Weakly bound anions

Yourshaw I, Zhao Y and Neumark D M 1996 Many-body effects in weakly bound anion and neutral clusters zero... 

Figure 3.6. Model to interpret CS planes in figure 3.4, (001) projection (a) weakly bound anion vacancies (b) elimination of pairs of the vacancies by shearing by, e.g., a jb along an octahedral edge.

Yourshaw I, Zhao Y and Neumark D M 1996 Many-body effects in weakly bound anion and neutral clusters zero electron kinetic energy spectroscopy and threshold photodetachment spectroscopy of Ar Br" n = 2-9) and Ar r (n =... 

In addition, for a weakly-bound anion it may be necessary to consider the possibility that the molecule s vibrational motion could access regions of the potential energy surface where the anion is no longer thermodynamically stable with respect to electron ejection (autodetachment). For weakly-bound anions in the gas phase, autodetachment induced by rotational motion may also need to be considered. ... 

In view of this discussion, it is tempting to conceptualize the weakly-bound anions of polar molecules as dipole-bound anions. Solution of the Schrodinger equation for an electron interacting with a point dipole reveals that bound states are obtained for dipole moments g > 1.625 debye, ° ° with no further molecular structure required. (In practice this threshold should be modified to something like g > 2.4 debye, owing to the possibility of rotational-to-electronic energy transfer, but the point remains that... 

In this section, we discuss the ways in which weakly-bound anions place special demands on quantum chemistry calculations. It is presumed, in this discussion, that the anion M is a bound species (VDE > 0) at the molecular geometry in question, such that the application of bound-state quantum chemistry methodology is appropriate. Referring to the situation in Figure 1, bound-state methods are appropriate for the description of the anion AB only for R > R. For R < R, the neutral molecule is lower in energy, and application of bound-state methods to M is not appropriate. Electronic structure methods for temporary anion resonances are discussed later in this chapter. 

On the other hand, 6-31(l4-,3-l-)G is not appropriate for high-accuracy calculations of the most weakly-bound anions. To converge VDEs to within 0.00l eV of the basis-set limit, Skurski et al. have shown that seven... 

In addition, attempts to optimize the geometry of M sometimes access geometries in which the anion is unbound, which may lead to convergence failure in the SCF calculation. The regions of the potential surface where this problematic behavior occurs can be expected to proliferate as the VDE of M approaches zero. Such problems are usually avoided - for all the wrong reasons - by the use of overly compact basis sets, which is perhaps why the literature is rife with weakly-bound anion calculations using inappropriate basis sets. The VDEs reported in these studies are certainly incorrect, and the structures may be bogus as well. 

Figure 24 Radial plots of the SOMOs for the two weakly-bound anions from Figure 22, illustrating the / -> c algorithm of Ref. 228. The dotted regions would be encapsulated by a 10% isoprobability contour and the hashed regions by a 50% isoprobability contour. Adapted with permission from Ref. 228 copyright 2008 American Chemical Society.

That said, and while KT EAs do still find some utility in stabilization calculations of temporary anion resonances (as discussed later in this chapter), for bound states of M there is little reason to rely on KT since Hartree-Fock calculations are nowadays computationally facile on large molecules, often in large basis sets. It is therefore easy to compute a ASCF value for the EA, which includes the effects of orbital relaxation, simply by computing separately the Hartree-Eock energies of M and M , assuming that the latter is bound. (If it is not, then neither the KT nor the ASCE value of the EA is reliable.) This raises an important point, namely, that one obtains a positive EA from KT only when lumo < for very weakly-bound anions there may be... 

Much has been made of the critical role of electron correlation effects in the description of weakly-bound anions.As with any electronic structure problem, electron correlation is always quantitatively important and occasionally qualitatively important. Cases where correlation is qualitatively important include certain anions with very small VDEs, where dipole binding effects alone (which might be reasonably well-described at the Hartree-Fock... 

Composite methods are designed for equilibrium thermochemistry, not for mapping out potential energy surfaces, so we now return to a discussion of CC methods and consider some cases involving prediction of VDEs for weakly-bound anions. Consider the case of (H20)J, for example. The best available calculation for the VDE of the trans isomer (depicted in Figure 5)... 

---