Townes-Dailey model

We assume that, on formation of B- XY, a fraction 5j (i = intermolecular) of an electronic charge is transferred from the electron donor atom of Z of the Lewis base B to the npz orbital of X and that similarly a fraction 5p (p = polarisation) of an electronic charge is transferred from npz of X to n pz of Y, where z is the XY internuclear axis and n and n are the valence-shell principal quantum numbers of X and Y. Within the approximations of the Townes-Dailey model [187], the nuclear quadrupole coupling constants at X and Y in the hypothetical equilibrium state of B- -XY can be shown [178] to be given by ... 

Since the Townes-Dailey model is the basis of most interpretations, we shall discuss here the results of this theory. For simplicity, we consider a single bond between an element and a halogen atom. 

The paper is divided into sections as follows. First, ab initio calculations on Br2 and then some approximate supermolecule calculations on H3N Br2 are reported. Then the Townes-Dailey model for nuclear quadrupole coupling constants and its connection with response theory are briefly reprised. Finally the model is applied to the complex H3N" -Br2, and empirical and ab initio results are compared. 

The Townes-Dailey model is as follows. An isolated molecule X2 has a coupling constant xo(X). If intermolecular charge transfer and the effects of... 

Below -140, a sudden reversible shift to 28,8 and 29,3 Mc/sec is observed which suggests that a solid state ttansition is taking place. The average of the frequencies observed, at room temperature is consistent, according to the TOWNES-DAILEY model and assuming 15% s-hybridization, with a charge distribution of +0,61 e on Au and -0,40 e on Cl,... 

A simple semiempirical theory for the interpretation of nuclear quadrupole coupling constants in terms of chemical bonding was developed by Townes and Dailey 2 24). This theory is treated in many summary papers on NQR, e.g. 39 4S 94.i43.is9) An extension of the Townes-Dailey treatment was given by Cotton and Harris 113-123>. Besides these simple model theories, a variety of more elaborate calculations have been done on the influence of intramolecular bonding on NQR spectra (see the discussion in 143- S9)). 

The C1 n.q.r. frequencies have been reported for the two trichloride salts Ph4AsCl2 and Et4NCl3. Using the Townes and Dailey model, the negative... 

The crucial point in the determination of the quadnipole coupling constants is the computation of the electronic contributions which in principle should include all the core as well as the valence electrons. The computation may be carried out at various levels ranging from empirical Townes-Dailey (TD) type models to ab initio molecular orbital calculations In the following section, we shall discuss two simple approaches which are often used to interpret nuclear quadnipole data of central ions and nitrogen ligands in transition metal compounds ... 

NQR data yield primarily informations on the p electron densities in the valence shells, from which the net resulting charge on the investigated atom can be easily inferred by means of some simple models, e.g. of the TOWNES-DAILEY theory. Usually the accuracy of experimental measurements is rather high, and is at present not matched by equally powerful and accurate interpretative theories. Therefore at this initial state of application in the field of coordination chemistry we are devoting more attention to the experimental results of NQR spectroscopy, their immediate interpretation and their close connection with X-ray structural data. 

In the following sections, we will first develop a set of equations (Townes-Dailey method) relating the observed NQR parameters to a bonding model of the nitro-group. Then we will use these equations to interpret the TNT data, and show how additional NQR data, much of it new, on molecules similar to TNT supports our interpretation. 

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