X conformation

Fig. 7.1 A simple representation of the two conformations of the five membered puckered ring in metal complexes of ethylenediamine and derivatives. Rings are viewed along the plane containing the two nitrogens and the metal. The C-C bond is nearer the observer than the two nitrogens. It is skewed down to the right for a 5 conformation and to the left for a X conformation.

The exact mechanisms of the complex formation and dissociation processes are not known. The overall process is represented by equation (X). Conformational changes may occur. Bimolecular processes might contribute, especially in solvents of low polarity (see, however, 143). A limited number of exchange rates have been reported, based mainly on NMR data (Table 13). Exchange kinetics are of prime importance in transport processes, where, however, more complex mechanisms may be operative than in the systems described here (see below andp. 145). 

A3.2.1. Infinite medium, D is constant, boundary conditions at x=-[Pg.571]

X Conformation Method/basis C1C2 C2C3 CX Change of Prediction by Reliability... 

One of the most extensively studied systems is that of [Co(dien)2]3+ (Table 7.1). The mer- isomer is chiral (C2) because of the two possible orientations of the proton at the secondary nitrogen atomst38,39], the unsym-fac-vsoiasi is also chiral (C2) while the sym-fac-isomer is achiral (Q). For the five-membered chelate rings all possible combinations of S and X conformations (see Fig. 6.5 for the SIX nomenclature) have to be considered, leading to a total of 40 isomers and conformers (some conformers are calculated to be unstable and may therefore be neglected144,451). The calculated distribution is based on partition functions (Eqs. 7.1, 7.2) ... 

Table X. Conformational Equilibria (AG°/kcalmol 1) of 3- and 4-Substituted Piperidine Derivatives and the Intramolecular Hydrogen Bonding Energy (AGHB/kcal mol-1) of Nipecotic Acid and Derivatives at 22 °C...

The optically active ligands, RR-ptn and SS-ptn, adopt skew boat conformations in which both methyls are equatorial (76). Thus RR-ptn adopts the X conformation, and SS-ptn forms the 8 conformation of the skew boat. The meso form of the ligand, RS-ptn, however, must adopt a chair conformation if both methyls are to be equatorially oriented. Tris complexes composed solely of RR- (or SS-) ptn are therefore restricted to the lela and 083 isomers. Molecular mechanics calculations have predicted that the lel conformation of complexes with the racemic ligands (e.g., A XXX) is significantly (4-7 kJ mol ) more stable than the 083 (A XXX) (77, 78). The tris chair form of RS-ptn was estimated to be very similar in strain energy to the RR-ptn leh complex (78). 

The crystal structure of a 108-nucleotide RNA-DNA Holliday junction has been solved at 3.1 and differs from a previously solved stacked-X conformation. The present structure differs due to a 135° rotation of the branches, and comparison of the two structures gives an insight into factors contributing to the flexibility of four-way junctions. 

V. Sasisekharan, P. K. Ponnuswamy. Studies on the conformation of amino acids. X. Conformations of norvalyl, leucyl, aromatic side groups in a dipeptide unit. Biopolymers. 1971, 10, 583-592. 

Since the CD spectrum of the one isomer of the [Co(i-sdda)-en]+ (Figure 12) shows a positive dominant CD peak and an overall CD curve similar to that of (+)-s-cis-[Co(edda)( -stien)]+, it might be expected to have the A-configuration. However, the A-s-cis-configuration would require the jt-stilbene-diamine backbone to have the X-conformation, forcing the large... 

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