7t-jt interaction

The peak shift due to aggregation is observed not only in LBK films containing azobenzene chromophores, but also for other chromophores with extended Ji-systems, such as viologen polymers. For monolayers of the poly(p-phenylene sulfonate) 9/ dioctadecyldimethylammonium bromide (DODA) complex, the peak shift due to aggregation results in a piezo-chromic effect—that is, upon compression of the monolayer, a significant shift of the poly(p-phenylene sulfate) A band is observed (see Figure 6.9). This photochromic effect has been shown to be based on the improved 7t-Jt interaction upon compression of the monolayer. ... 

The structural parameters are included in Table 15. Since these complexes are dimers with planes related to each other by the inversion center, they exhibit ji-jt interactions. Generally, the complexes which show strong ji-ji interaction may have very low values of mean plane separation and lateral shift [10]. Thus, the 7t-jt interaction in cation radical complexes is rather weak as judged by higher values of lateral shift and mean plane separation. A detailed study indicates that the unusual saddle conformations occuring in the 71-cation radical is not electronic but rather steric in origin. The unusual saddle conformation results... 

Monte Carlo calculations of interactions of 1 //-bcnzotriazole with water reveal significant electronic polarization of the heterocycle. The dipole moment is increased by 2.89 D for the ground state and 2.75 D for the excited state to the total values of 6.89 and 6.40 D, respectively. Direct measurements of dipole moments in water are not possible, but these numerical results are supported by experimental solvatochromic blue shift of the 7t —> Jt transition <2003IJQ572>. 

X-Ray diffraction studies of l-(4,6-dimethylethylsulfanyl-l-//-pyrazolo[3,4-r/ pyrimidinyl)-3-(5-methyl-6-methyl-sulfanyl-4-oxo-l,5-dihydropyrazolo[3,4-, pyrimidin-l-ylpropane 69, the Leonard trimethylene linker, showed unusual U-moieties formed due to intramolecular 7t—Jt stacking interactions, CH- -N, CH- -S, and S- -S interactions <2006JST106>. 

Chiral quaternary alkanes do not have any m-electrons or a heteroatom. Thus, the present chiral discrimination may involve the CH-7T interactions between the CH group of the chiral hydrocarbon and the m-electrons of the pyrimidine-5-carbaldehyde 11. This CH-jt interaction [109] maybe used to discriminate between the Re and Si enantiofaces of aldehyde 11, because the... 

T-7T Stacking or jt-jt interactions are important noncovalent intermolecular interactions, which contribute much to self-assembly when extended structures are formed from building blocks with aromatic moieties. In relation to the rich variety of jt-jt stacking in the crystal structure of silver complexes of phenylenediethynide, related silver complexes of phenylethynide and its homologues with different substituents (-CH3, -QC or the -CH3 group in different positions (0m-, p-) are investigated. 

Figure 7 The effect of turning on the jt interaction between a 7T-acceptor ligand and the metal. The unoccupied, and relatively unstable, it orbitals of the ligand are shown on the right. Their effect is to stabilize the filled Aj, orbitals of the complex and so increase A...

Transition metal-alkyne 7t-bonding interactions are similar to their alkene counterparts in that both can be considered as donation from the alkyne Jt-to-metal d orbitals, with concomitant back-donation from the metal d-to-alkyne tt orbitals leading to bent alkyne structures (Fig. 16) (304, 305) Commonly, the donor interactions are primarily ct type while the back-donation occurs mainly... 

To understand this stability another way, notice that as a result of the Jt interactions, there is a fairly large energy gap between the highest occupied (7t bonding) and the lowest unoccupied MO jt antibonding). 

The use of tt—jt interactions to form a supramolecular polymer is not Hmited to water, however. Many groups (including that of the present authors) have developed structures that self-assemble in other solvents partly or mainly as a consequence of 7T—7T interactions (Figure 34.16) [74-77]. 

The cooperativity of cation-it and n-n interactions has also been demonstrated taking a series of mono-valent cationic species such as, Li+, Na" ", K+, NH4+,PH4+, OH3+ and SH3+ where systematic quantum chemical studies were performed to estimate the effects of cation-n and n-n interaction on each other in cation-Tt-7T systems (Fig. 18.6b) [123]. In this study all possible orientations of onium ions to form cation-jt complexes have been explored and the most stable conformation has been taken further to evaluate its effect on k-k interaction. The results from this study indicate a notable increase of 2-5 kcal/mol in the n-n interaction energy in the presence of the cations. The cation-jt interaction energy is also enhanced in the presence of n-n interaction albeit to a smaller extent. The wide varieties of cations (which include both metal and inorganic ions) employed here underline the generality of the results. 

Peptide X-H...jt interactions are fimctional in stabilization of helix termini, strand ends, strand edges, 8-bulges, and regular turns. Side-chain X-H...7T hydrogen bonds are formed in considerable numbers in a-helices and j8-sheets [205]. The conformational influence of N - H...n and O -in organic molecules and also their role in crystal packing have been established [206,207]. The study on C- interactions has clearly established... 

In the next paragraph, the aromatic 7t-cation interaction in host-guest complex in C32 fullerenes will be analyzed. It is well known that in jt-cation interactions there are strong forces between the cation and the 7t-face of an aromatic structure. Electrostatic forces, polarization, and the charge distribution of aromatic species appear to play a dominant role in this kind of interactions [22,23]. 

On the other hand, when n lies close to Jt-HOMO, the interaction between 7t-HOMO and n is strong. Both orbitals contribute considerably to FMO. The combined orbital, Jt-HOMO - n is a component of FMO. The Jt orbital interacts with the n more strongly than with the jt-HOMO dne to the spatial proximity. The phase of is determined by the relation with n rather than jt-HOMO so as to be out-of-phase with n (viz. in phase with Jt-HOMO). As a result, is the opposite... 

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