Thermal Trans-Cis Isomerization of Cloro-Azobenzene System

FANNY C. ALVAREZ ESCALADA; VALENTINA REY; ANA E. LEDESMA

Coimbra

Congreso; XXXIV European Congress on Molecular Spectroscopy - EUCMOS 2018; 2018

Azobenzenes are molecules that contain two aromatic rings linked by an N=N bond. They can be present in two stereoisomeric forms: trans and cis. The trans → Cis isomerization in aqueous solution could be induced by light illumination with appropriate wavelength in the UV range or changes in the temperature or pH values. In this work, the thermal trans → Cis isomerization of 3,3?,4,4?-tetrachloroazobenzene (TCAB) was studied in the 18-60 ºC temperature range using different warm-up time and pH values in order to evaluate their behaviour in that conditions. We analysed the system by UV-visible and infrared spectroscopies. TCAB is a contaminant not commercially manufactured, it is a metabolite formed from the degradation of chloranilide herbicides, like propanil and remains deposited in the soil. For this reason, we evaluate the effects of the temperature and pH of media as probable degradation process. An ethanolic solution of TCAB (1 mg/4 ml) was prepared and kept in the dark. An aliquot (50 µL) was transferred into a 1 cm light path quartz cuvette containing 2000 µL of water and used to further analyses. When the temperatures increase, the thermal trans-cis isomerization ocurs and is carried out by the appearance of two isosbestic points at 258 and 381 nm. The UV-Vis spectra of the trans-cis conversion of TCAB in Water at 40ºC (as example) is showed in Fig. 1. The aggregates products were also observed in the visible region. The infrared spectra showed the structural changes induced by changes in the temperature. The isomerization is hightly dependent of the pH independent the used temperature.We observed a predominance of the cis isomer at hight temperature and basic and acid conditions.