Stable dimer

In the commercial Gorham process (2), the extremely reactive PX is conveniendy generated by the thermal cleavage of its stable dimer, Vo-di- -xyljIene (DPX), a [2.2]paracyclophane [1633-22-3] (3). In many instances, substituents attached to the paracyclophane framework are carried through the process unchanged, ultimately becorning substituents of the polymer in the coating. 

Type 125 0x0 forms are characteristic for 5-hydroxy-l,2,4-triazoles [76AHC(S1), pp. 379, 388], These forms are additionally stabilized by an electron-withdrawing substituent, R = NO2 (98MRC343). Both hydroxy and 0x0 tautomers are capable of forming stable dimers owing to the in-termolecular hydrogen bonds (126 and 127 [76AHC(S1), pp. 377,379). 

Garzon-Rodriguez W, Sepulveda-Becerra M, Milton S, Glabe CG. Soluble amyloid Abeta-(l-40) exists as a stable dimer at low concentrations. J Biol Chem 1997 272 21037-21044. 

Of the derivatives in Table 2.3, the dimer arising from the 9-methyl-10-methoxyanthracene has been found to be the most stable. Dimers produced from the others are quite thermally labile. 

The symbols A, (A - A)s, and Aa refer to acenaphthylene monomer, its singlet excimer, and its stable dimer(s), respectively. Experimental support for the singlet excimer mechanism [(10.27b)-(10.27d)] has been provided by Chu and Kearns,<47) who have shown that the cis (but not the trans) dimer in EPA (300-195°K) gives a broad fluorescence emission due to a singlet excimer. This excited state was also demonstrated to lead to dimer dissociation. 

At temperature below the decarbonylation level cyclopropenones are preferentially transformed to stable dimers, which do not eliminate CO at higher temperatures. Thus, thermolysis of diphenyl cyclopropenone at temperatures above 160 °C gives mainly diphenyl acetylene, whilst heating in the molten state to 145— -... 

Cramer has isolated this 77-crotyl complex in its stable dimeric form (1) by interacting butadiene with an ethyl-Rh111 chloride (4) ... 

Figure 2. Extraction of anharmonicity constants ojexe from the comparison of fundamental OH stretching spectra (center) with overtone spectra (top) and OD spectra (bottom) for the case of jet cooled trifluoroethanol (M) and its most stable dimer conformation, which features a hydrogen bond donor stretching band (Dd) and an acceptor stretching band (Da). The deuteration analysis yields slightly different constants than the overtone approach and underestimates the hydrogen bond effect on donor stretching modes [89].

Preliminary restricted Hartree-Fock calculations have been performed on the dimers 201-204 of H2Ge = C = PH.173 The most stable dimers are the bicyclic compounds 201, about 15 kcal/mol below 202, which are in turn favored by about 18 kcal/mol with respect to 203, and 43 kcal/mol with respect to 204 (Scheme 44). Very small differences are found between head-to-head or head-to-tail dimers for each series, and for the cis or... 

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

The geometries for the dimeric isomers are also optimized at the B3LYP/6-31+G level. The results were compared with Hartree-Fock and PM3 results. The stable dimers 4a, 4b and 4c are found to have Cj symmetry (Figure 5). 

The explanation offered for this unusual spectroscopic behavior is the existence of stable, dimeric aggregates. Due to the symmetry of l-/ -corannulenes, face-to-face association results in two topologically different dimers (dl and meso). Moreover, the lack of any signal coalescence in either or NMR up to room temperature suggests that these dimers are unusually stable, and do not dissociate on the NMR time scale. ... 

Many cycloaddition reactions have been carried out with ketenes and thioketones. The products are thiolactones (52). Hexafluorothioacetone and diphenylketene, however, do not undergo cycloaddition even after prolonged heating at 100°C. Good results can be obtained when the more stable dimer of this fluorinated thioketone (53) is used. Anionic monomer 54 could be released by the action of potassium fluoride in an aprotic solvent. Two-step cycloaddition to diphenylketene yields ketone 55. 

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 in other areas of the chemistry of nitroso compounds, the tacit assumption has been that the rearrangement of aliphatic nitroso compounds bearing hydrogens on the carbon atom which also carries the nitroso group rearranges to the oxime virtually instantaneously. Yet, recently, the existence of all types of aliphatic nitroso compounds has been well established. Without going into details here, nitrosomethane, nitrosocyclohexane, and nitrosodibenzoyl-methane have been prepared and appear to exist as reasonably stable dimers. 

Photodecomposition accompanying directly excited delayed fluorescence will clearly be less obvious. For example, at the same rate of light absorption in a 5 X 10-SM solution of anthracene, and with the same quantum efficiency of photodecomposition, a period 1000 times as long would be required to produce the same percentage decomposition. It was in fact found that after prolonged irradiation, a deaerated solution of 5 X 10-BM anthracene was almost completely decomposed and a crystalline precipitate was formed which had the appearance of dianthracene. It seems not unlikely therefore that one or other of the excited dimers formed by triplet-triplet quenching may be partly converted into a stable dimer. If this is so, mixed triplet quenching should result in the formation of stable mixed dimers. Experiments are in hand to test these hypotheses. 

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