AN FT-EPR INVESTIGATION OF THE ANOMALOUS CIDEP OBSERVED IN PHOTOINDUCED REACTIONS OF XANTHONE WITH ALCOHOLS IN THE PRESENCE OF HYDROCHLORIC-ACID

CIDEP spectra from free radicals produced by the photolysis of xanthon e (Xn) in 2-propanol were investigated with FT-EPR. The spectra were a ssigned to the 2-hydroxypropan-2-yl (2HP) and xanthone ketyl (XnH) rad icals. In pure 2-propanol and 2-propanol containing 10% H2O, the spect ra display low field emission/ high field absorption with net emission (E/A) type polarization. The observed CIDEP pattern is mainly due to the S-To radical pair mechanism (RPM) with minor contributions from t he triplet mechanism (TM) and radical triplet pair mechanism (RPM). Up on addition of HCl, the polarization changes to net absorption. The ri se time of the absorptive signals is determined by the response time o f the spectrometer (similar to 3 x 10(-8) s). Transient optical absorp tion measurements show that the triplet state of xanthone ((3)Xn) is quenched by HCl, and the change in spin polarization produced by HCl a ddition is attributed to this quenching process. The dependence of the CIDEP pattern and triplet xanthone lifetime on HCl concentration show s that both involve a diffusion-controlled process. The main 3Xn() qu enching process was found to be nonreactive, but it is proposed that i ts spin selectivity produces spin polarization in the products of the hydrogen abstraction reaction that runs in parallel with this quenchin g process.