Rotational conformation

Figure 4-20 Rotamers (rotational conformers ) of 1-butene (Kevins, Chen, and AlliiiceT, 1996 ).

Further evidence of steric crowding owing to bulky phosphines is found in RhCl(CO)(PBu2 alkyl)2. Study of the 3IP NMR spectra at low temperature, freezing in the rotational conformers shows separate signals for each (Figure 2.25) [62],... 

Fig. 6.—Allowed Rotational Conformation, Enclosed by Solid Line, of the Glucosidic Bond of 1,6-Anhydro- -cellobiose Hexaacetate (3), Calculated as a Function of the H-l,H-4 and H-l.H-5 Interatomic Distances. (Reproduced from Ref. 49.)...

Two wider ranging, more systematic investigations of conformational dependence have since been performed to establish whether the conformational sensitivity noted in the above PECD smdies may generally provide a means for identifying and distinguishing gas-phase structure of suitable chiral species. The B-spline method has been applied to the model system (l/f,2f )-l,2-dibromo-l,2-dichloro-l,2-difluoroethane [60]. Rotation around the C C bond creates three stable conformational possibilities for this molecule to adopt. The results for both core and valence shell ionizations reaffirm an earlier conclusion a and p are almost unaffected by the rotational conformation adopted, whereas the PECD varies significantly. Eor the C Ij ionization to show any sensitivity at aU to the relative disposition of the halogen atoms further reinforces the point made previously in connection with the core level PECD phenomenon. 

Caillet, J., P. Claverie, and B. Pullman. 1978. Effect of the Crystalline Environment upon the Rotational Conformation about the N-C and C-C Bonds ( and P) in Amides and Peptides. Theoret. Chim. Acta (Berl.) 47, 17-26. 

When your muscles contract it is largely because many C-C sigma (single) bonds are undergoing rotation (conformational changes) in a muscle protein called... 

Rotation of the oxygen around the Fe-O bond involves a small energy barrier ( 2 kcal mol ), suggesting that several rotational conformations could be available at room temperature. Indeed, our molecular dynamics simulations show that the 02 ligand undergoes large-amplitude oscillations within one porphyrin quadrant, jumping to another quadrant on the picosecond timescale. The dynamics of the FeCO unit are characterized by rapid mo-... 

A consideration of the analysis of rotational conformations in a molecule suggests that the existence of a preferred conformation is due to an optimized stabilizing balance between the mono- and polycentric energy terms, which vary upon rotation. 

V. A. Danilova, N. V. Istomina, and L. B. Krivdin, 13C-13C Spin-spin coupling constants in structural studies XXXVII. Rotational conformations of hydroxy groups in pyranose, furanose, and septanose rings, Russ. J. Org. Chem., 40 (2004) 1194-1199. 

The NOESY experiment has also been very useful for revealing the presence of rotational conformers of dimeric flavonoids and flavone C-glycosides (Figure 2.3). Strong exchange crosspeaks between equivalent protons of each conformer revealed the rotational equilibriums. This NOE phenomenon was first noted by Hatano et al. in two conformers of procyanidin dimers. The volume of the NOESY crosspeaks is related to the distance... 

More successful attempts to interpret yielding on a molecular level were based on an extension of the Eyring phenomenological flow theory by incorporating molecular characteristics [132,133]. The modification is based on changes in distribution of rotational conformation states of segments upon stress action and the effect of temperature on them. 

The distance measured by EPR is the point dipole distance between the two paramagnetic centres. When applied to structures of spin-labelled biomolecules the desired distance is between sites on the biomolecules. A key question is the conformation and conformational flexibility of the spin label. X-ray crystal structures of four spin-labelled derivatives of T4 lysozyme have been reported.54 Preferred rotational conformations of the linkage between the cysteine introduced by site-directed mutagenesis and the spin label were observed. The electron density associated with the nitroxide ring in different mutants was inversely correlated with the mobility of the label observed in fluid solution EPR spectra. 

Bichromophoric model compounds can provide such standards, since these compounds can be highly purified and characterized by techniques inapplicable to polymers. For example, the racemic compound 2,4-diphenylpentane models the syndiotac-tic dyad of PS, while the meso 2,4-diphenylpentane models the isotactic dyad of PS. The 2,4-diarylpentanes are the best models of aryl vinyl polymers, since the populations of the rotational conformers of the model compounds are similar to those of the dyads of the polymer. 

The conformational statistics of asymmetric vinyl chains such as P2VN are well-known 126). The rotational conformers of isotactic (meso) dyads are entirely different from those of syndiotactic (dl) dyads. Frank and Harrah132) have described each of the six distinct conformers for meso and dl dyads, using the t, g+ and g nomenclature of Flory 126). Excimer-forming sites (EFS) are found in the tt and g g+ meso states, and in the degenerate tg , g t dl state. Because the rotational conformers of compounds such as l,3-bis(2-naphthyl)propane do not match those of either the iso-or syndiotactic dyads of P2VN, the propane compounds make poor models of aryl vinyl polymers. However, the rate constants of fluorescence and decay of the intramolecular excimer in polymers can usually be determined from the propane compounds (but see the exceptional case of PVK and its models133)). 

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

At T = 77 K in MTHF, the kinetics of fluorescence decay of P-L-Q with a bridge containing one bicyclo[2.2.2]octyl is of a non-exponential character. This effect can be explained by the coexistence in the frozen solution of several rotational conformations of the P-L-Q molecule (rotation of the porphyrin fragment around the a bond in its meso position is meant here). The characteristic time of the fluorescence decay for the predominant portion of the P-L-Q particles at 77 K, r 1.1 x 10 1°s, virtually coincides with the value of r = l/k(e1 at 298 K, i.e. the rate of tunneling from P to Q is independent of temperature. The exponential character of the fluorescence decay curve at 298 K indicates that, at this temperature, the rate of rotation exceeds k(e1. ... 

Figure 5-7 Six rotational conformations about the 2,3 C-C bond of butane. The forward groups are shown here as rotating counterclockwise with respect to the rear groups.

The equilibria (relative stabilities) and equilibration (rate of interconversion) of the rotational conformations of ethane and butane were discussed in Section 5-2. If you review this material, it will be clear that forming a ring from a hydrocarbon chain will greatly reduce the number of possible staggered and eclipsed conformations. 

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