U-conformation

Fig. 1. Comparison of modes of bonding of allyl, pentadienyl, and cyclopentadienyl ligands. For pentadienyl the symbols refer to the shape of the group the W and U conformations resemble the letter symbols, the S sickle.

Molecular orbital calculations of varying sophistication have been used to try to predict the relative stabilities of the W, S, and U conformers. The earliest using the EHMO method predicted that the U conformer is the most stable (70), but MNDO (74) suggests that the W form is of lowest energy for the parent C5H7 anion. Most of these calculations relate to the gaseous ion, but similar conclusions were reached by inclusion of lithium or BeH (71-73). The merits and demerits of the calculations are discussed by Dewar et al. (74). 

Mattice, W., and Suter, U. "Conformational Theory of Large Molecules The Rotational Isomeric State Model in Macromolecular Systems". Wiley-Interscience, New York (1994). 

Figure 17.21 (part I) also shows the most stable conformer each of (DHQ)2-PHAL and (DHQD)2-PHAL. Its conformation first reflects the propensity of the oxygen atoms at the central heterocycle to conjugate with its -electron system leading to an -hybridization of these oxygen atoms. Second, the bulky sec-alkyl substituents are in search for the least hindered position. The result is that they try to avoid/>eri-interaction with the labeled H atoms of the central heterocycle (for the term />eri-interaction see the discussion of Figure 5.8), but do not tolerate a U-conformation in the non-hydrogen substituent-Csec-0-C=N substructure. 

Brandenburg, K., Kusumoto, S., Seydel U. Conformational studies of synthetic lipid A analogues and partial structures by infrared spectroscopy. Biochim Biophys Acta 1329 (1997) 193-201. 

For weakly coordinating potassium K+ counter-ions, the acidity of diethyl malonate (24) (in DMSO) increased by increasing the fraction of ion pairing, as shown in Table 1 for three counter-ions in DMSO solution (Table 1). This trend also continued by increasing the coordinating ability of the metal counter-ions from sodium to lithium counterions. From this study, it appears the ( ,W)-confoimer (26 ) is more preferred over the (Z,U)-conformer (26) as the repulsion between the C—O bonds is more energetically unfavourable than chelate formation might be favourable. 

Because the double bond cannot rotate, the cirand /ra/u conformations are not interconvertible. They... 

In Figure 4.24, the distances between the two N and M atoms are about the same depending whether the N=C—axes are parallel or not. This means if the atomic contributions on N and C are important for Cu- and Ag-containing chromophores, but less or absent for Pd- and Pt-containing species, the exciton phenomenon will be as important for the U-conformation for both types of chromophores. On the other hand in the Z-conformation, the N- N separation is about 5.8 A. If the atomic conttibution on Ihe N atom is important as indicated for Cu- and Ag-containing chromophores (Table 4.10), then energy migration is still possible. On the other hand, if this contribution is weak or nil, then transfer is very inefficient. In the cases... 

Laus, G., Bentivoglio, G., Kahlenberg, V, Wurst, K., Nauer, G., Schottenberger, H., Tanaka, M. and Siehl, H.-U, Conformational flexibility and cation-anion interactions in 1-butyl-2,3-dimethylimidazolium salts, Cryst. Growth Des. 12 (4), 1838-1846 (2012). 

Fig. 1 Probable crystal structure biological conformations of prevalent unsaturated fatty acids as suggested by de Jong [ISf A - oleic acid B - linoleic acid C - arachidonic acid left and middle - side and top views of spiral conformation, right - U conformation,... 

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