Significant Energy Difference

The stereochemical course of reactions at three-, four-, and five-member rings can be reliably predicted by assuming the relative congestion of the two faces. As the ring size increases above six so does the conformational mobility and hence the uncertainty of the stereochemical outcome. Even with seven-member rings, predictions are generally difficult. 

Intermediate in complexity are six-member ring systems that prefer either the chair or, much less commonly, the boat and twist boat conformations. As pointed out by Barton, knowledge of the conformation of a six-member ring system, and hence its axial/equatorial substitution pattern, is crucial to an understanding of the stereochemical basis for many reactions in ring systems.  

A significant energy difference for a conformer is considered to be 7.S kcal/mol. This corresponds to roughly 95% preponderance of one conformer at room temperature. Thus conformational homogeneity would be predicted for 2-methylcyclohexa-none (AE j =1.8 kcal/mol) but not for cw-2-methyl-5-phenylcyclohexanone (AE = 0.2 kcal/mol) (see page 44). 

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Sodium has 1 valence electron, and 10 bound electrons. The first two excited states are the 3 Pi/2 and the 3 P3/2 states. Transitions to these levels give rise to the Di and D2 transitions respectively. There are two h)q)erfine levels in the 3 ground state, and four h)q)erfine levels in the 3 Pa/2 excited state (Fig. 3). There is no significant energy difference between the h)q)erfine levels in the 3 Pa/2 state. Thus, the six permitted fines appear in two groups, producing a double peaked spectral distribution, with the peaks separated by 1.772 GHz. 

Sterically (Mechanics), there is no significant energy difference between the competing transition states 22 and 23. We therefore assume that the difference is electronic, and that conformation 22 makes electron density more readily available from the target C-H bond than does conformation 23. This interplay between steric and electronic effects will be important throughout this discussion of rhodium-mediated intramolecular C-H insertion. 

All -eliminations from the benzyl derivative in Figure 4.5 exhibit -stereoselectivity. This is true regardless of whether the elimination is syn- or anti-selective or neither. The reason for the preferred formation of the E product is product development control. This comes about because there is a significant energy difference between the isomeric elimination prod-... 

The configuration at the oxygen atom of a coordinated water molecule may be pyramidal or planar.23 There appears to be no significant energy difference between these two. 

Relatively few theoretical works have attempted to determine the exact distortion where the APES of the isolated molecule has minima. An early Hartree-Fock calculation by Koga and Morokuma on C q found no significant energy difference... 

The various approximate energies obtained for the 2pi/2 and 2p3/2 sublevels of atoms from Cl to Ba are given in Table 3. There are also given the most significant energy differences. 

To describe the effects of steric restrictions in another polymer, polyisobutylene, consider the Newman projections of the staggered conformations of two adjacent carbons in its repeat unit, as shown in Fig. 2.6. Here the chain substituent on the rear carbon shown is either between a methyl group and polymer chain or between two methyl groups on the front carbon. There is no significant energy difference between the conformers. Since no conformation is favored, polyisobutylene will tend to spiral into a helix (gauche conformers) as well to form into a zigzag (If),... 

A band gap occurs when there is a significant energy difference between two bands. 

A band gap occurs when there is a significant energy difference between two bands. The magnitude of a band gap is typically given in electron volts (eV) ... 

As can be seen from Figure 3.7, a much wider network of CH. ..n and CH...O interactions present in the TS16 compared to that in the TS17 resulted in significant energy difference for these two transition states. 

The basic requirement for an enantioselective synthesis is a significant energy difference between the transition states leading to the possible products. The greater the energy difference, of course, the greater the preference for one of the products. The extent of enantioselectivity is expressed in terms of the percent enantiomeric excess. 

The initial interaction energy from DFT work is 0.1 a.u., i.e., 2.7 eV, but differences as low as 0.1-0.2 eV may be significant, corresponding to weak (hydrogen bonding or polarisation) interactions. This is the value retained as threshold for significant energy differences to be resolved by QMC. 

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