THEORETICAL STUDIES ON HEATS OF FORMATION OF PYRIDINE N-OXIDES USING DENSITY FUNCTIONAL THEORY AND COMPLETE BASIS METHOD

The heats of formation (HOFs) for 11 pyridine N-oxide compounds are calculated by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86, PBE1PBE) methods with 6-31G** basis set and ab initio CBS-4M method. It is demonstrated that the B3PW91 method is accurate to compute the reliable HOFs for pyridine N-oxide compounds. It is also noted that the HOF is the smallest for the pyridine N-oxide which has the substituent group on the para-position, such as 4-NC-c-C5H4NO, 4-H2NOC-C5H4N-O, and 4-HO2C-c-C5H4NO. In addition, we think that the HOF of 2-HO2C-c-C5H4NO is much larger than that of 3-HO2C-c-C5H4NO and 4-HO2C-c-C5H4NO, which may be the result of intramolecular hydrogen bond formation and further measurements are needed to reexamine the HOFs for 2-HO2C-c-C5H4NO, 3-HO2C-c-C5H4NO, and 4-HO2C-c-C5H4NO.