Torsion around bond

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

Torsion around C—C and C O bonds connects different alcohol isomers. The analysis of interactions between torsional states which are concentrated in different torsional wells can provide important information on energy differences between conformations [101, 102]. Conformational isomerism in alcohols is so subtle that it cannot be easily separated from intermolecular influences in... 

Model building remains a useful technique for situations where the data are not amenable to solution in any other way, and for which existing related crystal structures can be used as a starting point. This usually happens because of a combination of structural complexity and poor data quality. For recent examples of this in the structure solution of polymethylene chains see Dorset [21] and [22]. It is interesting to note that model building methods for which there is no prior information are usually unsuccessful because the data are too insensitive to the atomic coordinates. This means that the recent advances in structure solution from powder diffraction data (David et al. [23]) in which a model is translated and rotated in a unit cell and in which the torsional degrees of freedom are also sampled by rotating around bonds which are torsionally free will be difficult to apply to structure solution with electron data. 

P2i2,2i Z = 4 Dx = 1.57 R = 0.13 for 1,860 intensities. The molecule is in the folded conformation, but differs greatly in a number of the torsions around the backbone bonds found in the sodium salt85 of CDP-choline. Furthermore, the conformation of the D-ribosyl group here is C-3 -endo (28.3°). The glycosyl disposition is anti (33.1°), and the orientation around the exocyclic, C-4 -C-5 bond is -57.4°. The pyrophosphate group has a staggered orientation around the... 

The similarities between the main excited-state decay paths for cytosine and the remaining pyrimidine nucleobases, uracil and thymine, have been noticed by several authors. In all cases the energetically favored decay takes place at an ethylenic-type intersection, associated to torsion around a C=C double bond, which can be accessed along a low barrier in cytosine, and along barrierless paths in thymine and uracil [35]. The main difference between cytosine and the other two pyrimidinic bases is the presence of an additional low-lying decay path which involves the lone pair on N3 and torsion around the N3-C4 bond, both of which features are absent in uracil and thymine. 

For cytosine, in common with other pyrimidine nucleobases, photostability is the result of an ethylenic-type conical intersection associated with torsion around a C = C double bond. This interpretation remains - but the barrier is reduced - when the solvent is included approximately. 

Schiff Bases. Isomerization data related to these molecules are reported in Table 1. Of special interest is the high value of the photosensitization parameter (< >jgQ = 1) observed for 11-cis PRSB, independently of the sensitizer s energy. A theoretical analysis (195) interpreted this effect in terms of destabilization of the first excited triplet of the 11-cis isomer due to its (39°) torsion around the 12-13 single bond. This characteristic distortion of the 11-cis isomer leads to a substantial decrease... 

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 topological torsion is defined as a linear sequence of four consecutively bonded non-hydrogen atoms k-i-j-l, each described by its atom type (TYPE), the number of n electrons (NPI) and the number of non-hydrogen atoms (NBR) bonded to it [Nila-kantan et al., 1987]. Usually NBR does not include k-i-j-l atoms that go to make the torsion itself therefore it is minus 1 for k and / atoms, and minus 2 for the two central atoms i and j. The torsion around the i-j bond and defined by the four indices k-i-j-l is represented by the following TT descriptor ... 

The simple at-a-glance predictive correlation that emerges from Tables 3 and 4 is as follows with the peptide backbone drawn in the plane of paper, substituents in the same half space (i.e., ///reconfiguration) are all matched to the same helical handedness. This correlation can be rationalized by analyzing the basic conformers in /3-amino acids.183184 263 264 Such an analysis reveals a strongly preferred backbone torsion around the Ca—Cp bond, driven in part by the tendency to minimize repulsive vicinal interactions (Figure 36)249 and possibly by internal electrostatic considerations.184 The sc conformation, favored for monosubstituted /32- or /33-residues (especially if the substituents are bulky)263... 

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