Molecular aggregates definition

For the label of specificity, criterion (2), we must appeal to a more limited area of study, to spectroscopic and diffraction data. The most definitive data are, no doubt, those which indicate atom positions in the molecular aggregate. Thus, x-ray diffraction, neutron diffraction, and certain nuclear resonance studies of solids can provide more or less direct evidence that there are H atoms which occupy positions of close approach (hence bonding distance) to two other atoms. Electron diffraction spectra can yield the same information for gaseous species. More easily obtained, however, are IR and Raman spectra, which reveal specific involvement of H atoms by peculiarities in their vibrationeil degrees of freedom in the molecular aggregate. Finally, high resolution proton magnetic resonance studies provide a sensitive index of the electronic environment of the H atoms. 

This general definition provides useful flexibility. The principle evidence for hydrogen-bond formation is association or chelation, i.e. the occurrence of molecular aggregates. We shall see that not only do these molecular aggregates form, but they do so in recognizable, and often predictable patterns. 

As described above, the /3-carotene molecules are randomly oriented in the SC film, but the above discussion suggests that the short-range order similar to that in of the single crystal exists in the SC film. Therefore, we can now conclude definitely that the SC film is an amorphous film, and this conclusion supports the assumption by Babaev and Al perovich (1973). As for the LB film, the optical absorption spectrum measured on normal irradiation with light polarized parallel to the x-axis ofthe substrate showed exactly the same spectral pattern with that parallel to they-axis, although the intensifies of these spectra were apparently different. This shows that there is only one transition moment in the mixed LB film. Saito et al. (1991 a,b) reported the optical absorption spectra of various molecular aggregates in the... 

A surface should be two dimensional and capable of fixing a bound chelate in a definite orientation. Such a strict stereochemical restriction precludes stacking of a chelate into molecular aggregates. 

Mavelli and Luisi (21) suggest that such autopoietic systems are a minimalist definition of a living system. They further suggest that self-reproduction is one of the possible kinetic aspects of autopoiesis and therefore autopoietic molecular aggregates can be considered as the most elementary chemical structures capable of simulating certain essential properties of the simplest living systems. In considerations of molecular evolution... 

While these definitions and descriptors may be useM in systematizing molecular conformations, no safe correlations have ever been found between any of them and solid state properties of organic compounds. The seemingly inevitable conclusion is that as far as the smdy of molecular aggregation in condensed phases is concerned, these descriptors are too qualitative to be of any real use. Condensed phase properties depend on subtle details of the arrangement of the electron density in the molecule and between molecules, so that the true physics of the interaction must be considered. 

Some caution has to be used with regard to the definition of an infective unit , which need not necessarily be a single molecule. In 029 transfection, Hirokawa (unpublished) has furnished evidence that a large portion of the infective units represent multi-molecular aggregates. 

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

Although, the true density of solid phase p=m/Vp (e.g., g/cm3) is defined by an atomic-molecular structure (/ ), it has become fundamental to the definition of many texture parameters. In the case of porous solids, the volume of solid phase Vp is equal to the volume of all nonporous components (particles, fibers, etc.) of a PS. That is, Vp excludes all pores that may be present in the particles and the interparticular space. The PS shown in Figure 9.17a is formed from nonporous particles that form porous aggregates, which, in turn, form a macroscopic granule of a catalyst. In this case, the volume Vp is equal to the total volume of all nonporous primary particles, and the free volume between and inside the aggregates (secondary particles) is not included. 

The most rigorous evidence that proteins had defined structures was probably the molecular weight determinations of Adair and Svedberg. From 1900, however the crystallization of increasing numbers of proteins, while not a very reliable indication of purity, suggested to Schulz that proteins were not colloidal aggregates but large molecules with definite structures. 

While CMC is assumed to be an observable and definite value in the case of surfactant monomers, there are frequent reports in the literature of the formation of aggregates or micelle-like associations in solutions of organic solutes so dilute as to preclude apparently the formation of micelles [208, 267-269, 272, 275,278]. Work with different types of commercial surfactants has indicated that molecularly non-homogeneous surfactants do not display the sharp inflection in surface tension associated with CMC in molecularly homogeneous monomers, but rather the onset of aggregation is broad and indistinct [253,267,268]. The lack of well-defined CMCs for non-homogeneous surfactants is speculated to result from the successive micellization of the heterogeneous monomers at different stoichiometric concentrations of the surfactant, which results in a breadth of the monomeric-micelle transition zone. 

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