Twisting around bonds

INDO methods. Dipole moments of states as a function of twist around bonds are examined and predictions made of photoisomerization effects. Energies, dipoles, and non-adiabatic couplings have also been computed for diradical and zwitterion states of ethylene and propene. Conformational analysis of flexible trans-dienes can be made by the technique of polarization spectroscopy in stretched polymer films as has already been mentioned. Vitamin D and its derivative have been selected as examples of the use of the technique. 

Each polytripeptide chain is twisted around a threefold screw axis and exists in a secondary structure, analogous to the left-handed polyproline II-helix, i.e. with transposition of the peptide bond (pitch 8.4 A, 3 amino acids) (Figs. 2,3). 

FIGURE 18.7 The Tr-bond (represented by the yellow electron clouds) in an alkene molecule makes the molecule resistant to twisting around a double bond. Consequently, all six atoms (the two C atoms that form the bond and the four atoms attached to them) lie in the same plane. 

The structure of DNA resembles a ladder that has been twisted around itself. The rungs of the ladder are composed of bases (guanine, thymine, cytosine, and adenine) that form hydrogen bonds. 

Anthracene has also been used as an acceptor (Fig. 10). In solution, 26 emits a single fluorescence band that is somewhat structured in nonpolar solvents and becomes broad and structureless with increasing polarity [58]. The strongly hindered molecule 27 also exhibits a similar behavior, but its absorption spectrum is better structured [59]. The rate of formation of a charge transfer state is higher for 27 than for 26. Based on this observation, it appears that the twist around the anthryl-phenyl C-C bond plays a significant role in the fluorescence profile of the probes [60]. Acridines, such as 28, behave similarly to anthracene except that acridine is a better electron acceptor [61]. 

Methodical details of crystal calculations have been published by several authors (8, 95-98). As applications we only mention the calculation of the crystal structure of diphenyl (98, 99) which has a planar structure in the solid state (100) whereas gaseous diphenyl is twisted around the CC-bond, which links both benzene rings, by about 40° (101). The calculations reproduce satisfactorily the experimental findings. A further application is the calculation of the crystal structures of a series of amides (102), with the object of deriving suitable functions for the description of hydrogen bonds (cf. previous Section). 

The planar conformations s-cis and s-trans cannot be appreciably populated owing to the repulsive steric interaction between the Cg—H and C5-methyl (in the s-cis) or between the same Cg—II and the methyl group on (Q in the s-trans). This repulsion is minimized by introducing a twist around the Q,—( 7 bond. Limiting conformations, with skew angles of about 40° and 140°, can be assumed, as shown in Scheme 8. In this way, an intrinsically dissymmetric chromophore is created. 

J. F. Letard, R. Lapouyade, and W. Rettig, Structure-photophysics correlations in a series of 4-(dialkylamino) stilbenes Intramolecular charge transfer in the excited state as related to the twist around the single bonds, J. Am. Chem. Soc. 115, 2441-2447 (1993). 

Sometimes transformation of aromatic componnds into ion-radicals leads to stereochemically unusual forms. Octamethylnaphthalene is a nonplanar molecule twisted around the bond that is common for the two six-membered rings. The nitrosonium oxidation results in the formation of the cation-radical with the centrosymmetric flatten chairlike geometry (Rosokha and Kochi 2006). According to the authors, such a skeletal transformation improves the overall planarity of octamethylnaphthalene. For example, the mean deviation of the carbon atoms in the naphthalene core for the flatten chairlike cation-radical (0.007 nm) is less than half of the corresponding value for the neutral twisted parent (0.016 nm). Within this flatten carcass of the anion-radical, the spin density can be delocalized more effectively. 

The previously mentioned ethylene molecule is planar that is, all 6 atoms lie in a single plane because the double bond is rigid. In Figure 6-13, the stiff double bond prevents the molecule from being twisted around the axis between the carbon atoms. If a reaction substitutes a different atom— like a bromine atom—for two of the hydrogens, the resulting compound can exist in either of two different structural configurations. 

Since discussions in the preceding sections are limited solely to the twisted double bond systems whose unsaturated centers are constrained within the ring, this last section will give some example of systems in which the double bonds are twisted by crowdedness around these unsaturated centers. 

Variation in the delocalization (resonance) electrical effect of a -bonded substituent X or active site Y attached to a -bonded skeletal group, G. This results from twisting around the X—G or Y—G bond as a result of steric effects exerted by an adjacent group. 

Figure 6.1. Structure of DNA double helix. DNA double helix of 20 A diameter with its two strands twisted around each other. The nitrogen-containing bases are perpendicular to the helical axis and about 3.4A apart from their next base of the same strand. The bases from the two strands opposite each other form hydrogen bonds and help to stabilize the helix. The helix makes a complete turn every 34 A. (Reproduced from Textbook of Biochemistry with Clinical Correlations, T. M. Devlin, ed., Wiley, New York, 1982.)...

The parameters of the double helix that is formed by DNA include a diameter of 20 A, a rise per nucleic acid residue of 3.4 A with ten residues per complete turn. The two chains that make up the molecule are antiparallel the chains grow by adding repeat units to the 3 group on ribose or the 5 group on ribose and therefore one chain is joined 3 to 5 by phos-phodiester bonds and in the other chain the riboses are joined by 5 to 3. The two polynucleotides are twisted around each other in such a way as to... 

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