Rotation barrier large

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

As with conformational energy differences, SYBYL and MMFF molecular mechanics show marked differences in performance for rotation/inversion barriers. MMFF provides a good account of singlebond rotation barriers. Except for hydrogen peroxide and hydrogen disulfide, all barriers are well within 1 kcal/mol of their respective experimental values. Inversion barriers are more problematic. While the inversion barrier in ammonia is close to the experimental value, barriers in trimethylamine and in aziridine are much too large, and inversion barriers in phosphine and (presumably) trimethylphosphine are smaller than their respective experimental quantities. Overall,... 

This difference in the properties arises from the overall stiffness of the polymeric chains. Flory explained these differences in the properties by conformational energy considerations (45). In H-T PIB, the energy difference in changing the chain conformation is not pronounced. This causes the chain to become flexible. In contrast, H-H PIB exhibits a large rotation barrier around its adjacent dimethyl substituted carbon atoms. Thus, H-H PIB can not change its conformations, which results in a stiff and regular chain that is crystalline (19). 

If A and B are large bulky groups they will bump together, attainment of the eclipsed conformation will be almost impossible, and rotation will be severely restricted. Even if A and B are hydrogen atoms (ethane), there will be a rotational barrier in the eclipsed conformation which amounts to 12 kj (3 kcal) per mole because of the crowding of the hydrogen atoms as they pass each other.5 9 This can be appreciated readily by examination of space-filling molecular models. 

Table VI-5 shows that the dissociation process, N02 - NO + O( D) takes place energetically below 2439 A. Usclman and Lee (985) have measured the production of O( >) as a function of incident wavelength near 2439 A. They have found that the contribution of rotational energy to dissociation is insignificant near the second threshold in contrast to the case near the first threshold at 3980 A where the contribution of rotational energy is substantial. They attribute the lack of rotational contribution to the presence of large rotational barriers at high J values in the excited state (987). The quantum yield of O( D) production increases to a plateau of about 0.5 0.1 towards shorter wavelengths, indicating that at least two processes, (VI-59) and (V1-60). occur concurrently below the second threshold wavelength.

A careful analysis of the NMR line shape provides the BH2 + /BH + ratio. Because of a large difference in activation energies between the rotation barriers in the mono-and diprotonated aldehyde, the observed rates are very sensitive to the concentration of BH+. Thus ionization ratios of the order of 10-4 could be measured and approximately 4 log units of Hq could be covered with the same indicator.45... 

---