Rotation saddle point

A low AH for a cooperative cluster rotation allows excitation of a cluster of atoms from normal to saddle-point positions. Such an excitation may, in turn, lower the energy of the saddle-point sites relative to the normal sites, thus effectively introducing a AHg(T) that collapses in a smooth transition. At temperatures T> T, the mobile ions become disordered over the normal and saddle-point sites. Such a situation appears to be illustrated by stoichiometric LijN and PbFj (Goodenough, 1984). 

Figure 6 The calculation of the effectiveforce in the Dimer method. A pair of images, spaced apart by a small distance, on the order q/ 0.1A is rotated to minimize the energy. This gives the direction of the lowest frequency normal mode. The component of the force in the direction of the dimer is then inverted and the minimization of this effectiveforce leads to convergence to a saddle point. No reference is made to the final state.

Figure 14. (a) Potential-energy surfaces, with a trajectory showing the coherent vibrational motion as the diatom separates from the I atom. Two snapshots of the wavepacket motion (quantum molecular dynamics calculations) are shown for the same reaction at / = 0 and t = 600 fs. (b) Femtosecond dynamics of barrier reactions, IHgl system. Experimental observations of the vibrational (femtosecond) and rotational (picosecond) motions for the barrier (saddle-point transition state) descent, [IHgl] - Hgl(vib, rot) + I, are shown. The vibrational coherence in the reaction trajectories (oscillations) is observed in both polarizations of FTS. The rotational orientation can be seen in the decay of FTS spectra (parallel) and buildup of FTS (perpendicular) as the Hgl rotates during bond breakage (bottom). 

Electron correlation effects have been found to be also significant as far as the internal rotation of thioamides is concerned57. MP2/6-31G(d) geometry optimizations57 indicated the eclipsed conformer (3) as the stable structure of thioacetamide, while this conformer was predicted to be a first-order saddle point at the HF/6-31G(d) level57. 

One important class of exceptions to the preceding discussion is provided by prereactive or postreactive van der Waals (vdW) complexes. The lifetimes for such resonances can be much longer than that for conventional transition state resonance trapped near the saddle point. In the discussion of the F+HCl reaction below, we shall see that resonances of this sort can have lifetimes much longer than the rotational period of the complex and thus may show INR signatures. 

K.T. Chung, B.F. Davis, Saddle-point complex-rotation method for resonances, Phys. Rev. A 26 (1982) 3278. 

The PES for rotation around the C—C bond of 58, M=Si, Ge (Figure 18) shows two minima the s-trans and the gauche, while the s-cis structure is a saddle point that connects two gauche enantiomers. The gauche rotamer is by ca 4333 and 3 kcal mol-1335 for 58, M = Si, Ge, respectively, less stable than the s-trans rotamer, an energy difference very similar to that in 1,3-butadiene. However, the rotation barrier about the central C—C bond in 58, M = Si and Ge, of ca 10-11 and 9.5 kcalmol-1, respectively, is... 

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