Asymmetry, torsional

Torsional Asymmetry. Rotation about single bonds of most acyclic compounds is relatively free at ordinary temperatures. There are, however, some examples of compounds in which nonbonded... 

The concept of torsional asymmetry is not jnst an interesting abstract idea. Some years ago, fertility in some Chinese rnral commnnities was found to be below normal levels, and this was traced back to the presence of gossypol in dietary cottonseed oil. Gossypol acts as a male contraceptive, altering sperm matnration, spermatozoid motility, and inactivation of sperm enzymes necessary for fertilization. Extensive trials in China have shown the antifertility effect is reversible after stopping the treatment, and it has potential, therefore, as a contraceptive for men. 

Gossypol is a biphenyl system with ortho substituents, so there is restricted rotation about the bond joining the aromatic rings, creating torsional asymmetry and the existence of enantiomers without a chiral centre. We are only asked about the shape, and this will have the planar aromatic systems at right angles to minimize interaction between the ortho substituents. 

Sikaroudi, H., Chandler, A. M. (1992). Structure-foundation interaetion in the earthcpiake response of torsionally asymmetrie buildings. Soil Dynamics and Earthquake Engineering, 77(1), 1-16. doi 10.1016/0267-7261(92)90022-6... 

P2J2A Z = 8 Dx = 1.931 R = 0.034 for 2,321 intensities. There are two molecules in the asymmetry unit, and both exhibit the syn disposition (—84.5°, + 76.1°) for the base. The conformation of the D-ribosyl group is 3T4 (28.5°, 39.4°) in molecule A and 3T2 (359.9°, 36.2°) in molecule B. The exocyclic, C-4 -C-5 bond torsion-angle is gauche+ for both molecules (55.2°, 59.7°). The purine bases of the crystallograph-ically independent molecules are paired by N-l-H 0-6 hydrogen bonds across a pseudo-two-fold axis. The bases are stacked such that the Br atoms are tucked under the pyrimidine moiety of the adjacent... 

Since the a //-distance varies so little with framework vibration, particularly in high barrier cases 09). The anti-peak is accordingly not suited for direct determination of 00. On the other hand, the torsional motion leads to an asymmetry in the anti-peak due to the functional relation between r and . For a low barrier case this asymmetry may be appreciable, while in a high barrier case it may be observed only as a shrinkage effect for the anti-distance. The asymmetry or the shrinkage may be used to derive a value for 00. 

The thermal asymmetry of the Ci. C4 anti-peak gives information about the torsional motion near anti and demonstrates that the potential well is wider in butadiene than for torsional motion about sp3-sp3 linkages. 

Because of the observed asymmetry in the anti-peak, Eqs. (13) and (14) were applied over a region corresponding to 2o

determined values of AH, torsional potential was estimated including V[-, V2-, V3-terms in the Fourier series expansion. The resulting... 

It is important to note that the proportional relationship between Amax, Amid, and Amin for these couplings is the same for 100% spin density, and for the present case with approximately 50% spin density. When this is so it indicates that there is no rocking motion at the radical site. This is good evidence therefore that the radical site is essentially planar. The best evidence for radical planarity comes from the analysis of the direction cosines associated with each principal values of the hyperfine coupling tensor. The direction of Amin (Table 18-2) is known to be associated with the direction of the >C-H bond, while the direction associated with the Amid indicates the direction of the n-clcctron orbital. These directions are easily calculated from the crystal structure, and are included in Table 18-2. One sees that the direction associated with Amid deviates only 2.0° from the computed perpendicular to the ring plane, while the direction of Amin, deviates only 2.8° from the computed direction of the C6-H bond. The errors listed on these values are at the 95% confidence level. This is very clear evidence that the radical shown here is planar in the solid-state. Any torsional motion of the C6-H would lead to asymmetries of the hyperfine coupling tensor, and would not produce the observed agreement between the direction cosines and the known directions obtained from the crystal structure. 

The effect of the biaryl torsional angle on enantioselection has been studied Harada T, Takeuchi M, Hatsuda M, Ueda S, Oku A (1996) Tetrahedron Asymmetry 7 2479... 

---