Potential surface single-minimum

This particular potential energy surface seems very clean-cut, because there is a single minimum in the range of variables scanned. The chances are that this minimum is a local one, and a more careful scan of the potential surface with a wider range of variables would reveal many other potential minima. 

Pc-Cu samples as well as single-crystal planes have been studied564-587 in contact with various aqueous electrolyte solutions. The data are somewhat controversial, since the main experimental difficulty with Cu is its great tendency to surface oxidation. The potential of the minimum in C,E... 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

The presence or absence of a homoaromatic interaction is often based solely on the distance between the non-bonded atoms. Distances greatly over 2.0 A are thought to lead to a p-p overlap that is too small to make any significant contribution. This simplistic approach is not necessarily reliable as shown by Cremer et al. (1991). Their calculations on the homotropylium cation [12] indicate a double-minimum potential energy surface with respect to variations of the C(l)-C(7) distance at the Hartree-Fock level of theory. At the MP4(SDQ) level of theory, only a single-minimum curve was found with the minimum at 2.03 A. The calculated potential energy curves are quite flat in this region. 

Figure 1. Hypothetical single-minimum potential surface for ion-molecule...

In this complex, the energy gap 2J separating upper and lower potential surfaces is estimated to be 0.19 eV. This is evidently too large for the weak-interaction limit to hold. In other systems, J may, in fact, become so large that we have a single minimum, with a stationary delocalized electronic ground state. 

Interestingly, some systems, such as (1) itself, show a tautomeric equilibrium between an H2 complex and a classical dihydride form others show a stretched H2. In the first case we have a double minimum on the potential energy surface (PES) (H2 and dihydride) and in the other a single minimum. The difference appears to lie in the motion of the heavy ligands that produces a barrier to the dihydrogen/dihydride tautomerism and where there are no such heavy atom rearrangements, the barrier disappears and a stretched H2 becomes possible. 

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