The above representative IR bands decreased its peak intensity slightly with the increase of temperature up to 165°C (Fig. 2A), since the atoms or molecules occupied the fixed site with very little vibrational motion in the solid state. Once beyond 165°C, however, the peak intensity of these specific IR bands decreased dramatically because aceta- minophen changed from solid state to liquid state.14) More- over, the peak positions of the CϭO stretching and N–H bending bands for acetaminophen shifted to a much higher or lower wavenumber when temperature was Ͼ165 °C, respec- tively.