Conformation isomerism

This is a simple form of isomerism in which the isomers have different stereochemistries but are otherwise identical, e.g. trans planar and tetrahedral NiCl2(Ph2PCH2PPh2)2 (cf. Fig. 3.7). Its occurrence is confined to relatively few metal ions, usually those with a d configuration. 

The flurry of recent interest in isomerism arose because of the suggested existence of an isomerism related to conformation isomerism, a isomerism which is variously called distortional isomerism or bond-stretch isomerism. The suggestion was made that a bond length in a given complex could have either of two very different values. This could arise, for instance, if there were two different bonding interactions, each leading to stability at a different internuclear distance. What was the ground state at one distance 

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The spectroscopic properties of the /V-nitrosamines, especially the nmr and mass spectra, vary widely depending on the substituents on the amine nitrogen (44—47). The nmr spectra are affected by the E—Z isomerism around the N—N partial double bond and by the axial—equatorial geometry resulting from conformational isomerism in the heterocycles (44,45). Some general spectral characteristics for typical dialkylnitrosamines and simple heterocycHc nitrosamines are given in Table 1. 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, many organic molecules undergo rapid proton shifts (tautomerism), and the chemical reactivity of the two isomers may be quite different It is not valid, however, to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions will not be controlled by the position of the facile equilibrium. 

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

Dipole Moments. The dipole moments of quinoxaline, several derivatives, some pyrazine analogs, and comparable homocyclic compounds have been measured in an attempt to rationalize the values in terms of mesomeric effects and/or conformational isomerism. ... 

The next homolog, 1,5-hexadiene (1,5-HD), is of special chemical interest because the molecule is capable of undergoing the so-called Cope rearrangement. A GED study of 1,5-HD was also recently reported6. Because of the increased conformational complexity of this molecule compared to that of 1,4-PD, the structural details of the various con-formers could not be resolved and only averaged structure parameters were determined from the gas phase. Molecules in the solid state are frozen, mostly in only one conformation, which may but must not represent the conformational ground state. Therefore, conformational isomerization is usually not discussed with X-ray structures presented in the literature. 

Conformational isomerism in linear conjugated polyene radical cations... 

In this chapter, it will be described how we have utilized endogenous and engineered Zn2+ binding sites to explore the structure and molecular function of Na+/Cl -depen-dent neurotransmitter transporters. The work has not only allowed the definition of the first structural constraints in the tertiary structure of this class of transporters, but also provided new insight into both conformational changes accompanying substrate translocation and mechanisms governing conformational isomerization in the translocation cycle. In the chapter, we will also review the theoretical and practical basis for... 

CONFORMATIONAL ISOMERIZATION IN THE TRANSPORT CYCLE CONVERSION OF THE INHIBITORY ZN2+ SWITCH IN HDAT TO AN ACTIVATING ZN2+ SWITCH... 

Infrared of rj5-C5H5Mn(CO)2S, S = sulphur ligand demonstrate Mn-S conformational isomerism. Isotopic data are included... 

We first consider the case of conformational isomerism of 1,3-butadiene, the simplest 4 N pi electron molecule. We will discuss the relative stabilities of three important points on the rotational surface the cis isomer (6 = 0°),thegauche isomer (6 = 45°) and the trans isomer (6 = 180°). We shall focus our attention entirely on the effect of pi interactions on conformational preference. 

This pattern is familiar to the theoretical organic chemist since it reflects nothing more than the fact that the B + C union in the all-c/ s geometry resembles an antiaromatic structure while the same union in the all-tarns geometry resembles a non-aromatic structure. We have already encountered such situations in our discussion of the conformational isomerism of 1,3-butadiene. 

In order to exemplify how the various computed indices can be used in testing the predictions of OEMO theory within the framework of the dynamic model we shall consider once again the problems of conformational isomerism of 1,3-butadiene and geometrical isomerism of 1,2-difluoroethylene. We first consider the cis and tram conformers of 1,3 butadiene. 

In this section we shall consider conformational isomerism in typical hexaatomic molecules, X4Y2 and X2H2Y2. A diagram which depicts the MO s spannend by the 2px lone pairs of the heteroatoms X and the MO s of the coupling unit Y—Y in planar Y2X4 is shown in Fig. 20. We distinguish two cases ... 

We note here that similar reasoning has been employed by Lowe97 as well as Hehre and Salem98 in their studies of conformational isomerism of propene. 

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