Interactions not involving hydrogen

The case is different with the approach of O or N atoms to Cl or S atoms (Fig. 12.11(b)) the latter have diffuse electron clouds, and coulombic energies, after an initial destabilizing trend, become less destabilizing or stabilizing due to penetration effects, while the higher polarizability introduces significant polarization and 

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Michelson (42) was the first to observe that oligo- and even dinucleotides showed hypochromicity without double helix formation. This first indication that base-base interactions not involving hydrogen bonds may play an important role in the stabilization of nucleic acids has now been amply confirmed. The phenomenon is called stacking and is due to the interaction of the tt electrons of the neighboring bases. These stacking interactions strongly influence optical parameters (decrease in absorption, increase in ORD and CD) (43). 

Figure 12.11 and Table 12.5 show the results for interactions not involving hydrogen atoms. A steady coulombic repulsion, increasing in the order F < O < N, arises on approach of the most electronegative atomic species (Fig. 12.11(a)). Overlap repulsion also increases in the same order. Attractive polarization and dispersion terms are very small due to the scarce atomic polarizabilities, and cannot overcome the repulsive terms. On the whole, the interaction energies are connected in a consistent and... 

In the case of spin-spin coupling involving hydrogen nuclei, the dipolar and orbital interactions are usually neglected. However, they are very important in some examples of coupling not involving hydrogen nuclei. 

Second sphere complexes are sometimes obtained from organometallics and crown ethers. In these cases the crown ethers do not interact directly with the metal center of the organometallic compound, but rather with the ligands coordinated to the metal. The interaction usually involves hydrogen-bonding. 

Then the reduction of stored NOx with hydrogen is addressed. The bulk of data points out that the reduction of stored nitrates occurs under near isothermal conditions through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt—Ba interaction was suggested, which plays a role in the NOx storage phase as well. 

Bi-0 2.54(l)-2.68(2) A], In contrast, the nine-coordinate capped square antiprism geometry for bismuth in [Bi(N03)3(H20)3] (18-crown-6) does not involve the expected multidentate ether coordination to bismuth, but rather a hydrogen-bonded interaction of the crown ether with the hydrated bismuth center chelated by bidentate nitrate groups [Bi-0 2.38(2)—2.56(2) A] 32, implying that the hexado-... 

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