Delocalization term

It was proven [96] that the molecular polarizability can be written as a sum of intrinsic atomic polarizabilities of the atoms in the molecule and a charge delocalization term. Thus, the xy element of the molecular polarizability tensor of molecule A can be decomposed as... 

Is it not true that the most naive interpretation of the estimates of electrostatic and delocalization energies for ice (+ 8 and + Skoal, respectively) suggest, not that the interaction is essentially electrostatic, but rather that the electrostatic and delocalization terms are of about equal importance ... 

A HF is further divided into Heitler-London (A HL) and delocalization term... 

It is noteworthy that the delocalization term, also referred to as the deformation component [20], contains charge transfer and polarization effects. The 4p term appearing in (20.3) comprises electron correlation corrections ... 

For a better understanding of the nature of the adsorption forces between TNB and the siloxane surface of clay minerals, the decomposition scheme of Sokalski et al. [199] was applied. The results of such energy decomposition are presented in Table 6. They are in complete agreement with qualitative conclusions presented above. One may see that two dominant attractive contributions govern the adsorption of TNB. As it is expected, one is an electrostatic contribution, and the other one is contribution, which includes components that originate from the electronic correlation. The electronic correlation related contributions include the dispersion component and a correlation correction to electrostatic, exchange, and delocalization terms of the interaction energy. 

Heisenberg portion of the Hamiltonian in earlier work (9) for the delocalization terms we assume that a single parameter B characterizes resonance interactions between each pair of sites. Hence, the spin Hamiltonian matrix becomes ... 

V.M.S. GIL and J.N. MURRELL, The relative importance of delocalization terms in proton spin-spin coupling constants. Theor. Chim. Acta, 4,114 (1966). F.B. VAN DUIJNEVELDT, V.M.S. GIL and J.N. MURRELL, The calculation of directly bonded CH and CC coupling constants using delocalized molecular orbital theory. Theor. Chim. Acta, 4, 85 (1966). 

The popular concept of bonding taking into consideration the ionic and covalent nature of chemical bonds agrees well with the partitioning presented above. Electrostatic forces represent ionic, while the delocalization term represents the covalent character of chemical bonds or interactions. Modem studies often consider the exchange repulsion interactions as a third component of the ionic-covalent concept of bonding [38]. 

In chapter i we discussed the interaction between two widely separated water molecules and concluded that it could conveniently be treated in terms of (i) the interaction between the permanent electric moments of the molecules, (ii) the polarization or delocalization terms due to the distortion of the electron cloud of one molecule by the field of the other, (iii) dispersion forces having their origin in co-ordinated electronic motion in the two molecules, and (iv) the repulsion due to the action of the Pauli exclusion principle when the two electron clouds begin to overlap. Most calculations of the cohesive energy of the ice crystal amount to an evaluation of these contributions for more or less realistic models. Among these it is convenient to distinguish two different types ... 

When the nitrogen atom and its three substituents lie in the same plane (e.g. in I and in II), the singly occupied molecular orbital (SOMO) is a pure n orbital, and the nitrogen atom lies in its nodal plane. As a consequence, according to (1) the nominally unpaired orbital cannot give any direct contribution to hfs (delocalization term). However, the presence of an electron with unpaired spin (say a) in the n system polarizes the electrons of the a system, d causes a slight excess of a electrons at the nitrogen nucleus in a planar / NO moiety only this spin polarization (indirect) effect contributes to A. ... 

Results presented in the Table 6 indicate that the physical nature of pKa shifts is dominated at least in 99.8% by electrostatic term, with very small contribution of delocalization term not exceeding 0.2 %, whereas exchange component is completely negligible. Inexpensive atomic multipole estimates of the electrostatic term ApK may reasonably approximate costly SCF... 

---