Cis- and trans-conformation

Figure 5 Molecular structures of the cis and trans conformations in polyfmethyl-methacrylate).

The reactions of Eq. (2) could not be carried to completion in a reactive olefin such as 1-hexene. After several increments of MeCl2SiH had been added to the mixture, succeeding increments were mostly consumed to form /i-C6H13MeSiCl2. When this product appeared in the solution, gas-liquid chromatography (GLC) showed that hexene in the solution was no longer only 1-hexene but a mixture of isomers which contained mostly 2- and-3-hexene, in both cis and trans conformations. 

It has been suggested that there are three major pathways for peroxynitrite to react with various substrates. With a pK value for peroxynitrite equal to 6.8 [103], peroxynitrite exists in the ionized or nonionized form depending on pH. Furthermore, the reactivity of peroxynitrite depends on its cis- and trans-conformations. Correspondingly, the following mechanisms for peroxynitrite decomposition have been proposed ... 

Figure 15 Sketch of the local environment along a polybutadiene chain of cis-and trans-conformers. For sp3-hybridized carbon atoms (indicated by the gray spheres), the chemical shift is different when they belong to a cis-monomer than when they belong to a trans-monomer. For sp2-hybridized carbon atoms (shown by black spheres) in a cis-monomer, NMR shows a different chemical shift whether they have another cis-monomer as a neighbor or a trans-monomer as a neighbor, and it is similar for the sp2-hybridized carbon atoms in the trans monomer.

Arguing as before, we see that the energy gap between the nA and 4 will be comparable for both cis and trans conformers but S A04 will be greater in the tram conformation due to a X-C4 bonding interaction ... 

The macrocyclic ligand 6,13-diamino-6,13-dimethyl-1,4,8,11 -tetra-azacyclotetradecane (diammac), illustrated in Scheme 5 for the two cis and trans conformations, is properly suited to fulfil the requirement of six donor atoms to satisfy the Cr(III) hexacoordination. In fact, in addition to the four central N atoms it has two amine groups (a sort of head and tail, from which the name scorpiands is given to these ligands). 

Let us start with the Ni(II) complexes of the already mentioned scorpiand diammac (6,13-diammino-6,13-dimethyl-1,4,8,11-tetraazacy-clotetradecane) in its two cis and trans conformations. In contrast to the previously mentioned chromium-, iron-, and cobalt-diammac complexes, in which the geometry of [M(fra s-diammac)]" + was substantially octahedral and that of the [M(cw-diammac)]" + was substantially trigonal prismatic, in the case of both [Nif/raws-diammac)]2+ and [Ni(m-diammac)]2 + the structural differences are attenuated and both can be viewed as more or less distorted octahedral geometries, with two sets of averaged Ni-N bond lengths of 2.07 A and 2.13 A, respectively.161 162 This is reflected by the fact that both the two complexes exhibit in aqueous solution a chemically reversible Ni(II)/Ni(III) oxidation ([Nif/raws-diammac)]2 + E° = + 0.67 V vs. SHE [Ni(m-diammac)]2 + ... 

Z/E ratios have been reported for [Ak—CH—CH=CH2] M+ with M = Li, Na, K and Cs, in various solvents at various temperatures - (set CP2, Table 10). These ratios are equivalent to the ratios Cz/Ce, where Cz and Ce are the concentrations of the cis and trans conformers, respectively. They are equal to the conformational equihbrium constants for the process depicted in equation 47 ... 

Figure 1. Schematic indication of nodal planes for cis and trans conformers in ethylene.

The import of diabatic electronic states for dynamical treatments of conical intersecting BO potential energy surfaces is well acknowledged. This intersection is characterized by the non-existence of symmetry element determining its location in nuclear space [25]. This problem is absent in the GED approach. Because the symmetries of the cis and trans conformer are irreducible to each other, a regularization method without a correct reaction coordinate does not make sense. The slope at the (conic) intersection is well defined in the GED scheme. Observe, however, that for closed shell structures, the direct coupling of both states is zero. A configuration interaction is necessary to obtain an appropriate description in other words, correlation states such as diradical ones and the full excited BB state in the AA local minimum cannot be left out the scheme. 

In 2-carbomethoxythiophenes, nearly equal amounts of the S,0-cis and trans conformers (157a and b) are present (82JOC3759) at low temperature. 

In acetyl and higher COAlk derivatives of furan (76ZN(A)1217 84JST(116)377 85JCS(P2)1839), for example, in the 2-formyl- and 7-formylbenzo[h]furans (84JCS(P2)1479), the conformational equilibrium was found to be solvent dependent. This behavior should characterize all situations in which the energy difference between cis- and trans-conformers is small (<8 kJ mol" ) and the more stable conformer in nonpolar media is the one with lower polarity. 

In earlier theoretical studies Shen and coworkers used Hartree-Fock self-consistent-field (HF) calculations with different basis sets to study water complexes of anionic ONO—O-30. Two stable ONO—O isomers, cis and trans, formed hydrogen bonds with H2O molecules at different positions. Second-order Mpller-Plesset perturbation theory (MP2) with a 6-311+G(d,p) basis set has also been applied to the study of ONO—O-, (H2O), (n = 1 or 2) complexes31. Koppenol and Klasinc studied the cis and trans conformers as well as the transition state for torsional motion of ONO—O- at the HF/6-31(d) level32. In their calculations, the trans conformer is slightly more stable than the cis form, and the rotational barrier was thought to be quite high. However, correlated methods (MP2) were also used to study this molecule, and they predict that the cis conformer is more stable than the trans conformer33,34. 

Because the double bond cannot rotate, the cis and trans conformations are not interconvertible. They... 

Figure 1.3 The peptide bond. Distances are in A (1 A = 0.1 nm). Proline residues are found in both cis and trans conformations.

An investigation of the vibrational spectrum of cyclopropylcarbonyl fluoride was carried out by Durig and coworkers using HF/3-21G theory115. The authors could assign all frequencies of cis and trans conformations and analyse normal modes in terms of potential energy contributions using appropriate symmetry coordinates. The calculated conformational stability and rotational barriers [HF/6-31G(d) and HF/3-21G] were compared with results obtained from the far-infrared spectrum. 

This method is more efficient in the resolution of cis and trans conformational isomers of fatty acids than the usually used GC methods with packed columns, and there are no problems with derivatization of short-chain fatty acids or heat-labile polyunsaturates. Another advantage over GC methods is that the separated fatty acids are not destroyed during their detection, which enables further analysis to be performed. The trans isomers are generally eluted after the corresponding cis isomers. Positioning of the double bonds in the proximity of the carboxy group of an acid usually leads to a shift in the direction of the methyl end of the carbon chain. 

Morton et al.135,141) were the first to study the poly(butadienyl)lithium anionic chain end using (b). They found no evidence of 1,2-chain ends and concluded that only 1,4-structures having the lithium cr-bonded to the terminal carbon were present. A later study by Bywater et al.196), employing 1,1,3,4-tetradeuterobutadiene to minimize the complexity of the spectrum that arises from proton-proton coupling, found that the 1 1 adduct with d-9 fert-butyllithium in benzene exists as a mixture of the cis and trans conformers in the ratio 2.6 1. Glaze et al. 36) obtained a highly resolved spectrum of neopentylallyllithium in toluene and found a cis trans ratio of about 3 1. 

Ab initio calculations on the structures of ethylchlorocarbene and chloromethyl-chlorocarbene show that the equilibrium between cis and trans conformers is shifted moderately to the trans isomer for the ethyl derivative and strongly to the cis for the chloromethyl (due to a stabilizing carbene n — rearrangement transition states are product-like and the Z-isomers are stabilized by a tiirough-space interaction of the p orbitals localized on the terminal atoms. 

Mukheijee et al [56] also used the half projected Hartee-Fock (HPHF) method due to Smeyers and co-workers [70-74], which is based on variational formulation with two DODS determinant for the representation of the wave function and is proved to be very effective in calculating singly excited states of molecules. Table 5 displays the energy values only in the cis and trans conformations of the two excited states. 

Figure 11.12. Multidentate organic ligands 7 and 8 (with cis and trans conformations) used in the construction of silver(I) tube-like complexes.

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