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

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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 N O 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 N O bond for pyridine N-oxides compounds. In addition, pyridine N-oxide substituted with CONH 2 is stabilized relative to that substituted by CO 2H, which showed that the carboxyamide derivatives have an increased double bond character in the N O bond possibly due to the negative charge on the oxygen atom in the N O bond being partially distributed to the CONH 2 group.