Meaningful Atomic Charges

The meaning of n can be inferred from Eq. (5.4) where a measures, in a way, the sensitivity of charge variations to substituent effects. Small n values indicate strong substituent effects. If inductive effects did no exist, the charges would be those corresponding to n = oo (i.e., a = 0), and all H atoms would carry the same charge. No theoretical method leads to this extreme result. 

The corresponding correction for hydrogen is as follows, from obvious charge normalization considerations  

The parameter p is deduced from a multiple regression analysis using MuUiken charges and the appropriate experimental NMR shifts. For the STO-3G charges used in this example, it is p = 30.12 me. 

Energy calculations [108], on the other hand, offer an empirical but quite reliable evaluation 35.1 me, which is henceforth adopted. Similar considerations 

The comparison between the net atomic charges deduced from optimized 

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Now, the problems associated with the search for meaningful atomic charges, such as those required in applications of our bond energy formulas, are manifold. One... 

One way to decide if a trend in BE values within a given series of compounds is meaningful is to perform complementary experiments such as XANES. The absorption edges measured from XANES also depend on the same factors as in XPS, providing information about atomic charge and coordination environment [3,71]. 

L. C. Allen has suggested that a more meaningful formal charge can be obtained by taking into account the electronegativities of the atoms involved. Allen s formula for this... 

We conclude that the adiabatic mode intensities and effective charges derived from them are the localized counterparts of those effective charges derived from measured intensities. They should be more appropriate for the description of the properties of individual bonds. In particular, they should lead to chemically more meaningful effective charges where future work has to show how effective charges, atomic monopole and dipole contributions, and the charge flux are related. 

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