited state (exciton) is split by resonance interaction of the local excitations. Exciton coupling may take place between identical chromophores (A=B) or non-identical chromophores (A B) but is less effective when the excitation energies are very different, i.e. when the relevant UV-visible bands do not overlap.

Although exciton coupling leads to shifted and broadened, if not split, UV-visible spectra of the composite molecule, when the chromophores are held in a chiral orientation, exciton coupling can be detected even more clearly in the CD spectrum as two oppositely-signed CE s typically corresponding to the relevant UV-visible absorption band(s). The signed order of the CD transitions correlates with the relative orientation of the relevant electric dipole transition moments, one from each chromophore, and hence the absolute configuration of the composite molecule (Exciton Chirality Rule.)[2] For the bis-anthra-... 

Orally administered fluorescein is readily absorbed in the small intestine. By contrast fluorescein coupled to dilaurate cannot be absorbed unless the composite molecule is cleaved intra-duodenally by the pancreatic cholesterol esterase to form lauric acid and (absorbable) fluorescein. After its absorption, fluorescein is partly glucuronidated in the liver and then excreted in urine, predominantly as fluorescein diglucuronide. Thus, in pancreatic-insufficient... 

Figure 26-4. Principle of the Pancreolauryl and the NBT-PABA (V-benzoyl-i.-tyrosyl-P-aminobenzoic acid, bentiromide) test. The composite molecule consisting of a substrate and a marker molecule cannot be absorbed but can be cleaved intraduodenally by pancreatic enzymes (cholesterol esterase and chymotrypsin, respectively), leading to release of an absorbable marker substance. Following absorption and hepatic conjugation, the marker is excreted in urine. In pancreatic-insufficient patients, decreased secretion of pancreatic enzymes results in incomplete cleavage of the composite molecule. This results in decreased absorption and subsequent excretion of the marker, which can be measured photometrically.

Most of the assemblies produced in the above studies tend to be near-perfect squares, although the hybrid systems show rhomboidal distortions. These respective structures are relatively rigid since they are cations (mainly with 8+ charges), they also show considerable potential as anion receptors. X-Ray studies indicate that in some cases,these composite molecules stack in the solid state to produce channels and hence such species are of potential interest as artificial zeolites . 

The composite molecule approach to the molecular orbitals of MeAsHj (Figure 3) provides further aid to the interpretation of its PE spectrum, namely ... 

FIGURE 3. Qualitative MO assignments of MeAsHj based on the composite molecule (CH4 + AsHj--------- CH3ASH2) approach... 

Most observed kinetics of multibasin protein dynamics is a consequence of averaging over an ensemble of many activated barrier crossings with multiple time scales. The direct observation [53-55] of dynamical behavior of a single molecule, so far buried in an ensemble average, should provide us with a new magnifying glass, which enables us to explicitly see the dynamical behavior inherent to the composite molecules in complex systems. 

M. Gay-Lussac has shown in an interesting Memoir that gases always unite in a very simple proportion by volume, and that when the result of the union is a gas, its volume also is very simply related to those of its components. But the quantitative proportions of substances in compounds seem only to depend on the relative number of molecules which combine, and on the number of composite molecules which result. It must then be admitted that very simple relations also exist between the volumes of gaseous substances and the numbers of simple or compound molecules which form them. The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases is always the same for equal volumes, or always proportional to the volumes.3... 

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