This paper deals with the investigations of terephthalic acid (TPA) molecules deposited on a low reactive Ag(111) surface and studied using scanning tunneling microscopy (STM) at low temperature and DFT calculations. These investigations show that two deprotonation states energetically equivalent can be produced at the single molecule level. On self assemblies, the mobility of H atoms at 77 K favours the motion of created defects in the layer. STM observations and DFT calculations show that the most stable structures are obtained when only one hydrogen atom is removed from an O-HÁ Á ÁO bond and when these deprotonated molecules are located in adjacent TPA rows.