Donor-acceptor complexes stability

Factors Affecting the Stability of Donor-Acceptor Complexes ... 

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

In the 1,3-thiazole system, the presence of sulphur is a factor stabilizing the bromonium ion, which may be obtained from a donor-acceptor complex. 

As expected, separation of the acyl groups leads to increasing stability of the diacyl dications. Malonyl fluoride (175) reacts with excess SbFs to give the donor-acceptor complex (176), which is in equilibrium with the dicationic species (177, eq 54).67... 

Very little is known about the nature of the weak interactions of CAs in solutions where a vast majority of their chemical reactions has been studied. Particularly, the study of donor-acceptor complexes of CAs by modern physical-chemical methods is still of great interest. Besides, complexation of CAs with donors or acceptors of electron density is a useful tool for modifying the stability, reactivity and spectral properties of CAs. Systematic investigations of the redox properties of CAs are needed in order to elucidate the role of electron transfer in the transformations of CAs. 

Boranes are strong reducing agents and the neutral molecules, inflame spontaneously in air, although the anions [BnHn]2- have remarkable kinetic stability. Diborane itself reacts with Lewis bases to give donor-acceptor complexes with BH3, which is a soft Lewis acid and forms adducts with soft bases such as CO (1). More complex products often result from unsymmetrical cleavage of B2H6, for example,... 

The stretching frequency of the Ge—O bond in Ge(OCgH2(CH2NMc2)3-2,4,6)2 recorded in benzene solution, has been reported to lie at 1040 cm . However, this germylene is additionally stabilized by intramolecular coordination of the Ge center by the N atoms of two dimethylaminomethyl groups attached to different benzene rings in the orf/zo-positions and in fact can be considered as an intramolecular donor-acceptor complex. ... 

The formation of donor-acceptor complexes between bipyridinium salts (electron acceptors) and xanthene dyes (electron donors) (e.g., eosin. Rose Bengal) has been studied extensively. Crystal structures of these complexes have been identified, and the structural features of the donor-acceptor complexes in solutions have been characterized using NMR spectroscopy. The xanthene dye/bipyridinium donor-acceptor complexes are stabilized by... 

T vo different molecules M and Q can also form complexes with a definite stoichiometry (usually 1 1). If complexation is present already in the ground state and leads to CT absorption (Section 2.6), which is absent in the individual components, the complex is referred to as a charge-transfer or donor-acceptor complex. If, however, the complex shows appreciable stability only in the excited state, it is called an exciplex (excited complex)-. 

In order to get some information about the fate of silylenes S-la-d in the presence of a second molecule of methyl halide 6a-d, we checked whether silylenes S-la-d can experience stabilization by donor/acceptor interaction with 6a-d. Indeed, the two partners form weak donor/acceptor complexes 7a-d. That means that if S-la-d is created in a matrix at very low temperature in the presence of an excess of a methyl halide 6a-d, complexes like 7a-d should be present. Such a solvation might be the prerequisite for the direct synthesis of silanes 8a-d. These compounds represent the global minima on the CiHaSiX potential energy surfaces and are expected to be generated fl-om the two educt molecules S-la-d and methyl halide 6a-d in rather exothermic (AH, = 65-95 kcal moP ) processes. In the chlorine series (Scheme 1) the value is 73.9 kcal moP. ... 

These points prompt a few words of caution regarding the results discussed in this section and some comments on theoretical results in general. In particular, besides considerations of the size and effect of basis sets, one needs to examine the energy dependence on electron correlation. For donor-acceptor complexes, such as Lewis acid-carbonyls, electron correlation seems to be crucial. It is also instructive to consider the dipole moments of various structures in theoretical studies. Assumption of a gas phase environment may overestimate destabilization due to a large dipole, as dipoles can often be stabilized quite effectively in solution. At times the interpretation of theoretical results are the source of trouble and confusion. Thus two different groups propound apparently conflicting and opposite views on the effect of Lewis acids on the carbonyl dipole. 

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