N-C torsional angle

As in the case of monocyclic polyether ligands, the preferred conformations about C-C and C-O bonds are synclinal and antiperiplanar, respectively. The C—N—C—C, C—C—N—C torsion angles may lie in either of these low-energy ranges. 

Cl-C-N-C torsional angle it is zero when the Cl-C bonds are in the same plane and syn to the N-C bond. 

In order to test the speed of the D C method relative to conventional MO, single-point PM3 calculations were carried out on a series of a-helix polyalanine structures. Standard bond lengths and bond angles were used for each peptide unit, with a C-C-N-C torsional angle of —57° and an N-C-C-N torsional angle of —47°. In the D C calculations, inner and outer buffer layers of 4.0 A and 2.0 A were used, along with a matrix cutoff of 7.0 A. All computations were performed on a 32-Mbyte IBM RS/6000 Model 370 running AIX version 4.1. 

Fig. 7.11 N-acetyl N -methyl alanine amide. Identification of the 4> (N-C(a)) and i / (C(a)-C ) torsional angles.

Now change the Start value for the Co-N-C-C torsion angle to 40° and repeat the calculations. This generates the next slice of the potential energy surface and the results from the . dat file should be added to the file containing the previous slice. 

The vibrational spectrum of the tetramethylammonium cation in the region 150 -550 cm l contains botii torsional and vibrational modes. The vg and V19 vibrational modes of E and T2 symmetry involve C-N-C bond angle bending. These modes are Raman active and have been studied for TMA+ in several zeolite environments, although little change in frequency is observed (51). The V4 and V12 torsional modes involve partial rotation about C - N bonds and form respectively a singlet (A2) and a triplet (Ti) which are both Raman inactive. These torsional modes are directly observed in the HNS spectra and prove to be sensitive to the character of the TMA+ cation (see Table 1) environment(52). 

The organic part consists also of four independent monomer units similar to the monomer structure of 1. The main differences in the chains can also be observed on the N-C-C-C torsion angles. The four independent monomers tend to align in a unique direction for packing optimization. 

Fig. 9.9. Histograms of C(0)-N-C-C torsion angles r (absolute values) for N-aryl amides (top secondary, bottom tertiary)...

Table 7. C-N - Se=C Torsion Angles for Spiral Ribbons in Selenourea...

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