Torsional angle change

The tetramethylammonium ion appears to exhibit some specificity in its binding to poly(dA-dT) since the dTjsdA and dA dT resonances are partially resolved in the presence of this counterion. This specificity is probably also reflected in the selective glycosidic torsion angle change (monitored at one sugar H-l resonance) in the poly(dA-dT) duplex on proceeding from 1 M Na+ solution to 1 M TMA " solution. 

Steric hindrance Strain in a molecule caused by two atoms in that same molecule that would (under normal bonding conditions for each) approach closer than normal. To alleviate this closeness of approach the atoms readjust their positions slightly with resultant bond angle and torsion angle changes. 

Table 14 The torsion angle changes upon protonation of the carbonyl group in 3-ax- and 3-eq-chlorocyclohexanones...

Calculated barriers are for rigid rotation, where no bond length or angle changes occur except for the torsional angle change about the internal axis. 

We refer to this process as a torsional angle change, or an internal rotation. Such a change will produce changes in distances between nuclei located on opposite ends of the torsional axis, hut these distances typically remain several times as great as a bond length throughout the entire process. 

Gordon and Quin (1976) have noted that trivalent and tetravalent oigan-ophosphorus compounds (e.g., phosphines, phosphites, phosphonates) may show upheld or downfield shifts with increasing temperature. These shift variations may also relate to P—O and P—C torsional-angle changes. 

The K -NMR spectra were also consistent with this conformational analysis (Table VII). In flippii from a chair to a boat, the C-5— C-4—O—P torsional angle changes from trans to gauche. In 8a-11a, the carbon-phosphorus coupling constant Vjp is 8.8-9.8 Hz (trans), whereas in 11b, V5P is 6.1 Hz (gauche). In 8b- 11b, Vjp is intermediate (7.8 - 8.5 Hz), consistent with the rapid equilibration between chair and IxMit conformations. 

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