Highly symmetric reaction paths

Figure 2. Initially, in a thermal reaction, there are two electrons in each of the ethylene -orbitals, and it is apparent that if the reaction follows the symmetrical reaction path, the initial state correlates with a highly excited state of the product. Configuration interaction between the two Aig states leads to an avoided crossing, but there is still a considerable activation energy (Figure 3a). The thermal reaction is...

The work on the formic acid dimer focused on the double-well potential for a highly symmetric system. An attempt to locate a double-well potential for a less symmetric system was made by Zielinski and Poirier (1984). They studied the formamide dimer and isolated a possible structure for the transition state for a double-proton transfer along the reaction path to the formimidic acid dimer (a dimer of the enol form of formamide) using the 3-21G basis set. The proposed transition state is only slightly less stable than the formimidic acid dimer. In other words, a very asymmetric double-well potential was found with a very shallow well on the formimidic acid dimer side of the reaction. It will be interesting to see the shape of the function for a double-proton transfer between formamide and amidine, which would more closely mimic the double-proton transfer that may be possible for the A-T pair. 

The 0 K IR spectrum in the range 3600-3800 cm i is shown in Fig. 3.12 and is presented with no energy shift. As seen, the spectrum is fairly complex but does show intensities in qualitative accord with experiment, shown in Fig. 3.6. Our analysis indicates that the relatively intense lines at 3620 cm are the monomer OF -symmetric stretch and those at 3710 cm i are the asymmetric stretch. We stress that these are preliminary results and likely not well converged. More accurate DMC and Multimode-reaction path calculations for the four monomer stretches indicate that the single reference energies are 40-100 cm high [28]. 

For another perspective we mention a second approach of which the reader should be aware. In this approach the dividing surface of transition state theory is defined not in terms of a classical mechanical reaction coordinate but rather in terms of the centroid coordinate of a path integral (path integral quantum TST, or PI-QTST) [96-99] or the average coordinate of a quanta wave packet. In model studies of a symmetric reaction, it was shown that the PI-QTST approach agrees well with the multidimensional transmission coefScient approach used here when the frequency of the bath is high, but both approaches are less accurate when the frequency is low, probably due to anharmonicity [98] and the path centroid constraint [97[. However, further analysis is needed to develop practical PI-QTST-type methods for asymmetric reactions [99]. 

The highly symmetric spiro cation 57 has been shown to afford only the fused dibenzazepine-dibenzazocine derivative 58 (and none of the regioisomeric amine 59) on treatment with the phosphazene base P4-t-Bu. This is the product of a [1,2]-Stevens rearrangement of the intermediate ylide, the geometry of which is important in determining reaction path selectivity. In the presence of enantiopure BINPHAT as a counterion, the rearrangement led to some enantioselectivity in the reaction <04SL1565>. 

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