Piroxicam (PRX) is a nonsteroidal anti-inflammatory drug. The thermal decomposition process of PRX was investigated with thermogravimetry and differential scanning calorimetry. The gaseous products generated by thermal decomposition were characterized with thermogravimetric analysis coupled with Fourier transform infrared spectroscopy. The residues of the thermal decomposition at various temperatures were identified with infrared spectroscopy. The molecular bond orders were calculated using an ab initio method from the GAMESS program of quantum chemistry. The mechanism of thermal decomposition for PRX was discussed. The results indicated that the thermal decomposition of PRX is a two-stage process with the initial temperature of 198 °C either in nitrogen or air atmospheres. The thermal decompositions of the first stage in two atmospheres are the same process. The main part of the molecule, including sulfamide, amide, benzene ring and pyridine ring, decompose simultaneously and to form gasifiable small molecules and carbonaceous residue in the first stage. The second stage in nitrogen is a slow thermal pyrolysis process of carbonaceous residue. The forepart of the second stage in air is a slow thermal pyrolysis process as like as in nitrogen, and the later period of the second stage is an oxidation (combustion) reaction process of carbonaceous residue. PRX is stable under ambient temperature and air atmosphere, and it can be preserved for long-term storage under ambient temperature and in air atmosphere.
The thermal decomposition processes for ten pyrimidine nucleoside analogs were measured with thermogravimetry and differential scanning calorimetry. The IR spectra, high-performance liquid chromatography, and liquid chromatography–mass spectrometry of pyrimidine nucleoside analogs and their residues of thermal decomposition at various temperatures were determined. The molecular bond orders of pyrimidines and pyrimidine nucleoside analogs were calculated with an ab initio method from the GAMESS program. We then discuss mechanisms of thermal decomposition in these pyrimidine nucleoside analogs. The results indicate that there are four types of mechanisms. The decomposition mechanism depends on the relative strength of the peptide bond and the amide bond within pyrimidine ring and whether or not accompanied by oxidation reaction. The substituent groups affect the thermal stability and the thermal decomposition mechanism of pyrimidine nucleoside analogs. Increasing the number of electron-donating groups on the pyrimidine ring and furan ring will enhance the peptide bond, and will elevate the temperature of thermal decomposition. There is a positive correlation between the molecular bond orders calculated by quantum chemistry and the thermal decomposition temperature of pyrimidine nucleoside analogs. The stronger the weakest bond order, the higher the decomposition temperature. The molecular bond orders thus can be used as a basis to judge molecular thermal stability for analog compounds with similar molecular structure, size, and energy.
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