Observation process common configurations

The traditional S 2 reaction is a concerted process, as illustrated in eq. (2.8). The electron pair of Nu hinds to the carbon at which displacement is occurring. simultaneously with the departure ol 1. . along with the electron pair ol the C — I, bond. In the transition slate for the reaction, iwo lobes ol an orbital of approximately p hybridization on (he central carbon overlap with orbitals of Nu and I. , respectively. A molecular orbital scheme would assign the iwo electron pairs noted above io the two lowest molecular orbitals (A and ID o the linear ihrce-center subsystem. Trout this picture, the observed inversion of configuration follows. Most commonly die... 

In the Varicol process, hnes are shifted asynchronously. In this case, the column distribution between zones does not stay the same during the period, because lines are shifted at different times, so that the allocation changes accordingly. Since the number of columns in one zone is not constant over a period of time, the configuration of columns contains non-integral numbers. For example, a configuration of O.5/1.5/1.5/O.5 in a 4 column system is possible. A Varicol process is more adaptable than an SMB one as more flexible options are available for the repartition of columns. It is commonly observed that Varicol is 15-25% more productive than SMB (typically 5 or 6 columns are used in Varicol , whereas 6 to 8 columns are used in SMB) [11, 18]. 

In order to further illustrate the scope of the NeNePo technique and the abihty of our theoretical approach to treat more complex systems, two examples, Ag4 and Au4, have been chosen for the presentation, because they exhibit qualitatively different structural properties in the anionic state and have common properties in the neutral state. In the case of the silver tetramer, the global minima of the anion and of the neutral cluster assume related rhombic structures. Therefore, after photodetachment at low temperatures (T 50 K), which ensures that only the rhombic isomer is populated, the pump step reaches the nonequilibrium rhombic configuration close to the global minimum of the neutral species, as shown on the left-hand side of Fig. 4. Notice that the well-defined initial structure is a necessary condition to observe the time scales of the processes involved in the geometric relaxation of the neutral state, and therefore the experiments should be performed at low temperatures. 

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