Orbitals fragment

ADF uses a STO basis set along with STO fit functions to improve the efficiency of calculating multicenter integrals. It uses a fragment orbital approach. This is, in essence, a set of localized orbitals that have been symmetry-adapted. This approach is designed to make it possible to analyze molecular properties in terms of functional groups. Frozen core calculations can also be performed. 

For a theoretical consideration of the metal-silicon interaction in silylene complexes, the fragment orbital description proves to be very useful [148], This approach has been extensively used in the organometallic chemistry of carbon and allows a basic understanding of the interrelations also by means of a qualitative description. 

Employing a C2 symmetry in the case of the thiirene 1-dioxide and remembering that the spiro-operator that mixes the fragment orbitals gives nonzero matrix elements only if these orbitals are symmetric to the C2 operation53, the net result is stabilizing. On the other hand, thiirene 1-oxide suffers a homoconjugative destabilization. 

A series of papers by Shustorovi ch(63) and/or Baetzo1d(64) summarized in a recent article(65) have addressed the problem of chemisorption on metal surfaces in terms of electron accepting and donating interactions. Saillard and Hoffmann (66) developed qualitatively identical pictures of these interactions but starting from fragment orbital type analysis. These papers are only a few of the theoretical discussions that consider hydrogen activation, however we will use their approach because it address the problem in a fashion that can interpolate between the organometallic cluster and the bulk. 

Fig. 2 Simplified electronic interaction diagram between the empty Cp2Ti and occupied dithio-lene fragment orbitals. The interaction is stabilizing only when 0/0...

Scheme 6 Extended Htickel fragment analysis of the interaction between the partially occupied Cp2M and occupied dithiolene fragment orbitals in d1 [Cp2M(dt)]+ complexes, M = Mo, W (adapted from [69])...

Of course, it is the entire molecule that receives an electron on reduction. However, the nitro group is the part where the excess electrons spend the majority of their time. Consideration of quantum-chemical features of the nitrobenzene anion-radical is of particular interest. The model for the calculation includes a combination of fragment orbitals for Ph and NO2, and the results are represented in Scheme 1.1. The left part of the scheme refers to the neutral PhN02 and the right part refers to the anion-radical, PhN02 (Todres 1981). 

This chapter considers ion-radicals with detained and released single electrons. Some ion-radicals contain fragment orbitals that suspend an unpaired electron preferentially. Other ion-radicals are characterized by the delocalization of an unpaired electron along orbitals, which encompass the whole molecular framework more or less evenly. This chapter considers the material, comprised from this point of view, using the terms detained and released electron. Such an abstraction helps us to analyze these two intrinsic features of organic ion-radical reactivity. 

The ketone receives an electron in the antibonding n MO to form n anion-radical, which is transformed into a (-C-C1) moiety by an intramolecular electron transfer to the antibonding a fragmental orbital of the -C-Cl bond. The (-C-C1) moiety then fragments to form a chloride ion and the corresponding radical (Scheme 7.64). 

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