Torsional rotation, hindered

R. H. Hunt, W. N. Shelton, F. A. Flaherty, and W. B. Cook, Torsion rotation energy levels and the hindering potential barrier for the excited vibrational state of the OH stretch fundamental band V of methanol. J. Mol. Spectrosc. 192, 277 293 (1998). 

When determining the range of likely helical shapes from intrinsic properties of amylose, this variability in monomer shape is almost as important as hindered rotation about the bonds linking the monomers. This conclusion is supported by conformational analyses of maltose such as shown in Figure 5 of the introductory chapter of this book. There are relatively small ranges (about 40 ) of allowed torsional rotation within one kcal/mol of the minimum (one must correct for the fact that there are two glucose residues in maltose when making such a coiqparison). ... 

A vibrational degree of freedom may be replaced by internal rotation (torsion) around a a bond. In this case the microwave spectrum of the molecule is modified by torsion-rotation interaction. By studying this effect on the rotational spectrum, the internal rotation potential barrier can be determined. The hindering potential of CH3N3 was found to be V3 = 695 20 cal/mole (the subscript 3 stands for the 3-fold axis of the hindering potential). The potential is rather small but is not smaller than the value expected from a hyperconjugation effect . 

The temperature dependence of the Pake pattern can be used to deduce that the bound dihydrogen ligand undergoes a torsional or hindered rotation motion around an axis perpendicular to the metal-dihydrogen axis. The bound hydrogen is characterized as a rigid planar rotator. In some cases, the potential surface for this rotation can be characterized by these measurements. 

Variational RRKM theory is particularly important for imimolecular dissociation reactions, in which vibrational modes of the reactant molecule become translations and rotations in the products [22]. For CH —> CHg+H dissociation there are tlnee vibrational modes of this type, i.e. the C—H stretch which is the reaction coordinate and the two degenerate H—CH bends, which first transfomi from high-frequency to low-frequency vibrations and then hindered rotors as the H—C bond ruptures. These latter two degrees of freedom are called transitional modes [24,25]. C2Hg 2CH3 dissociation has five transitional modes, i.e. two pairs of degenerate CH rocking/rotational motions and the CH torsion. 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

Figure 7.10. Hindered rotation potentials for two inequivalent methyl groups I and II in a-toluene calculated by the atom-atom potential method. The values of V, V6, and 80 are equal to 29.8meV, -14.9meV, and 17.2° (I) and 23.0meV, -1.8meV, and 35° (II). Solid and dashed lines indicate the levels of torsional vibrations for CH3 and CD3 groups. The same potentials under pressure 4.6 kbar are shown in the right. (From Cavagnat et al. [1986].)...

In Figure 4, A gives a graph of the contribution due to three torsional Debye modes with 6 = 200° K., while B represents the heat capacity contribution of three hindered rotational degrees of freedom with ... 

The four librations (torsional oscillations or rocking motions) arise because the crystal-field potential prevents the I2 molecule from rotating as it would in the gas phase. There are some special crystals, called plastic crystals, in which symmetric molecules that interact weakly can still undergo hindered rotation in the solid phase, but l2( ) is not one of these. The librational motions for each I2 occur about two axes (a, /3) perpendicular to the 1—1 bond direction. The librations of the two I2 molecules in the same unit cell are coupled—giving rise to SL, AL and SL, AL vibrations, where SL denotes symmetric libration (angle displacements in phase) and AL denotes antisymmetric libration (angle displacements out of phase). 

The value of (cos 0) will depend on temperature, ofcour.se, since the molecule will have sufficient torsional motion to overcome the energy barriers hindering rotation when the temperature is sufficiently high. 

Vibrational fundamentals are from the assignment of Savoie and Giguere (7) as modified by Chackalackal and Stafford (9). Data include Raman spectra of the liquid (1 ) and infrared spectra of all three phases (7) plus the superheated vapor (9). Spectra of the superheated vapor helped to clarify the vibrational distinction between the monomer and associated molecules. Savoie and Giguere derived a barrier of about 1.3 kcal mol" for hindered interal rotation of the OH group and used this to calculate C = 17.64 and S = 71.38 cal k" mol" at 298.15 K. We calculated C = 17.98 and S = 71.02 cal k" mol" using the OH-torsional frequency (265 cm" ) instead of hindered rotation. 

Other estimated values that have been reported include 200 cm ( ) and 37 cm (5 ). The inactive torsional frequency is treated as a hindered Internal rotation. We use an estimated potential barrier of 8.0 kcal mol ( ) to calculate heat capacity contributions for hindered rotation from the table of Pitzer and Brewer (9). Contributions below 201 K could not be obtained by... 

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