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A number of different levels of theoretical calculations have been used for assigning the Mulliken charge to the atoms of mono-substituted pyridines, including both quantum chemical and empirical schemes. A reasonable acceptance for any successful theoretical scheme of calculation is that atomic charges produces should vary in a manner consistent with chemical intuition and more specifically, that these variation should be correlated in a sensible way with experimental observations. The Mulliken charge was calculated using six different theoretical levels (AM1, PM3, MNDO, HF, DFT, MP2). After that the Mulliken charges were tested for their ability to represent variation in the Effect of Atomic Charge and Some Physical Properties on pKa of Substituted ... 26 pKa values of 14 mono – substituted pyridines. In all cases the Mulliken charge at the nitrogen and the two α – carbon atoms were taken as a representative regression parameters. The Mulliken charge at nitrogen atoms calculated by AM1 and HF levels yielded the most successful correlations with pKa. Also using other theoretically calculated independent variable such as the heat of formation, leads to improve the correlation with values of pKa. Introduction Although the atomic charge in a molecules is not a proper quantum chemical observed quantity. The idea of assigning charge to atoms has been proven an immensely variable for explanation of a variety of chemical phenomena. On the other hand the atomic charges are important concept in understanding of many type of chemical reactions and they are important parameters in the study of molecular structure properties and activities of reacting molecules. The value of the electron density also it can be used to suggest the most active bonding set in the reacting molecules. According to the above reason different theoretical methods have been proposed for evaluation the electron density (or charges) in the molecules including quantum chemical and empirical methods (1-5). A reasonable assessment of any calculation scheme can be achieved when the calculated values are very in a manner consistent with the chemical intuition and correlated in a sensible way with experimental observations, such as, Hamitt constant and pKa's value of the compounds. Many works have been published on the correlation between the value of the pKa's of some organic molecules, such as anilines (6,7) and benzoic acid (8) and the electron density at the dissociating atoms in the molecules of functional group. On the other hand pyridine is an important molecule provides a good proto-type for assessing substituent effects on the pKa values of amines. Further more pyridine is relatively small size molecule makes accurate quantum mechanical calculation feasible and allow the discovery of quantum chemical descriptors with accuracy comparable to or better than the empirical Hamett constant. The present work used the most popular theoretical schemes at different levels for the calculation of atomic charge at pyridine atoms together with fourteen substituted pyridine in order to high light the best level can be used for such type of compound. The value of atomic charge and some calculated parameters are then correlated with pKa's of the compounds. Methods To determine the lowest energy conformations for each molecule, geometry optimizations were performed at the AM1 level for several starting geometries in which one or more torsional angles were Mahmmoud S. Saieed & Zaheda A. Najim 27 systematically varied. When multiple low energy geometries were found for compound, HF/(6-31G) single-point energy calculations were performed to determined the most stable conformer. The final geometry optimization for each of the compounds was conducted at the DFT/B3LYP/(6-31G) level. The optimized geometries were used to obtain the Mulliken charge and heat of formation. On the other hand the pKa values for all substituted pyridine were taken from literature(9). Results and Discussions The Mulliken charge at different atoms of pyridine (Figure I) are calculated using six different levels of theory (AM1, PM3, MNDO, HF, DFT and MP2). These results are tabulated in Table I. Figure (I): The numbering of pyridine ring atoms Table I: The Mulliken charges at different pyridine atoms using different theory levels Max. deferent MP2 DFT/B3 LYP 6-31G HF/6-31 MNDO PM3 AM1 Methods Atoms 0.5439 -0.3865 -0.3673 -0.5329 -0.2305 -0.079 -0.1381 N 0.0650 -0.007 -0.0013 -0.0542 0.0541 -0.062 -0.072 C2 & C6 0.1282 -0.1399 -0.1098 -0.2380 -0.1239 -0.1513 -0.1815 C3 & C5 0.1508 -0.1494 -0.1169 -0.1588 -0.0080 -0.0645 -0.0931 C4 These results showed that the charge is highly concentrated at the nitrogen atom which is the most electronegative atom in the pyridine. Further more the charge are suddenly decreased at the two αcarbon atoms to the nitrogen (C2 & C6) followed by small increase in the charge as the carbon atom becomes far away from the nitrogen. This result is consistent with fact that the electronegative atom acts as an electronwithdrawing group.(EW. G) (10) which attracted the electron from atoms near to it and this effect decreases as the distance between the atoms increases. Also there is no possibility for conjugation between the lone – pair electrons and π – electron of the aromatic ring(11). Effect of Atomic Charge and Some Physical Properties on pKa of Substituted ... 28 A plot (Figure II) of the charges against the different theoretical levels of calculation show that variations in the charge of carbon atoms are very small compare to the variations in nitrogen charge. On the other hand it is well known that there is a large difference between the electronegativity of the nitrogen and carbon atoms in pyridine. So that we thought that the theoretical methods gave a large differences in charge between αcarbon and the nitrogen could be consider as a best level of calculations. Figure II shows that maximum deference is shown by HF level of calculation while AM1 & PM3 shows the minimum difference. This results is in accordance with our expectation. MP2 DFT HF MNDO AM1 PM3 -0.65 -0.55 -0.45 -0.35 -0.25 -0.15 -0.05 0.05 0.15 m u ll ik e n .c h a rg e

Effect of Atomic Charge and Some Physical Properties on pKa of Substituted Pyridines