Ground state covalent character

Figure 1. Schematic diagram of the energies of the anion ground state (M S), the lowest covalent state (M-S ), ion-pair state (M S ) and cation ground state (M+S ) versus /j M = alkali metal atom S = solvent molecules). The ion-pair state is expected to correlate with the ground state of the solvated electron plus the solvated M+ ion in bulk fluids. The ground-state electronic character of the neutral cluster changes from covalent to ion-pair type at a certain critical size.

A larger covalent radius such as of sulfur, selenium, or tellurium reduces the ring strain d-orbital participation in thiophene is not significant in the ground state, and thiophene exhibits a pronounced aromatic character that is substantiated by its physical and chemical properties.103... 

So far we carried out only a very preliminary study of the excited states [62] for four typical 7r-electron systems CBT, benzene, hexatriene and naphthalene. In all cases we found that the PPP-VB method can successfully describe the low lying excited states using only a small subset of VB structures. For states of a predominantly covalent character, the same OEAO basis as for the ground state can be used. However, for higher lying states with significant ionic character, a re-optimized OEAO basis is preferable. 

Covalent bonding depends on the presence of two atomic receptor sites. When the electron reaches one of these sites its behaviour, while in the vicinity of the atom, is described by an atomic wave function, such as the ip(ls), (l = 0), ground-state function of the H atom. Where two s-type wave functions serve to swap the valence electron the interaction is categorized as of a type. The participating wave functions could also be of p, (l = 1), or d, (/ = 2) character to form 7r or 6 bonds respectively. The quantum number l specifies the orbital angular momentum of the valence electron. A common assumption in bonding theory is that a valence electron with zero angular momentum can be accommodated in a p or d state if a suitable s-state is not available. The reverse situation is not allowed. 

The very low isomer shift of Na2[Fe(CN)5NO] 2H2O, a compound that has been often used as calibration standard, can be understood on the basis of the strong covalent effects present in the Fe-NO bond. The ground state of the iron in the nitropnisside molecules is (Ax, (Axy) The electrons in the orbitals with (d, dj, ) character are delocalized by back donation to the 2p orbitals of the NO ligand. In addition, as in the cyanides shown in Table 2,... 

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