Homomolecular

Almost all the crystalline materials discussed earlier involve only one molecular species. The ramifications for chemical reactions are thereby limited to intramolecular and homomolecular intermolecular reactions. Clearly the scope of solid-state chemistry would be vastly increased if it were possible to incorporate any desired foreign molecule into the crystal of a given substance. Unfortunately, the mutual solubilities of most pairs of molecules in the solid are severely limited (6), and few well-defined solid solutions or mixed crystals have been studied. Such one-phase systems are characterized by a variable composition and by a more or less random occupation of the crystallographic sites by the two components, and are generally based on the crystal structure of one component (or of both, if they are isomorphous). 

Most hydrogen-bond-containing crystal structures are homomolecular, i.e. both the hydrogen-bond accepting and donating functionality exist in the same molecule. Examples of such co-crystals are urea and thiourea derivatives as well as non-cyclic imides156,179,180. 

A new wave of interest and productive research was aroused by synthesis of tricyclic condensed derivative benzobis(l,3,2-dithiazole) (BBDTA) which was shown experimentally to afford at least three distinct oxidation states dication, radical cation, and diradical <86SM233, 86SM239). This has been helpful for developing an approach to ferromagnetic organic metals in which homomolecular stacks are formed from donor-acceptor complexes in which the donor is a triplet and the acceptor a radical ion derived from the donor <86SM233>. Initial experiments showed conductivities lO " -10 S for complexes of BBDTA radicals with TCNQ <86SM239>. 

It is emphasized that the terms excimer2 and exciplex3,4 are reserved here for homomolecular and heteromolecular excited double molecules formed after the act of light absorption by one component in a process of photoassociation, in the absence of spectroscopic or cryoscopic evidence for molecular association in the ground state. Recent findings indicate that excimer (or exciplex) formation may also result from triplet-triplet annihilation,5,8 cation-anion combination7 (doublet-doublet-annihilation), and electron capture by the (relatively stable) dimer (or complex) cation8 these processes are discussed in Section VII. 

As will be discussed in Sect. 5, linear free energy relations allow comparisons between the kinetics of heterogeneous (electrode) and homogeneous homomolecular charge transfer reactions. 

The nitroaniline crystal structures discussed above were homomolecular. For most of these structures only one polymorph is known. If the bulk structure of a particular molecule, say PNA, is not a useful one then few options are known for modifying its packing pattern. For example, we tried to force PNA into a new polymorph by recrystallizing it from many different solvents. We obtained an interesting array of different crystal morphologies, none of which were polymorphic. The crystal morphologies that were obtained are shown in Figure 1. 

If both A and B are polyatomic relaxing gases, there will also be two collision processes, corresponding to (1) and (2), for vibration-translation energy transfer from B in homomolecular and heteromolecular collisions. In addition there can be a vibration-vibration transfer between A and B, making five transfer processes in all... 

The mixtures of the second section in Table 6, which were investigated earlier (when erroneous conclusions were drawn)77, all show double dispersion. The details for one mixture, SF6+C2F4, are shown in Fig. 16. There is near-resonance between the lowest (344 cm-1) mode of SF6 and the first harmonic of the lowest (190 cm-1) mode of C2F4. C2F4 shows very efficient homomolecular vibration-translation transfer, and the estimated vibration-vibration transfer rate (ZAB=70) falls between this and the slower vibration-translation transfer rate of SF6 (ZAA = 1005). Double dispersion is observed, and the predicted linear variation with concentration of the two relaxation times. The remaining mixtures in this section, all of which involve B components whose homomolecular relaxation is very rapid, behave similarly. 

Comparison of the collision numbers given above for vibration-vibration transfer with those for vibration-translation, given in Section 4, shows that in many cases vibration-vibration transfer between two resonant or near-resonant modes is much more efficient than vibration-translation transfer from either. This applies equally to homomolecular and heteromolecular collisions, and carries the interesting consequence that the quickest route for vibrational excitation of upper levels from the ground level by homomolecular collisions is an initial vibration-translation excitation to the v = 1 level, followed by successive vibration-vibration transfers to higher levels. Because of the selection rule, Av = 1,... 

This review will be concerned with recent progress made towards an understanding of conduction phenomena in typical homomolecular crystals, e.g. anthracene and the phthalocyanines, with certain charge-transfer complexes, selected biological systems, certain novel one-dimensional systems and other materials which serve to illustrate a particular theoretical approach or the value of an experimental technique. Little attention will be given to experimental procedures other than when these are not in common use and have not been adequately described in the earlier reviews. 

Few reliable measurements of thermoelectric power in homomolecular crystals are available. The data on the phthalocyanines appear to be the most representative, and in the studies of Hamann (1972 and references therein) and co-workers trapping effects and variation between differently prepared samples of the same material have been taken into consideration. Since the validity of a... 

On appreciating such difficulties, recent studies (Parkinson et al., 1974) on homomolecular organic crystals combine the more conventional TSC experiments and trap population by irradiation followed by thermal detrapping... 

In contrast to the conventional organic homomolecular crystal composed of neutral molecules held together by van der Waals forces, weak donor-acceptor complexes are formed by the interaction between 7r-electron donors, such as... 

Belsky, V. K., Zorkii, P. M. (1977) Distribution of organic homomolecular crystals by chiral types and structural classes, Acta. Crystallogr. Sect. A, 33A, 1004-1006. 

Norde W, Giacomelli CE (2000) BSA structural changes during homomolecular exchange between the adsorbed and the dissolved states. J Biotechnol 79(3) 259—268... 

Nucleation processes, especially in nature, are rarely homomolecular and homogeneous usually two species are involved (binary heteromolecular nucleation), and foreign surfaces may also be present (heterogeneous... 

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