Barrier interconversion

At - 120 , the speed of bond rotation is considerably slowed down. At this temperature there is not enough thermal energy to overcome the barriers, interconversion is therefore very slow so slow that separate signals are observed in the fluorine nmr, one for each environment. 

The configuration of pairs of isomeric 4-aryIidene-5-pyrazoIones, (Z)- and (E)-(117), was determined by H NMR data (72G491). When R is H, the E configuration is preferred when it is a methyl or a phenyl group, the Z configuration predominates. The presence of an exocyclic sulfur atom as in (118) lowers the interconversion barrier and the products... 

Molecules that are chiral as a result of barriers to conformational interconversion can be racemized if the enantiomeric conformers are interconverted. The rate of racemization will depend upon the conformational barrier. For example, -cyclooctene is chiral. E-Cycloalkenes can be racemized by a conformational process involving reorienting of the... 

Next, examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation ... 

The multiple minima nature of the bending energy and the low barriers for interconversion resemble the torsional energy for organic molecules. An expansion of bend in tenns of cosine or sine functions of the angle is therefore more natural than a... 

The interconversion must be relatively easy, i.e., the energy barrier separating the tautomers must not be too high. No fixed limit exists, but with dG of less than 25 kcal they would certainly be considered as tautomers, whereas with an energy barrier of about 40 kcal they would not be considered as tautomers. However, frequently such energy barriers can be lowered by using a suitable catalyst. 

However, no tautomeric interconversions between 1,2-dihydro- and 1,4-dihydropyridine analogs have been observed, probably because a high energy barrier is inherent to this transformation. 

The substitution of a heteroatom for an a-sulfoxy methylene group substantially increases the preference for an axial orientation of the sulfoxide oxygen320, despite the smaller space requirement of the sulfur with its lone pairs, compared to that of a methylene group321, at least in the case of 1,3-dithiolane oxides. The substituting heteroatom, therefore, should decrease the conformation stability (i.e. lower the barrier to chair-chair interconversion). 

Atropisomerism occurs in other systems as well. A sulfoxide (16), for example, forms atropisomers with an interconversion barrier with its atropisomer of 18-19kcal mol . The atropisomers of hindered naphthyl... 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

Wehle, D. Fitjer, L. Tetrahedron Lett., 1986, 27, 5843, have succeeded in producing two conformers that are indefinitely stable in solution at room temperature. However, the other five positions of the cyclohexane ring in this case are all spiro substituted with cyclobutane rings, greatly increasing the barrier to chair-chair interconversion. 

As expected, the hexagonal chair form of Se with 03a symmetry, occurring in the solid hexasulfur, is the most stable form of hexasulfur, due to its minimal strain. The boat conformer of C2V symmetry is 50 kj mol less stable than the chair form [54]. The Dsa—>C2v interconversion requires to overcome a barrier of ca. 125 kJ mol A structure of C2 symmetry, which is a local minimum at the HF/3-21G level [49, 50], is not a stationary point at higher levels of theory [54, 55]. 

Infrared and Raman spectra of 47f-pyran and 1,4-dioxin have been analysed <96JST255> and interconversion barriers for dihydropyrans and some dioxins have been determined <96BKS7>. 

The experimental barrier for interconversion of the cis and trans isomers of [PhNPPh2] is 8 kcal mol and the barriers of other derivatives have been calculated to be in the range 7-32 kcal mol where the larger barriers are associated with the presence of substituents on the nitrogen that are moderately electron-withdrawing [50,87]. 

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