Quantum Chemical Descriptors for Basicity Scales

In this method, the Lewis bases are characterized by three HF/6-31G computed properties  

-Pvmin and 7s,min are closely related to the electrostatic, polarization and charge-transfer components, respectively, of the Morokuma decomposition of proton affinity. It has been shown [197] that the spectroscopic scale Ziv(OH) (related to methanol), the phenol affinity, the diiodine affinity and the proton affinity of 42 nitrogen, oxygen and sulfur bases can be correlated by the triple-scale Equation 1.133  

The regression coefficients a, p and y, the intercept s and the statistics are given in Table 1.22. Taking into account the variety of bases and BDPs, the correlations are satisfactory. The calculation of affinity scales purely from the electronic structure of bases appears a promising method. 

An even better method, but of greater computational difficulty (see above), is the calculation of the properties of model complexes. For example, AEe, AH or AG calculated for the complexafion of the small hydrogen-bond donors H2O [198] or HE [199] were found to be successful descriptors of the basicity towards larger ones (that is of greater computational cost) such as 4-FC6H4OH. 

Ultimately, the affinity or the basicity scales themselves might be computed. This has been done successfully for the methanol affinity in the gas phase [41]. However, the MP2/aug-cc-pVTZ//B3LYP/6-3H-G(d,p) costly level required to obtain good agreement with experimental affinities has limited the scale to a few small Lewis bases. Proton and cation affinities and basicities in the gas phase are now computed on a routine basis (see Chapter 6). Nevertheless, the size of Lewis bases and cations and the number of bases studied are inversely proportional to the level of theory, that is, to the agreement with experimental data. As far as extended solution basicity scales towards usual Lewis acids 

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