7T-stacking

A detailed NMR study and determination of the X-ray structure of the ligand has suggested the occurrence of 7t-stacking of the 2,6-diisopropoxybenzene ring and coordinated aldehyde [5 c]. Because of this stacking, the si face of the CAB-co-ordinated a,/ -unsaturated aldehyde is sterically shielded (Fig. 1.1). 

Two supramolecular features predominate (1) the formation in one direction of the continuous zigzag chains of 7t-stacked C60 units with a C5—C6 ring centroid—ring centroid separation of 3.39 A, and (2) the formation in another direction of continuous -C60-pz-C60-pz- zigzag sandwiches (Fig. 34). 

An interesting question then arises as to why the dynamics of proton transfer for the benzophenone-i V, /V-dimethylaniline contact radical IP falls within the nonadiabatic regime while that for the napthol photoacids-carboxylic base pairs in water falls in the adiabatic regime given that both systems are intermolecular. For the benzophenone-A, A-dimethylaniline contact radical IP, the presumed structure of the complex is that of a 7t-stacked system that constrains the distance between the two heavy atoms involved in the proton transfer, C and O, to a distance of 3.3A (Scheme 2.10) [20]. Conversely, for the napthol photoacids-carboxylic base pairs no such constraints are imposed so that there can be close approach of the two heavy atoms. The distance associated with the crossover between nonadiabatic and adiabatic proton transfer has yet to be clearly defined and will be system specific. However, from model calculations, distances in excess of 2.5 A appear to lead to the realm of nonadiabatic proton transfer. Thus, a factor determining whether a bimolecular proton-transfer process falls within the adiabatic or nonadiabatic regimes lies in the rate expression Eq. (6) where 4>(R), the distribution function for molecular species with distance, and k(R), the rate constant as a function of distance, determine the mode of transfer. 

On the other hand, noncovalent adsorption leaves CNTs electronic and optical properties almost intact, as the type of modification is through van der Waals, 7T-7T stacking, H-bonding and electrostatic interactions, that do not determine any pyramidalization process. Such benefit is traded off by a decreased stability of CNT-functionalized systems, as a consequence of the weaker interactions. 

The large aromatic and hydrophobic character of CNTs make them ideal surfaces for noncovalent interaction vfith molecules via Van der Waals, 7t-stacking or hydro-phobic forces [39, 44]. 

Crystallographic studies show that pyrazole-fused derivative 136 forms staircase-like 7t-stacked rods <2003TL9161>. AMI calculations have been reported for the pentacyclic system 137 <1996H(43)1991>. 

The technique of noncovalent derivatization employs the noncovalent intermolecular forces (hydrogen bonding, 7t-stacking, UpophUic-lipophilic interactions, and electrostatic interactions) to trap the molecular species in organized matrices. 

Fig. 14. 7t-Stacking and charge-transfer dominated recognition of flat aromatic—hete...

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