Dihedral angle, 156 Gaussian

Sources for bond lengths, bond angles, and dihedral angles include the published literature, standard references like the CRC series, and previous calculations. Z-matrices may also be created by the NewZMat utility from data generated by a wide variety of drawing packages. Refer to the Quick Start for a sample conversion operation for your version of Gaussian. 

Ulmschneider, J.P., Jorgensen, W.L. Monte Carlo backbone sampling for polypeptides with variable bond angles and dihedral angles using concerted rotations and a Gaussian bias. J. Chem. Phys. 2003, 118, 4261-71. 

A second theory proposes that the propagating radical exists with a Gaussian distribution of dihedral angles, centered about a single preferred conformation. 

Sometimes one wants to hold one or more geometrical parameters constant during an optimization. For example, to find the electronic energy [/ as a function of the torsional angle in butane (Fig. 15.13), we would do several calculations, in each of which the dihedral angle D(CCCC) is held constant at a different value, while all other internal coordinates are optimized. The procedure for doing this in Gaussian is described in Problem 15.41. 

Do include a space before each of the words Variables and Constants. (In Gaussian 94 and 98, the keyword Opt must be replaced by Opt=Z-matrix.) Set up a Z-matrix for a partial geometry optimization for C2H5 in which the dihedral angles are held fixed at values corresponding to (a) the staggered form (b) the eclipsed form. Then perform HF/6-31G optimizations on these two conformations and take the energy difference to find the predicted barrier to internal rotation in kcal/mol. 

Fig. 3. In bulk, polymer chains either form helices (with a regular sequence of dihedral angles along the chain) that are packed in a regular polymer crystal (a) or they are in a disordered amorphous state where the average chain can be described as a random Gaussian coil (b)...

Both the chemical shift map and the energy map reflect the symmetry of the molecule as can be seen especially for (i92,i93) = (t, t), trans states are less sensitive to small variations of the dihedral angle than gauche states. In the simulations of the NMR spectra, a unique Gaussian broadening function was used for all conformational states. By this assumption, the number of parameters was kept small, but in the maps it can be seen that this assumption is in fact a simplification. 

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