Probing the Reactive Center for Site Selective Protonation in a Molecule by the Local Density Functional Descriptors

Probing the Reactive Center for Site Selective Protonation in a Molecule by the Local Density Functional Descriptors

Sandip Kumar Rajak (University of Kalyani, India), Nazmul Islam (University of Kalyani, India) and Dulal C. Ghosh (University of Kalyani, India)
DOI: 10.4018/978-1-4666-1607-3.ch009
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In this study, the authors have explored the efficacy of the local density functional descriptors like the fukui functions (f), the local softness (s) and the local philicity as probe for the reactive centers and site selectivity of the chemico-physical process of protonation of some molecules having multiple site for protonation, viz CH3NCO (Methyl isocyanate), CH3NCS (methyl isothiocyanate), NH2OH (hydroxyl amine), NH2OCH3 (o-methylhydroxylamine), CH3NHOH (N-methylhydroxylamine), NH2CH2COOH (glycine), CH3CH(NH2)COOH (alanine) and OHCH2CH2NH2 (ethanol amine). The authors have seen in terms of the numerical values of the local descriptors measures the reactivity (nucleophilicity) of a particular atomic site of a donor center towards a proton. In all cases, it can be said that the dynamic chemico-physical process of site selectivity can nicely be correlated in terms of the computed values of the local descriptors. Thus, it is found that the theoretical descriptors of the DFT can be efficiently exploited to study the mechanism of site selectivity in a chemical reaction.
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The Preferred Site Of Protonation

If a molecule has more than one donor sites i.e lone pairs and if a proton approaches such a molecule, the proton will not find all the donor sites to be attacked equally likely. The proton will select the most preferred site in the structure of the molecule. In the dynamic process of protonation reaction, the preferred site may be identified by the attaching proton in a kinetic and thermodynamic process. But the selection of preferred site will be decided by the thermodynamically controlled process. When the proton is fixed at the preferred site, the enthalpy change is more and when the proton fixes at non-preferred site liberation of energy is less. The procedure follows the hierarchical steps: (i) calculate energy of the molecule first, and (ii) then attach proton at different probable sites to generated protonated species one after another, and (iii) then compute the energy of the protonated species theoretically and/or experimentally.

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