Conformations structural parameters

Conformations, structural parameters, and charge distributions of 2-acetylthiophene 137, di-2-thienyl ketone 170, and higher oligomers 171 and 172 (Figure 41) have been determined by ab initio calculations at the HF/6-31"" level of theory <1998JST(455)131>. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

The replacement of carbon by other elements produces changes in several structural parameters and consequently affects the conformational characteristics of the molecule. In this section, we will first describe some stereochemical features of heterocyclic analogs of cycloalkanes. For the purpose of elaborating conformational principles, the discussion will focus on six-membered rings, so that the properties may be considered in the context of a ring system possessing a limited number of low-energy conformations. 

Although perhaps not strictly a conformational study, the structure of the gem-disulfoxide 184 was determined by X-ray crystallography169. A comparison of the structural parameters so measured with 1H and 13C NMR chemical shifts led the authors to conclude that chemical shifts should allow configurational assignments to be made to related sulfoxides. 

Thus P is a structural parameter ranging between 0 and 1 that acts at the initial moments of the oxidation process of every segment the higher the degree of closure (v), the lower the probability (P) of any spontaneous conformational changes and the greater the anodic overpotential required to create a relaxation nucleus. Under these conditions both magnitudes are related by... 

Conformationally dependent chirality is a common feature of molecules that contain carbon atoms with identical substituents. Another example is presented in Fig. 7.10. In dihydroxymethane, the conformation in which the H-O-C-O angles are trans, has a plane of symmetry, and the carbon atom is of the type C(a2b2). In contrast, when one of the O-H bonds is rotated out of the plane, the structural parameters change in an asymmetric way. The system is now of type C(aa bb ) and the mirror images cannot be superimposed. 

In order to study this question in a more systematic way, we have recently optimized 144 different structures of ALA at the HF/4-21G level, covering the entire 4>/v )-space by a 30° grid (Schafer et al. 1995aG, 1995bG). From the resulting coordinates of ALA analytical functions were derived for the most important main chain structural parameters, such as N-C(a), C(a)-C, and N-C(a)-C, expanding them in terms of natural cubic spline parameters. In fact, Fig. 7.18 is an example of the type of conformational geometry map that can be derived from this procedure. 

The bis-adducts are stable in the solid state at low temperatures. They dissociate in solution at ambient temperature. In the case of Me2Sb-SbMe2 a stable complex was formed only with the sterically hindered and less electrophilic t-Bu3Al. X-ray crystal structure analyses for [( -Bu)3M(R2Sb-SbR2)M( -Bu)3] (R = Me, Et M = Al, Ga) revealed that the distibine ligands adopt the anti conformation (Structural parameters of distibine complexes are summarized in Table I. 

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. 

Tetravinylmethane (TVM) is a very interesting compound with respect to its conformational and structural parameters. All the assumptions on the symmetry of TVM are based on D2d and S4 conformations10,11. Surprisingly, none of these conformations is observed in the crystalline state instead, C symmetry was found in an orthorhombic crystal lattice (space group Pbca). If one of the vinyl groups (C3-C8-C9) is rotated by ca 150°, the Ci symmetry can be transferred to S4 symmetry (or vice versa). This is evident from Figure 2 where DVC is also shown in the same projection which demonstrates that the C1 symmetry is no coincidence of packing effects. 

The structural parameters for the Cs symmetric bisected conformation obtained at B3LYP/6-311+G are similar to those obtained at B3LYP/6-31G51 level. At B3LYP/6-311+G level, C1-C2 bond slightly elongated (1.579 A) as... 

The conformations of the furanose ring in 250 nucleoside and nucleotide structures were analysed by Bartenev et al. (1987). These authors made the assumption, referred to above, that intermolecular interactions have a random effect on the structure in the crystal, and that the probability JVg of a structure crystallizing in a non-ground-state conformation is the same as the probability of it arising in thermal equilibrium at ambient temperature T in solution (6). (A difficulty arises immediately with the definition of the temperature, because structural parameters for molecules in crystals are... 

Structural parameters. Variations of bond lengths and bond angles in the Cl—N2—C3—N4—C5 moiety as a result of the aforementioned lpN-0c n overlap were introduced into MM2 by deriving conformationally dependent correction terms for r° and 6°. This treatment circumvented the electronegativity correction to bond lengths implemented in MM2-87 (Sections II.B.l and n.C.l). 

The results, presented in Table 8, show that in most cases the conformer with the lowest steric energy indeed corresponds to the experimentally most favored one. In addition, several molecules containing the N—C—N moiety were retrieved from the Cambridge Structural Database and calculated with the new parameter set. A comparison between MM2 and X-ray geometries (selected structural parameters only) for two conformers of 1,4,5,8-tetraazadecalin (25, 26) is provided in Table 9 and shows good fit between the experimental and calculated data. 

Recently a very detailed study on the single chain dynamic structure factor of short chain PIB (M =3870) melts was undertaken with the aim to identify the leading effects limiting the applicability of the Rouse model toward short length scales [217]. This study was later followed by experiments on PDMS (M =6460), a polymer that has very low rotational barriers [219]. Finally, in order to access directly the intrachain relaxation mechanism experiments comparing PDMS and PIB in solution were also carried out [186]. The structural parameters for both chains were virtually identical, Rg=19.2 (21.3 A). Also their characteristic ratios C =6.73 (6.19) are very similar, i.e. the polymers have nearly equal contour length L and identical persistence lengths, thus their conformation are the same. The rotational barriers on the other hand are 3-3.5 kcal/mol for PIB and about 0.1 kcal/mol for PDMS. We first describe in some detail the study on the PIB melt compared with the PDMS melt and then discuss the results. 

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