Theoretical study of the proton affinities of 2-, 3-, and 4-monosubstituted pyridines in the gas phase by means of MINDO/3, MNDO, and AM1

Proton affinities (PAs) of 2‐, 3‐, and 4‐monosubstituted pyridines in the gas phase are calculated using the MINDO/3, MNDO, and AM1 methods. The following substituents are considered: F, Cl, CN, CH3, CF3, CHO, NO2, NH2, N(CH3)2, OCH3, and SCH3. The results are compared with experimental values. It is found that all MINDO/3 PAs are ca. 6% too high (mean value) compared to the experimental results; on the other hand, the MNDO values are ca. 7% too low (mean value). However, a much better agreement has been observed for the AM1 method where the theoretical values are only ca. 2.4% too low (mean value). Correlations between the calculated proton affinities on one hand and the charges on the acid H atom and Hammett constants on the other hand are studied. Particularly good linear relationships are found for the 4‐monosubstituted compounds within the AM1 formalism.