Hiickel-Lewis methods

Abstract A simple Hiickel Hamiltonian is used and modified to describe localized states, where the electron pairs are confined to bonds between two atoms, or to lone pairs. The electronic delocalization can be considered either as a mixmre of these localized states, or through a standard Huckel calculation. The two Huckel-Lewis methods described here attempt to find the coefficients of the mixture, based on energy or overlap consistence with the standard Huckel results. After the description of the two methods, test examples are used to show advantages and drawbacks of the different approaches. In any case, the results are compared to the NBO-NRT approach which is used on the electronic density obtained from standard DFT hybrids calculations such as B3LYP/6-31+G(d). This chapter ends with an introduction to the HuLiS program in which the two methods are implemented. 

This section introduces the context and the notation of the Hiickel-Lewis (HL) methods. The notations that follow can be found in Table 13.1. 

The molecular orbitals obtained by the Huckel method are delocalized over the molecule, and are used in a Slater determinant to build the reference wave function i Pref) of our Hiickel-Lewis family of methods. Hence, for ethylene = l tSl and for a four-7t-electron molecule likeacrolein, it reads Thus, we... 

HL-CI The Hiickel Lewis Configuration Interaction (Cl) is formally equivalent to the Hiickel method for the interaction between several localized structures one can diagonalize a Hamiltonian in the basis of localized Lewis structures. In the following, this will be referred to as the energy based scheme. 

We want to compare here the weights of each Lewis structure given by the two different Hiickel-Lewis schemes (HL-CI and HL-P) discussed in Sect. 13.2 with those obtained by the NRT method [9, 13, 14] (see Table 13.2). 

Equation 2 was proposed by Lewis and Randall (6) even before the advent of the Debye-Hiickel theory for the evaluation of E° by the extrapolation of LHS of Equation 2 against f(m) because by definition y —> 1 as m —> 0. However, such extrapolations failed to yield precise values of E° because the plots did not produce the desired straight lines. The most common method of extrapolation is to substitute an expression for ln y from the Debye-Hiickel theory (7), and the two most frequently used substitutions are... 

Understanding Delocalization Lewis Drawings and Mesomerism Versus Molecular Orbitals (Hiickel) Method... 

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