Potential surfaces torsional

FIGURE 9. MM2-85 calculated torsional potential surface for the two-angle driver of the side chain of (S)-4-hydroxy-2-[(dimethylamino)methyl]indan 70. Reproduced with permission from Reference 108... 

Fully relaxed single-bond torsional potentials of oligothiophenes 16 (n = 0-2) under the interaction of the parallel external electric field (EF) constructed by point charges have been evaluated with semi-empirical AMI and PM3 calculations <2004SM(145)253>. Consistent evolutions of the torsional potential surfaces have been observed for three lineal oligothiophenes (Figure 43) as the EF increases. In particular, the equilibrium molecular geometries are deformed toward planar conformations, and the torsional barriers around the central bond are elevated. These... 

Fig. 4.5 Schematic drawing of ground- and excited-state potential surfaces along the 11 -ene torsional coordinates of the chromophore of rhodopsin (A), 8-membered rhodopsin (B), 7-membered rhodopsin (C) and 5-membered rhodopsin (D). This figure is modified from Mizukami et al. [40].

The temperature dependence of the Pake pattern can be used to deduce that the bound dihydrogen ligand undergoes a torsional or hindered rotation motion around an axis perpendicular to the metal-dihydrogen axis. The bound hydrogen is characterized as a rigid planar rotator. In some cases, the potential surface for this rotation can be characterized by these measurements. 

As an example, our first move in this direction has been to solve for the restricted-rotor energy levels of the CH3-NC/CH3-CN system given a theoretical potential for the torsional motion, and the calculated rate constant is very similar to that given in [80.P1] in this trial [82.C] the remaining molecular frequencies were taken to be the experimental ones, but there is no reason to think that equally acceptable ones could not have been generated from an ab initio potential surface. We expect to... 

Thus, there are 3iV - 6 coordinates that represent vibrational degrees of freedom of the molecule, and they are necessary to define its structure. If we want to know its structure, what we have to determine are the 3A - 6 coordinates that define the positions of all of the atoms when the molecule is in its ground state. If the molecule is a little bit complicated, there may be several different stable structures (conformations). In that case we will wish to know the sttuctures of all of the conformations. We will also need to know their relative energies so that we can calculate a Boltzmann distribution among them. Also, we frequently wish to know the torsional barriers on the potential surface that separate these structures from one another. 

Figure 8.3. Ramachandran plot of the CBA potential surface by MM40 (without the external anomeric torsional effect). (From Lii, Chen, Johnson, French, and Alllnger. Copyright 2005 by Elsevier. Reprinted by permission of the authors.)...

Figure 3.29 Model potential surface for isomerization of a cyanine dye. The branching space is shown in Figure 3.28(c). The x-axis corresponds to the reaction coordinate X3), in this case a complex mixture of conrotatory and disrotatory C=C torsion. Notice that the reaction path is quasi-parallel to the seam itself. From Figure 3.28(c), the branching-space vectors correspond to skeletal deformation rather than torsion. The reaction path encounters the seam at the point MEP-CI where one C=C bond is rotated 90°. (See the color plate.) Adapted from Hunt and Robb. ...

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