Predicted Valence States

The following equilibrium term energies T and angles tte have been derived from two theoretical calculations  

Apparently from a bending potential energy curve for an optimized P-H bond length. - From the total energy at optimized geometry. 

The vertical excitation energy was calculated by the MRD-Cl method to be AE = 5.50 eV [50]. AE = 9.3 eV (75000 10000 cm ) was estimated from the spin-rotation interaction constant of ground-state PHg and PDg [35]. 

Two further states, 2 and 2 result from the excitation of an electron from the 2bi(27iu) MO into the unoccupied 6ai(40g) and 3b2(30u) MO s. Electronic configurations for bent (Cgv) and linear (D h) shapes, equilibrium term energies Te, and angles ae from UHF calculations 

---

As was the case with lanthanide crystal spectra (25), we found that a systematic analysis could be developed by examining differences, AP, between experimentally-established actinide parameter values and those computed using Hartree-Fock methods with the inclusion of relativistic corrections (24), as illustrated in Table IV for An3+. Crystal-field effects were approximated based on selected published results. By forming tabulations similar to Table IV for 2+, 4+, 5+ and 6+ spectra, to the extent that any experimental data were available to test the predictions, we found that the AP-values for Pu3+ provided a good starting point for approximating the structure of plutonium spectra in other valence states. However,... 

Prediction of the energy level structure for Pu2+ (5f ) is of particular interest since no spectra for this valence state of Pu have been reported. On the basis of what is known of the spectra of Am2+ (26), Cf2" (27), and Es2+ (28), there appears to be evidence for a very small crystal-field splitting of the free-ion levels. Such evidence encourages use of a free-ion calculation in this particular case. The parameter values selected are indicated in Table V. Based on the systematics given by Brewer (19), the first f- d transition should occur near 11000 cm-, so the f- -f transitions at higher energies would be expected to be at least partially obscured. A... 

The E-state indices [72, 73] were developed to cover both topological and valence states of atoms. These indices were successfully used to build correlations between the structure and activity for different physicochemical and biological properties [72]. New applications of this methodology are also extensively reviewed in Ghapter 4. Several articles by different authors demonstrated the applicability of these indices for lipophilicity predictions [74—83]. 

Electronic ligand effects are highly predictable in oxidative addition reactions a-donors strongly promote the formation of high-valence states and thus oxidative additions, e.g. alkylphosphines. Likewise, complexation of halides to palladium(O) increases the electron density and facilitates oxidative addition [11], Phosphites and carbon monoxide, on the other hand, reduce the electron density on the metal and thus the oxidative addition is slower or may not occur at all, because the equilibrium shifts from the high to the low oxidation state. In section 2.5 more details will be disclosed. 

The purpose of this scale is to aid in qualitatively predicting the effect of 3d TM ion chemical substitutions on the valence of Mn in octahedral or tetrahedral sites of a ccp oxide. This scale can be useful for choosing chemical substitutions that will keep octahedral Mn in a relatively immobile valence state (i.e., near +4) over the range of an electrochemical cycle where the coexistence of Mn + and Li vacancies would allow the rapid transformation of a metastable ccp oxide (e.g., as with 7-LLMn02 or o-LLMn02). 

Electron configuration of an atom indicates its extranuclear structure that is, arrangement of electrons in shells and subshells. Chemical properties of elements (their valence states and reactivity) can be predicted from electron configuration. 

Whenever the atoms under consideration in a given molecule are in the same valence states as in the reference molecule, the relaxation process is such that the potential created by the other atoms at the kth nucleus is the same as would be predicted by leaving the pertinent intemuclear distances and the shapes of atomic electron densities as they are in the reference molecule, with the electron populations changed as required by the new situation. 

With atoms such as carbon and silicon, the valence-state electronic configuration to form four covalent bonds has to be (s)1(px)1(py)1(ps)1. Repulsion between the electron pairs and between the attached nuclei will be minimized by formation of a tetrahedral arrangement of the bonds. The same geometry is predicted from hybridization of one s and three p orbitals, which gives four sp3-hybrid orbitals directed at angles of 109.5° to each other. The predicted relative overlapping power of s -hybrid orbitals is 2.00 (Figure 6-10). 

The mode of 02 coordination was for some time a subject of controversy. It now seems settled that the ligand molecule is bonded via one O atom, with an Fe-O-O angle of c. 120°. This is consistent with the prediction of a rather crude VB description of the 02 molecule, which allocates two lone pairs to each atom the valence state of O is (sp2)2(sp2)2(sp2)1(p)1. This description is inconsistent with the observed paramagnetism of 02 (which can readily be explained in terms of MO theory). However, oxygenated haemoglobin Hb.402 is diamagnetic and we may fairly describe the Fe-0 bond in terms of a filled sp2 hybrid donor orbital from the O atom. 

Predicting Transition State Structures with the Valence Bond State Correlation Diagram Model... 

The predicted course of reaction between a heteronuclear pair of atoms is shown in Figure 7.2. Promotion is once more modeled with isotropic compression of both types of atom. The more electropositive atom (at the lower quantum potential) reaches its valence state first and valence density starts to migrate from the parent core and transfers to an atom of the second kind, still below its valence state. The partially charged atom is more readily compressible to its promotion state, as shown by the dotted line. When this modified atom of the second kind reaches its valence state two-way delocalization occurs and an electron-pair bond is established as before. It is notable how the effective activation barrier is lowered with respect to both homonuclear (2Vq)i barriers to reaction. The effective reaction profile is the sum of the two promotion curves of atoms 1 and 2, with charge transfer. 

Coulson has recently discussed the bond angles in these molecules in a review paper,550 using two simple models. An ionic model would predict these molecules to be linear, but in a second model, in which valence states arising from s2- sp, s2- sd excitations were permitted, the first would lead to linear, the second to bent structures. From a consideration of the excitation energies, it was shown that when the central atom is heavy and the attached halogens are as electronegative as possible, the bent situation is favoured, in agreement with the experimental results. 

---