Lewis structure open-shell

The Lewis structure concept may superficially appear to lose its usefulness for open-shell species. The electrons of radical or excited-state species cannot be strictly paired as in a conventional closed-shell Lewis structure diagram. Moreover, the concept of structure itself seems to lose its validity in many radical species, which tend to be characterized by floppiness, large-amplitude vibrations, and general lack of structural rigidity compared with closed-shell species. The reactivity and instability of radical species might seem to preclude a useful role for the Lewis structural concepts. 

However, we can recognize that the essential feature of a Lewis structiu-e is the localized 1-c, 2-c bonding pattern rather than electron pairing per se. Moreover, the a and 3 electrons of open-shell species necessarily experience different Coulomb and exchange forces and may hence lead to different spatial distributions, spin orbitals, and localization patterns. The familiar open-shell concept of different orbitsds for different spins can therefore be extended to a different Lewis structures for different spins (DLSDS) picture, where as usual we associate a Lewis structure with a specified pattern of one-center and two-center (spin-)NBOs. Because the NBO... 

For open-shell systems, where 1-electron density matrices for a and spin sets are no longer equivalent, it is straightforward to analyze each spin density separately for the distinct hybrids and Lewis structures of opposite spin. This emphasizes that the NBO Lewis structure concept refers to a particular localized bonding pattern of one- and two-center interactions rather than to electron pairing per se. Since Coulomb and exchange interactions for a and fi electrons differ appreciably in open-shell systems, the associated spin polarization effects are most naturally described in terms of different Lewis structures for different spins, the default NBO procedure. 

OPEN-SHELL LEWIS STRUCTURES AND SPIN HYBRIDS... 

Open-Shell Lewis Structures and Spin Hybrids 83... 

Table 4.4 Optimal spin-NBOs for spin-Lewis structures of molecular O2 [Text (4.46a,P)], showing occupancy (and parenthesized degeneracy) for each distinct Ic (no) or 2c ((Too, T oo) feature of the open-shell Lewis structure.

How can we understand the chemical origins of these spin density patterns The starting point is the different Lewis structures for different spins NBO description of open-shell systems (cf. Section 4.5), which leads to the two distinct spin NBO representations of the Tempone nitroxide bonding pattern, as shown in (7.6a,p) ... 

In the vertical (trans conformer) geometry of the ground singlet species, the open-shell NBO Lewis structures of the excited triplet species are found to be represented by... 

Abstract The concept of stable superatoms, molecular species which mimic the shell closures emphasised by Lewis and Kossel, has become an important paradigm of stability in cluster chemistry. In this review we discuss recent work, both experimental and theoretical, on the family of endohedral clusters M Ex, where M is a transition metal ion and E is a member of group 14 (Si, Ge, Sn, Pb). The structural chemistry within this family is very varied, ranging from deltahedral motifs for the heavier tetrels to open 3-connected stmctures such as the hexagonal prism in Cr Sii2- We explore the arguments that have been presented to rationalise these structural trends and their implications for chemical bmiding. 

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