Chemical potential nucleophilicity index

The nucleophilicity scale based on the stretching frequency of the hydrogen bond between a nucleophile and an acid (Nu-HX) 97 was tested against experiment using the hard acids HF, HCN, and BF3 and the soft acid BH3.98 The correlation with the hard acids is excellent but fails when a soft acid is used. A new nucleophilicity index, or = 7 (/xa - /t. )2/I M + b)2] 7a, where //A, and //B, are the chemical potentials of the nucleophilic and electrophilic molecules, respectively, and ijA and i B are their respective hardnesses, has been proposed. This gives the relative nucleophilicity that is... 

The electronic chemical potential /x, chemical hardness 17, and global electrophilicity 10 for the dipoles 83-86 are displayed in Table 11. Also included in Table 11 are the values of local electrophilicity and the values of the Fukui function for an electrophilic attack and for a nucleophilic attack fk at sites k for these dipoles. The two dipo-larophiles present similar electrophilicity values, 1.52 eV (14) and 1.49 eV (15) (see Table 1). According to the absolute scale of electrophilicity based on the co index,39 these compounds may be classified as strong electrophiles. 

B3LYP/6-31 lG(d) calculations have used frontier molecular orbitals, chemical potential, and Pearson s electrophilicity index co to study the reactions between allylic and aliphatic alcohols and ethylacetoacetate. All three methods predict the correct product substitution by the R group of the alcohol at the methylene carbon when the alcohol is electrophilic, and transesteriflcation of the ethylacetoacetate when the alcohol is nucleophilic. The results agree with the existing experimental evidence. 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

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