The ground state basicities of a series of substituted acetophenones: A theoretical study

D Dinda, B De
1999 Indian Journal of Chemistry   unpublished
MNDO calculations with complete geometry optimization are carried out on a seri es of p-substituted acetophenones and their o-protonated counterparts. The gas phase o-protonation turns out to be exothermic case and the local stereochemical disposition of the proton is found to be almost the same in each case. The presence of p-substituent is seen to cause very little change of the protonation energies ePE) relative to the unsubstituted acetophenones. Electron releasing p-substituents increase
more » ... by 0.3 eV and the electron withdrawing 17-substituents decrease it by 0.5 eV. Computed protonation energies are sought to be correlated with a number or computed system parameters such as, the net charge on the carbonyl oxygen, charge on the proton and the computed hardness of the unprotonated species. Basicity of carbonyl molecules in their electronic ground state is well-recognised I-J. The H-bonding ability of the carbonyl oxygen, its involvement in proton transfer process in the ground state as well as excited states are some of the consequences of basicity4-9. Many reactions that the carbonyl systems undergo are acid catalysed and invol ve an attack on the carbonyl oxygen by a proton in the primary steplD-U. The solvent shifts of the nn* transition in the carbonyl are also often modelled taking the basicity of the carbonyl chromophore into cognizance. Systematic experimental data on the basicity of different carbonyl systems even in the ground state are scarcely available. One is therefore compelled to turn to theory to obtain some quantitative idea about the relative basicity of a series of structurally related carbonyl molecules. In an effort to understand the nature and origin of variation in the relative magnitude of the basicities to be expected in a series of aromatic carbonyls namely, acetophenones, we have calculated the gas phase ground state basicities of a number of para-substituted acetophenones by the MNDO method l4. We have then analysed the computed protonation energy (PE) values to understand whether the pre-protonation charge di stribution local to the chromophore or post-protonation rel axation of charge density or both are important in shaping the overall basicity of the acetophenones. We have also looked into the possible origin of the small shift in the protonation energIes as one goes from the unsubstituted to the para-substituted acetophenones. The possibility of correlating the PE's with the global hardness of the molecules is also explored. Results and Discussion The calculations have been performed at the standard LCAO-MO-SCF method at the MNDO level of approximation 15. Complete geomet ry optimization has been carried out on the molecules both before and after protonation. The molecules studied are li sted in Table I along with their respective abbreviated names and computed protonation energies. Table II summarizes the computed net charge on the carbonyl oxygen atoms in the equilibrium ground state of the unprotonated free base molecules as well as the net charge carried out by proton in the equilibrium ground state of the protonated bases. The computed net charge on the carbonyl oxygen atom in the protonated bases are also reported for all the bases. The computed net charge on the proton is small il1 each case and is in the range 0.25-0.29 showi ng that a rather 'large migration of electron density to the added proton has taken place. That thi s mi gration is not local and originates from all-over the molecule is clearly reflected in the computed net charges on the carbonyl oxygen atom of the protonated bases. The oxygen atom still carries a net negative charge, albeit depleted relative to the unprotonated base. It can, therefore, be anticipated that the proton affiniti es of these carbonyl bases cannot be modelled or described by local