Ethylene bond rotation

While conformation II (Fig. 2.34) of Uke-y -amino acids is found in the 2.614-helical structure, conformation I, which similarly does not suffer from sy -pen-tane interaction, should be an appropriate alternative for the construction of sheet-like structures. However, sheet-like arrangement have not been reported so far for y-peptides composed of acyclic y " -amino acid residues. Nevertheless, other conformational biases (such as a,/9-unsaturation, cyclization between C(a) and C(y)) have been introduced into the y-amino acid backbone to restrict rotation around ethylene bonds and to promote extended conformation with formation of sheets in model peptides. Examples of such short chain y-peptides forming antiparallel (e.g. 152 [208]) and parallel (e.g. 153-155 [205, 208]) sheet-hke structures are shown in Fig. 2.38. 

Ratera et al. (2003) discovered valence tautomerism in the ferrocene connected through the ethylenic bond with perchlorotriphenylmethyl radical. As ascertained by Moessbauer spectroscopy, this species in the solid state exhibited a thermally induced intramolecular electron transfer resulting in the formation of ferrocenium and perchlorotriphenylmethyl anion moieties. The authors used the initial species in its trans form. If the cis form would be available, the possibility of rotation around the ethylenic bond would be interesting to disclose. According to the authors, the interconversion of the cation-radical and anion centers proceeds gradually. At ambient temperature, equilibrium composition of the tautomers is achieved. This peculiarity is important with respect to potential technical applications. 

It would also be interesting to check the ability of the ruthenocene acrylonitrile cation-radical to rotate around the ethylene bond Ruthenocenyl is weaker than ferrocenyl as a donor substituent (Laus et al. 2005). The particular property of rotating around the ethylenic bond in cation-radicals is a method of elucidating an electronic structure. 

The calculation of the electro-optical parameters describing Raman intensities is not yet very advanced, because of the paucity of data. Nevertheless, some success was achieved in calculations of the intensity of infrared absorption. The results on trans and gauche bond-rotation in ethylene glycol146 could be taken as a model for carbohydrates. Indeed, similar electro-optical parameters (/aCH, /aOH, /aCC, and /aCO) were calculated. This leads to the expectation that calculations of the intensity of the vibrational spectra of carbohydrates may be accomplished in the near future. In addition, the delicate problem of accounting for molecular interactions in calculating infrared intensities could be approached as it was for v(CCC) and i CO) vibrations in acetone.149 This will allow interpretation of weak, as well as strong, i.r. bands, in order to determine the structural properties of molecules. 

In general, the ethylene bond in organic cation radicals is weakened, and the barrier to rotation becomes significantly less than that of the neutral ethylene derivative. This particular property of the ethylene bond in cation radicals has been used to probe for the mechanism of many reactions (Todres 1987). 

Fig. 3. Important valence orbitals of some metal fragments. The energy scale markings are eV. (Reprinted, with permission, from Ethylene Complexes, Bonding, Rotational Barriers, and Conformational Preferences, Albright, T. A. et al. J. Am. Chem. Soc. 101, 3802, Fig. 1, copyright, 1979, by the American Chemical Society)...

The most effective approach to interpreting the barriers for a wide range of compounds lies in the consideration of the relative interactions within the Dewar, Chatt, Ducanson model of metal alkene bonding. An extended Hiickel MO approach has explored the interactions of the valence orbitals and examined the important interactions. A comprehensive extended Hiickel MO treatment of ethylene bonding and rotational barriers by Albright, Hoffmann et a/. presents an excellent analysis and the reader is referred to their paper for further discussiou. We have found that the following approach, which considers oifly three orbitals on the metal and the n and y orbitals of the alkene, provides the essential elements for understanding the barriers to rotation. Naturally, steric effects and secondary interactions with other orbitals modulate these primary iuteractious. 

Theoretical calculations have been made on stilbene which are relevant to photoisomerization dynamics. MNDO calculations of stilbene potential energy properties shows no evidence of a doubly excited "phantom" state but a singly excited state with adiabatic rotation around the central ethylene bond has only a small barrier on this path23T Calculations of dipole moments, optical spectra, and second order hyperpolarizability coefficients of some mono- and disubstituted stilbene molecules allows the design of useful nonlinear optical molecules 38. 

Fig. 48. Formation of a bond in ethylene a bonds omitted). If the two 2p orbitals are parallel there is substantial lateral overlap (for clarity not shown in the Fig.) giving a 71 bond rotation of the CIL groups lessens the bonding.

This represents rotational stability about the ethylenic bond produced by s/ 2-orbital-hybridized carbon atoms. Since free rotation about the carbon-to-carbon axis is not possible, two stable forms of the molecule can now exist. The six components of both forms are in the same plane. The orientations of substituents X and Y relative to each other, however,... 

Once the importance of the nature of the spacer unit in the PET process was realized, it was natural to probe a few more systems with different spacer units to understand the effect of M-F and M-R communications on the PET process and their photophysical behavior. Thus, the systems shown in Fig. 33 were designed (129) with the aim to understand the PET process and its manipulation to create new fluorescent signaling systems for transition metal ions. An examination of Table VIII shows that the efficiency of the PET process in the metal-free state depends on the nature of spacer and the amount of fluorescence recovery varies not only with the metal ion but with the spacer as well. In the case of Lg, the fluorescence enhancement is negligible as the fluorophore can come close to the metal center through rotation of the ethylene bonds. In Lio, the fluorophore is integrated with the receptor and the enhancement is slightly higher only in case of Zn(II) ion as input. 

Incorporation of the BAPTA ring into a fluorophore through a rotating trans-ethylenic bond gives an increase in fluorescence quantum yield (fluo-3) or a shift in emission wavelength on binding (indo-1) (Figure 3). 

The insertion of ethylenic bonds between the thiophene rings represents another approach to lower E. The combined effects of reduced rotational freedom and lower overall aromaticity of the TT-conjugated system result in a decrease of BLA and E from 2.20 eV for PT [8] to 1.74 eV for polythienylenevinylene PTV [37,38]. 

Consider the C—C double bond in ethylene. Imagine rotating one —CH2 group in ethylene relative to the other —CH2 group, as pictured in Figure 9.30. This rotation destroys the overlap of the orbitals, breaking the ir bond, a process that requires considerable... 

Only a relatively small number of polymers have sufficient mobility to be rubbery at room temperature. The molecular mobility depends heavily on the composition of the polymer backbone, which often contains a significant proportion of simple hydrocarbon species, such as those derived from ethylene, butadiene or isoprene. These species are small and are able to undergo bond rotation with relative ease, since they do not suffer problems due to steric hindrance [2] or the presence of strong dipoles. The rubber molecule is also able to undergo extension easily because the forces acting on the material are relatively weak secondary intermolecular forces, i.e., those acting between molecules, and not the primary inter-atomic forces, i.e., those existing within a molecule [3]. 

PROBLEM 12.13 There is another kind of rotation possible in 1,3-butadiene, that about the C(l)—C(2) or C(3)—C(4) bonds. We might guess that the barrier to this rotation would be little different from that for rotation in a typical double bond but, as a former president of the United States once said, that would be wrong. In 1,3-butadiene, it takes only 52 kcal/mol to do this rotation, some 14 kcal/mol less than the barrier of 66 kcal/mol for rotation in ethylene. Explain. Hint Look at the transition state for C(l)—C(2) bond rotation in 1,3-butadiene. 

Look again at Figure 20.24, from which we derived the information that the thermal 2 + 2 dimerization of a pair of ethylenes was forbidden. It s only forbidden if we insist on smushing the two ethylenes together head to head exactly as shown in the figure. If we are more flexible, and allow one ethylene to rotate about its carbon-carbon O bond as the reaction takes place, the offending antibonding overlap vanishes. What orbital symmetry really says is that for the concerted thermal dimerization of two alkenes to take place, there must be this rotation (Fig. 20.28). 

The CH2 scissors deformation of the vinyl and vinylidine groups give rise to a medium intensity band in the infrared and Raman spectra near 1415 cm In cw-1,2-dialkyl ethylenes, the in-plane CH rock where both CH bonds rotate in the same direction (clockwise or counterclockwise) appears near 1405 cm in the infrared, and the rock where the CH bonds rotate oppositely appears near 1265 cm in the Raman spectrum. In trans- y2-dialkyl ethylenes, the in-plane rock where both CH bonds rotate in the same direction appears in the Raman spectrum near 1305 cm , and die rock where the CH bonds rotate oppositely appears near 1295 cm in the infrared spectrum. In monoalkyl ethylenes the CH rock shows up in the Raman spectrum near 1295 cm where all the ethylene CH bonds rotate in the same direction to some extent. ... 

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