Theoretical studies on bond dissociation energies for some pyridine N-oxide biological compounds by density functional theory and CBS-4M method

Density functional methods (B3LYP, B3PW91, B3P86, MPWB1K, MPW1B95, TPSS and PBE1PBE) and complete basis method are employed to investigate the bond dissociation energies (BDEs) of the terminal NO bond for 12 pyridine N-oxides compounds. It is demonstrated that the B3PW91/6-31G∗∗ is the most accurate method to compute the reliable BDEs for the compounds studied. In order to test whether the non-local BLYP method suggested by Ref. [Y.R. Luo, Handbook of Bond Dissociation Energies in Organic Compounds, CRC Press, Boca Raton, 2003] is general for our study and whether B3PW91 method is sensitive to the basis set, the BDEs for 12 pyridine N-oxides compounds are also calculated by using BLYP/6-31+G∗ and B3PW91 method with 6-31+G∗, 6-31+G∗∗, 6-31G∗, 6-311+G(2df,2p) and cc-pVTZ basis sets for comparison. The obtained results show B3PW91 method with a moderate or a larger basis set, such as 6-31G∗ and 6-31+G∗∗, may be more suitable to calculate the BDEs of the NO bond for pyridine N-oxides compounds. In addition, pyridine N-oxide substituted with CONH2 is stabilized relative to that substituted by CO2H, which showed that the carboxyamide derivatives have an increased double bond character in the NO bond possibly due to the negative charge on the oxygen atom in the NO bond being partially distributed to the CONH2 group.