Energy barrier to rotation

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

Another group that frequently - and perhaps surprisingly, in view of secondary amide characteristics -exhibits rotameric behaviour is the secondary carbamate (R-COO-NHR1), though the energy barrier to rotation tends to be a little lower than in the amide case. 

There is a large energy barrier to rotation associated with groups joined by a double bond. 

Torsional barrier the energy barrier to rotation of a single bond. 

The MM2 force field3 is probably the most extensively parameterized and intensively used force field to date. It reproduces a variety of molecular properties such as geometry, dipole moments, conformational energies, barriers to rotation and heats of formation. Of particular importance for calculations of amines is that MM2 treats lone pairs on sp3 nitrogens (and oxygens) as pseudo atoms with a special atom type and parameters. A closely related force field, MM2 7, was derived from MM2 by Osawa and Jaime. MM2 uses the same potential functions as MM2, but employs a different set of parameters in an attempt to better reproduce barriers to rotation about single C—C bonds. 

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other... 

It follows that the preferred conformation of ethane is a staggered one but, since the energy barrier to rotation is relatively small, at room temperature there will be free rotation about the C-C bond. 

Figure 2. Determination of the energy barrier to rotation about the N—N bond of the nitrosamine by enantiomeric interconversion...

The entropy of a molecule is composed of the sum of its translational, rotational, and internal entropies. The translational and rotational entropies may be precisely calculated for the molecule in the gas phase from its mass and geometry. The entropy of the vibrations may be calculated from their frequencies, and the entropy of the internal rotations from the energy barriers to rotation. 

This operation involves no reshuffling of the atomic arrangement (or constitudon) within the unit but docs represent an isomerization. How readily such a conversion occurs then depends upon the size of the energy barrier to rotation. Looking at specific examples, the compound CH(-N=N—CHj, exists in two different and stable stereochemical arrangements (3. cis and 4, tram) cotrcsponding to 1 and 2 above ... 

Figure 27-3 Proton nmr spectra of 2,2,3,3-tetrachlorobutane in 2-pro-panone solution at different temperatures. The curves on the left are experimental curves and those on the right are theoretical spectra calculated in accord with the uncertainty principle for different values of At. The large peak at —44° corresponds to 1, the smaller one to the enantiomers 2 and 3. The change of At with the temperature indicates that the energy barrier to rotation is about 14 kcal mole-1.

Isomerization of the retinal Schiff s base can occur when the molecule is excited with light, because the C-l 1-C-12 bond loses much of its double-bond character in the excited state. The valence bond diagrams of figure S2.7 illustrate this point. In the ground state of rhodopsin, the potential energy barrier to rotation about the C-l 1-C-l2 bond is on the order of 30 kcal/mol. This barrier essentially vanishes in the excited state. In fact, the energy of the excited molecule probably is minimal when the C-11 -C-l2 bond is twisted by about 90° (fig. S2.8). The excited molecule oscillates briefly about this intermediate conformation, and when it decays back to a ground state it usually settles into the ail-trans isomer, bathorhodopsin. 

Similar arguments can be developed for the glass transition, with the difference that it is the rate of transition between states that is more important than the energies of the various minima and so it must be the height of the energy barriers to rotation which determines the glass transition temperature. 

TABLE 2. Free-energy barriers to rotation through the 90° twisted state (A (71. kcal mol 1) and... 

In sulfenamides (R S—NR2R3), the cation radicals keep an unpaired electron occupying a -rr -orbital. This orbital is localized between the sulfur and nitrogen atoms. In other words, somewhat S=N double-bond character exists in such species. A consequence of this double-bond character is an increase in the energy barrier to rotation about the S-N bond. Restricted rotation about the S-N bond is known for the neutral sulfenamides (Kost Raban 1990). The energy barrier to this rotation is greater for the derived cation radicals than for the parent compounds (Bassindale Iley 1990). 

The increase in fluorescence intensity of TO upon binding to dsDNA is due to the restriction of rotation around the monomethine bridge upon intercalation of the dye into the double helical structure as the benzothiazole and quinolinium rings adapt to the propeller twist of the base pairs [49]. The monomethine bridge has a low energy barrier to rotation and hence is free to rotate in solution, allowing for the electronically excited dye to relax by non-radiative decay [49]. The quantum yield of free TO in solution has been reported to be 2 x 10-4 at 25 °C [43]. The binding constant for TO is 106 M 1 while that of ethidium bromide is 1.5 x 105 M 1 [59]. 

CH3GeH2F relative to CH3GeH3 does not increase the barrier to rotation about this bond, although within the series CH3EH2F (E = C,Si,Ge), the potential energy barrier to rotation about the C—E bond does decrease with increasing E—C bond length, as expected (145). 

The added bulk of hydrogen-bonded groups about I may decrease k by increasing the energy barrier to rotation about the a C-0 bond which is necessary for P-phenyl quenching. This hypothesis would also account for another phenomenon which we reported in our earlier paper (3) in 40% aqueous alcoholic solutions of I both < >d and the triplet lifetime decreased, relative to the values observed in the pure alcohol. In alcohol solvents, I reacts from both Sx and Tt. Substitution of smaller water molecules for alcohol molecules could reduce the barrier to rotation and increase klf which would decrease the triplet lifetime, and decrease the fraction of triplets which decay by bond cleavage. 

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