Molecular torsion

From the decrease in quadrupole splitting in Te with increasing temperature, Violet and Booth 16) inferred a molecular torsional frequency corresponding to 33 cm.". This suggests that the Mossbauer eflFect may be a method of examining molecular motions in Te compounds at frequencies corresponding to the far-infrared. 

Figure 24. Conformational states (folded and unfolded) for Wilcox molecular torsion balance...

In good solvents at ambient temperature, the excited state (67 ) will quickly relax to the planar form, so that only the 0-0 emission from Si is detected in steady-state emission. If the same experiment is performed at low temperature and in a viscous solvent, the molecular torsion of 67 in attaining its planar form is hampered by the medium, and planarization is slow on the timescale of the fluorescence lifetime (355 ps). Emission will not only occur from the potential minimum of the lowest excited state, but from virtually all frozen ro-tamers resulting in a broad and blue-shifted spectrum. Only after planarization of 67 is complete narrow emission from the lowest excited-state conformation will reoccur. Consequently, planarization of the excited state rather than energy migration is likely to govern the emission behavior in PPEs such as 12. 

Fischer, F. R., Schweizer, W. B. and Diederich, F. (2007) Molecular torsion balance Evidence for favorable orthogonal dipolar interactions between organic fluorine and amide groups. Angew. Chem. Int. Ed, 46, 8270-8273. 

Figure 106 A molecular torsion balance for determining weak aromatic-aromatic interactions [139]...

It should also be noted here that the torsional mode assignment for CH3 CC13 has recently been confirmed in the low-temperature solid phase by Durig and co-workers,19 who have demonstrated in an extensive series of infrared and Raman studies of molecular torsional vibrations that forbidden or weakly active modes can very often be definitively observed in crystal phases due to the lower symmetry of the crystal field. 

Fig. 36. Pulse sequence for direct determination of H— C— H molecular torsion angles by heteronuclear dipolar modulation of C DQ coherence. ...

For searches containing only one distance constraint, molecules passing step 2 are considered to have satisfied the constraint and are added to the hit list. If, however, two or more distance constraints exist, specific molecular torsions are tweaked to verify that all the constraints can be met simultaneously. To do this, the rotatable bonds between matched atoms are determined (here defined as all acyclic bonds, excluding amide bonds and bonds between two unsaturated atoms), and the molecule is subjected to torsional optimization to minimize the RMS deviation between the constraint and fitted model values, according to the following equation ... 

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