Isomeric molecules

The tree can be constructed in five steps and, correspondingly, the generating function of the structurally isomeric molecules can be built up from five factors (multiplication of the generating function in the case of independence cf. Sec. 17). 

Hirsutene (1) and A9(,2,-capnellcnc (2), the parent members of the hirsutane and capnellane families of triquinane natural products, respectively, are isomeric molecules that possess four contiguous stereogenic centers, one of which is quaternary. The linearly fused tricyclopentanoid frameworks of compounds 1 and 2 are obviously very similar, differing only with respect to the positions of the three methyl groups. An asset of Curran s tandem radical cyclization strategy is that it provides a unified entry into a wide variety of linear condensed cyclopentanoid natural products. As a result, it is possible to devise nearly identical retrosynthetic pathways for these structurally related molecules. 

Early experiments [1] using (+)-apopinene and deuterium showed, however, that in the isomerized molecules the deuterium content was very low and the isomerization was much faster than deuterium incorporation into the allylic position. Therefore it seemed probable that isomerization takes place through an intramolecular hydrogen shift. A sigmatropic 1,3-hydrogen shift was suggested, in which the allylic endo-H shifted top shift) [2]. 

Rauhut and Semsel [141] and Steinfatt [142] have suggested an unusual mechanism whereby the proposed intermediate 1,2-dioxetandione forms a structural (isoelectronic) isomer as outlined in route B in Figure 6. Route A represents a dark reaction. The isomerized molecule is then thought to decompose into singlet oxygen and carbon trioxide [143],... 

Constitntional isomers have different bonding patterns that is, the atoms of two strnctnrally isomeric molecules are not identical in terms of their connectivity, the... 

Becanse of the high concentration of isomeric molecules (>0.1 mol dm ), this equilibrium is established instantaneously. The IP of trans-AtcaYm is 0.02 eV lower than the IP of cw-decalin (9.24 eV versus 9.26 eV). Therefore, the electron-transfer eqnilibrinm is shifted slightly to the left side. Thns, in terms of charge-transfer kinetics, the two ions behave as a single species. It shonld be worth noting that decalin has only two isomeric forms, cis and trans. On the contrary, n-nonane exists in the multitude of conformations. The rate constant of electron exchange between parent nentral molecules of nonane and its cation-radicals is much lower, namely, 2 orders lower than the diffnsion-controlled limit (Borovkov et al. 2007). 

In isomerizations, zeolites have special merit in their ability to admit straight-chain but not branched-chain molecules into the pores. Thus, normal alkanes up to n-Ci4H30 can penetrate the pores of zeolite 5A to reach the cavities where the C—C or C—H bonds may be catalytically broken the fragments, on reemerging from the pores, can recombine as isomerized molecules. The reverse process is not possible, since the isomers, having... 

If, on the other hand, the resonance integral is very small, it may be convenient to refer to the substance as containing tautomeric or isomeric molecules, with electronic structures that are represented essentially by I or II alone. The relation of resonance to tautornerism and the distinction between tautornerism and isomerism are discussed in the last chapter of this book. 

Heats or free energies of formation can be used to compare directly the energies of isomeric carbocations. Such a comparison is similar to the more familiar comparisons of energies of isomeric olefins, such as cis- and trans-butene. It depends on the energies of formation of isomeric molecules or ions being based on the same combination of elements. Energies of isomerization can also be measured directly, and Bittener, Arnett, and Saunders have measured the enthalpy of isomerization of secondary to tertiary butyl cations in CH2C12 as solvent.39... 

In this language, the implications of the W-H rules are that allowed reactions will proceed in a concerted fashion and that forbidden reactions will not, the argument in the latter case being that some other, non-concerted mechanism is likely to provide a more stable pathway. It is now fairly clear that there are exceptions to both of these generalizations. In this connection the concept of orbital isomerism is helpful. This was introduced by Dewar, Kirschner, and Kollmar,39 who postulated that a group of isomeric molecules can be divided into sets in such a way that members of any one set can be interconverted by paths which involve no crossing between the highest... 

Constitutional isomerism. Molecules can have the same molecular formula but different arrangements of atoms. 

The application of molecular mechanics to metal complexes developed in parallel with its application to organic molecules. Indeed, the earliest report that considered the importance of nonbonded interactions in determining the relative stabilities of isomeric molecules was a 1944 study of the six isomers of the coordination compound [Co((5)-pn)2(N02)2] (pn = propane-1,2-diamine)[1]. A number of similar, though more detailed, studies in the 1950s and 1960s l-2 4] led to the full application of molecular mechanics to metal complexes. The common theme... 

Fig. 2. Geometry-based classification of isomeric molecules. Upper half the conventional classification. Lower half the isometry-based classification. SP, superimposable SC, same constitution NSP, nonsuper-imposable mirror images I, isometric. Adapted from [18] and [19].

FIG. 11-2. The structure of the isomeric molecules ethyl alcohol, C2H5OH. and dimethyl ether, (CH3)20. 

---