Isomer, conformational, and

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Stereoisomers, on the other hand, are compounds with the same molecular formula, and the same sequence of covalently bonded atoms, but with a different spatial orientation. Two major classes of stereoisomers are recognized, conformational isomers and configurational isomers. 

How to draw conformational isomers and to flip cyclohexane rings... 

Volume additivity methods generally do not take into account crystal packing efficiency or molecular conformation effects and thus will afford identical calculated densities for positional and conformational isomers and for compounds that possess different multiples of the same functional group composition. As an example, a volume additivity calculation predicts that l,3,5-trinitro-l,3,5-triazacyclohex-ane (RDX), l,3,5,7-tetranitro-l,3,-5,7-tetraazacyclooctane (a-HMX), and /3-HMX all will possess the same crystal density, 1.783 g/cm [32]. In fact, the experimentally observed densities of these three compounds differ markedly (i.e., 1.806 [33], 1.839 [34], and 1.902 [35], respectively). 

Stereoisomers are isomers having the same molecular formula and the same connectivity, but different spatial arrangements. There are three classes of stereoisomer cis-trans isomers, conformational isomers and enantiomers. 

M. Guerra, G. F. Pedulli, M. Tiecco, and G. Martelli, Conformational isomers and rotational harries in the ketyl radicals of thiophen and thienothiophens. J. Chem. Soc., Perkin Trans. II, 562 (1974). 

It is common, however, for liquid-phase systems to include many specific absorbing species. Such species could include isotopic variations, conformational isomers, and solvent-solute interactions resulting in varied-lifetime transient associations between molecules. Distributions resulting from these effects give the Voigt profile utility in studying liquid spectra. We must understand, however, that the functions introduced here are only rough approximations when applied to the spectra of liquids because of the complexities just mentioned and others beyond the scope of this work. 

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

Table 1.1 lists minimum lifetimes for observation of separate species and the appropriate spectroscopic methods. The time scale of nuclear magnetic resonance (NMR) experiments is particularly long, and many conformational isomers and some constitutional isomers (see below) interconvert rapidly within the time of observation and appear to be more symmetric than simple bonding considerations would imply. We will expand on these ideas after the next two sections. 

Compare the strain energies of the two conformational isomers and calculate the proportion of each expected in solution at room temperature. Using HyperChem measure the contacts between the methyl group and the remainder of the complex. Consider how this contributes to the difference in strain energies. 

Hindered rotation about the P-C bonds in phosphine ligands can also produce conformational isomers and DNMR spectra below room temperature.There have been moderately successful attempts to estimate relative conformational stability using molecular mechanics methods with force fields adjusted for metal compounds.These different conformations can produce conformational isomers that are diastereomers in complexes with chiral metal centers, such as in (39) and (40). Propeller chirality, which is designated as P and M, has also been studied in triphenylsilane and triphenylstannane. ... 

Calixarenes were developed later than crown ethers and cyclodextrins but have stillbeen extensively researched. Macrocycles of calix[n]arenes are constructed by linking a number of phenol residues via methylene moieties (Fig. 2.16). Like crown ethers, the name calixarene reflects the structures of these molecules, since a calix is a chalice. Calixarenes with various cavity sizes have been designed, each of which has conformation isomers, and their phenolic hydroxyl groups are often modified. These structural characteristics allow us to create calixarene derivatives with various structural modifications. 

Figure 2.17. Conformation isomers and ion binding behavior of calix[4]arene...

The operation required—rotation—is the same for interconversion of geometric and conformational isomers, and it has been suggested that they be called collectively ro/a/Zo/itf/(or torsional) isomers. Geometric isomers are thus double-bond rotational isomers, and conformational isomers are single-bond rotational isomers. 

Now, Just how did we arrive at the conclusions of the last few paragraphs Most of us—perhaps without realizing it—judge the equivalence of protons by following the approach of isomer number (Sec. 4.2). This is certainlv the easiest way to do it. We imagine each proton in turn to be replaced by some other atom Z. If replacement of either of two protons by Z would yield the same product— or enantiomeric products then the two protons are chemically equivalent. We ignore the existence of conformational isomers and. as wc shall see in Sec. 13.13, this is just what wc should do. 

Ring reversal in 7-methoxy-7,12-dihydropleiadene (see accompanying figure) can be frozen out at —20°C. Two conformations are observed, in the ratio 2 1. When the high-frequency (low-field) part of the I2-CH2 AB quartet in the minor isomer is doubly irradiated, the intensity of the 7-methine proton is enhanced by 27%. Double irradiation of the same proton in the major isomer has no effect on the spectrum. What are the two conformational isomers and which is more abundant ... 

Isomers Compounds that have the same molecular formula but which have different constitutions (constitutional isomers), configurations (enantiomers, diastereomers), or conformations (conformational isomers), and therefore have different chemical and/or physical properties. 

Hindered rotation about the P-C bonds in phosphine ligands can also produce conformational isomers and DNMR... 

A quantitative description of sudi sittiations can be obtained for the simpk thermomechanical system whose molecules can assume only two conformational isomers The thermodynamic states for this system are usimlly uniquely ddined by temperature T, pressure p and the internal variable x, where x is the concentration of one conformational isomer and 1 — x is the concentration of the othw. The differences between the response functions under conditions of frozen and completely thawed conformational isomerism are given by Davies rebtionships 2.43) ... 

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