Conformations lowest energy

It is quite common to do the conformation search with a very fast method and to then optimize a collection of the lowest-energy conformers with a more accurate method. In some cases, single geometry calculations with more accurate methods are also performed. Solvent effects may also be important as discussed in Chapter 24. 

It is generally recognized that the flexibility of a bulk polymer is related to the flexibility of the chains. Chain flexibility is primarily due to torsional motion (changing conformers). Two aspects of chain flexibility are typically examined. One is the barrier involved in determining the lowest-energy conformer from other conformers. The second is the range of conformational motion around the lowest-energy conformation that can be accessed with little or no barrier. There is not yet a clear consensus as to which of these aspects of conformational flexibility is most closely related to bulk flexibility. Researchers are advised to first examine some representative compounds for which the bulk flexibility is known. 

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

A rigorous conformational search on the active compounds was performed to determine their lowest energy conformation(s). 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Do any or all of the reactive conformations correspond to the lowest-energy conformations What, if anything, does this tell you about the rate of interconversion of conformers relative to the rate of elimination ... 

Examine all of the low-energy (within. 004 au or 3 kcal/ mol of the lowest-energy conformer) conformers of cis-2-methylcyclohexyl tosylate. Identify every conformer that can undergo anti elimination of OTs and H+, and predict the alkene that will be produced. What alkenes will be obtained from the cis tosylate ... 

During studies on ditryptophan derivatives, an interesting acid-induced cy-cHzation has been discovered. The 10-membered ring 37 was thus subjected to acidic conditions to produce the indolocarbazole derivative 38 (Scheme 6). Interestingly, calculations performed on the precursor 37 indicated that the lowest energy conformation resembled that of the diastereomer of 38, which was never observed. An additional experiment furnished the parent system 1 on treatment of 38 with a catalytic amount of acid. A TFA-induced formation of an indolo[2,3-<3]carbazole was also observed from a related acyclic 2,2 -connected tryptophan dimer (99JOC8537). 

Conformational studies of c/5-6,11/)-H- and /rnns-6,ll/)-H-6-methyl-2,3,4,6,7,1 l/)-hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones (136 and 137) by means of the MM2 method implemented in the HyperChem 4.5 suggested, that in the lowest-energy conformations heterocyclic moiety adopted trans-fu ed ring annotation in both cases, with a pseudo-equatorial and -axial methyl group, respectively (97LA1165). 

A. In vacuo lowest energy conformation showing the peptide carbonyls to be slightly librated into the channel by about 10°, Reproduced with permission from Ref.53). 

Chair cyclohexane (Section 4.5) A three-dimensional conformation of cyclohexane that resembles the rough shape of a chair. The chair form of cyclohexane is the lowest-energy conformation of the molecule. 

Scheme 2. Lowest energy conformations for dihedral b of PCH3. Structures (1), (2) and (3) in turn correspond to the three energy minima in Fig. 2...

Scheme 3. Lowest energy conformations for the mesogen 5CB (4-n-pentyl-4 -cyanobiphenyl)...

---