Structural Effects of n-o Interactions

We will illustrate the importance of n-o orbital interactions by discussing the geometrical and conformational preferences of the following types of molecules  

The flrst R2 AARi molecide which we shall examine is diimide, N2H2- Tlus molecule can exist in cis and tram geometries and the stabilizing interactions which obtain in the two geometries are specified below  

Ilie dominant interaction is the n—o interaction and since it is maximized in an anti anangement, it is concluded that HN=NH will be expected to exist in the sterically crowded cis geometry. Of course, if the n—o interactions are weak they will not be able to reverse the tmns over cts preference dictated by steric effects. 

The relative magnitude of charges, overlap populations and total energies of the geometric isomers of N2H2 as predicted on the basis of each of the three important effects thought to determine molecular structure (see Section 6.0) are shown in Table 42. Comparison of the ab initio data with the various predictions reveals that 

Property Sigma conjugative effect Nonbonded interaction effect Electrostatic or steric effect Ab initio 

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In all these structures the Sn-C-C=C angle is close to optimum for g-tc interaction between the C(allyl)-Sn g bond and the n system. In the two crystallographically independent molecules of cyclopenten-2-yl-triphenylstannane 197 and cyclohepten-2-yltriphenylstannane 199, the stannyl substituent takes up a pseudoaxial orientation. Although cyclohex-2-enyltriphenylstannane 198 exists in the solid state with the stannyl substituent in a pseudoequatorial orientation (Sn-C-C=C 120°), it exists in solution predominantly in a pseudoaxial conformation. Analysis of the structural parameters in 198 and 199 provides some tentative structural evidence for the presence of the o-Jt interaction in these compounds the Sn-C(allyl) bond distances, which are 2.189(5) and 2.182(5) A, respectively, are slightly longer than the normal Sn-C(aliphatic) distance, which is typically of the order of 2.13 A, consistent with the expected structural effects of electron donation from the Gc Sn orbital into the n orbital. 

On Rh surfaces, only desorption of N2 and NO is observed in the TDS. Figure 6 shows TDS for NO on two stepped Rh surfaces, Rh(533) [structure 4(111) X (100)] and Rh(410) [structure 4(100) X (100)]. The adsorption of NO and the effect of preadsorbed O and N on the adsorption of NO have been studied on these surfaces and on many other Rh surfaces by Janssen et al. [28). The N atoms are markedly more strongly bound on (100) terraces than on (111) terraces. The presence of steps does not affect the thermal stability of Nads on Rh. At higher NO exposures, repulsive N-N and N-0 interactions lower the thermal stability of Nads- Following saturation NO exposure, N2 desorbs in a single state from (100) terraces at 750 K. From (111) terraces several desorption states of nitrogen appear at temperaturs between 450 and 700 K. An important observation is that recombination of Nads and Oads to give NO is more favorable than the 2 Nads N2 reaction when Rh with precovered Oads is exposed to NO. 

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