The hydrogen bonding and amino-imino tautomerization of the alkoxy-aminopyridines and amino-methoxypyrimidines with acetic acid

The hydrogen bonding and amino-imino tautomerization of the systems of 2-amino-3-methoxypyridine (2A3MOP), 2-amino-6-methoxypyridine (2A6MOP), 2-amino-6-n-propoxypyridine (2A6NPOP), 2-amino-6-iso-propoxypyridine(2A6IPOP), 2-amino-4-methoxypyrimidine (2A4MOPM), 4-amino-2-methoxypyrimidine (4A2OPM), 4-amino-6-methoxypyrimidine (4A6MOPM), 2-amino-4-methoxy-6-methylpyrimidine (MMPM), and 2-amino-4,6-dimethoxypyrimidine (DMOPM), with acetic acid (AcOH) in n-hexane at room temperature were investigated by means of the UV absorption and fluorescence spectroscopy. From the UV absorption spectra the presence of the dual hydrogen-bonded complexes that linked by a 1:1 molar ratio with AcOH were found, since the enthalpy changes accompanying the hydrogen bond formation between 2A3MOP, 2A4MOPM, 4A2MOPM, 4A6MOPM, or MMPM, and AcOH were ca. 42.8-61.1 kJ mol−1 in n-hexane. The fluorescence spectra of the 2A3MOP/AcOH, 2A4MOPM/AcOH, 4A6MOPM/AcOH, and MMPM/ AcOH systems revealed that the imino-tautomers were produced through double proton transfer in the amino hydrogen-bonded 1:1 complexes in the S1 state, but the imino-tautomer formation for the 4A2MOPM/AcOH system was not found on account of the steric hindrance due to the inversion of the methoxy group in the S1 state. The imino-tautomer for the MMPM/AcOH system fluoresces most intensely among these systems investigated. On the other hand, not only the formation of the corresponding amino dual hydrogen-bonded complex and but also that of imino-tautomer were prevented for the 2A6MOP/AcOH, 2A6NPOPM/AcOH, 2A6IPOP/AcOH, and DMOPM/AcOH systems, because of the steric hindrance of the methoxy group in both the S0 and S1 states. The theoretical approaches by an ab initio molecular orbital calculation were in accord with the experimental results.