Stacking complexes

Table I. Comparison of Binding Constants for Stacked Complexes at...

More recent studies have indicated a probable mechanism that relies on the interaction of thalidomide with DNA. The phthalimide double-ring structure (Fig. 7.74) is relatively flat and can intercalate with DNA forming a stacked complex. Furthermore, observations were made that thalidomide... 

Metallic properties are also observed in columnar stacked complexes in which the intrachain separation is too long for conduction to be associated with the formation of a band from overlap of the metal dz2/pz orbitals. For these complexes the conduction process involves carriers present in the delocalized MOs extending over die whole complex. The conduction process in these compounds may be described by a hopping mechanism in which the on-site Coulomb repulsion energy has been reduced by the delocalization of the charge over the whole molecule. 

Because of the efficiency, the density functional theory also became an object of interest for the purpose of evaluation of intermolecular interaction energies for nucleic acid base complexes. Most standard functionals provide qualitatively a good picture of interactions in H-bonded nucleic acid base pairs. However, they completely fail for stacked complexes. This is due to the inability of current functionals to describe correctly the dispersion energy. Several routes were proposed to alleviate this deficiency with different rates of success [9-12],... 

The Step 2 product (2.98 mg) and 1,3-dinitrobenzene (1.80 mg) were dissolved in 10 ml of THF and then diluted 100 times. This stock solution was used for absorption spectrum measurements in a 10 mm quartz cell at ambient temperature. The intensity was 0.140 at 242 nm, which changed to 0.108 after the addition of 1,3-dinitrobenzene. By changing the 1,3-dinitrobenzene concentration from 2.5 x 10 Mand5.0x 10 M at 242 nm, the absorption intensity changed from to 0.077 and 0.059, respectively. This hypochromic effect demonstrated that the fluorene ring of the polymer and 1,3-dinitrobenzene formed a stacked complex. 

The relative energies of hydrogen bonded versus stacked arrangements of the A-T pair are sensitive to solvent. The stacked monohydrated complex 40 is the fifth lowest energy complex, but for the dihydrated complexes, the stacked form 41 is the most stable. The population of the stacked complexes increases in going from the monohydrated to dihydrated forms. Compared to the G-C pairs, the A-T population is much more favored toward the stacked and T-shaped forms. The preference for the stacked arrangement over the hydrogen-bonded ones in aqueous solution is consistent with experiments. 

Work directed at preparing and characterizing stacked complexes has been developed since the late 1980s, and the syntheses are carefully controlled. The principles of the approach are outhned below, but may be summarized as an example of the method. The archetypal sandwich compound is ferrocene. 

The discussion of the electrochemical properties of metallacarbaboranes involves the compounds that are best subdivided into clusters and stacked complexes. 

Fig. 6.2 Views of the two polymorphic structures of 6-II 6-III. In both cases the view is on the plane of the tetracyanoquinodimethane molecule 6-II. (a) The red, transparent, mixed-stack complex, a semiconductor (b) the black opaque, segregated stack complex, a conductor. (From Bernstein 99 b, with permission.)...

Detection of n-n interactions has largely relied on NMR-based techniques, such as chemical shifts variations,and Nuclear Overhauser Effect Spectroscopy (NOESY) or Rotating-Erame NOE Spectroscopy (ROESY)7 Diffusion-ordered NMR spectroscopy (DOSY) has also been used to detect n-n stacked complexes. ... 

Complexation is the association between two or more molecules to form a nonbonded entity with a well-defined stoichiometry. The two types of complexation that are most useful for increasing the solubility of drugs in aqueous media are stacking and inclusion. Stacking complexes are formed by the overlap of the planar regions of aromatic molecules, while inclusion complexes are formed by the insertion of the nonpolar region of one molecule into the cavity of another molecule (or group of molecules). 

Fig. 16 Some compounds that are known to form stacking complexes.

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