A study of the reaction mechanisms and reactive intermediates involved in halogenated compounds : trichloroethylene oxide, halogenated benzophenones, and halogenated quinoline-based phototriggers

Abstract

UV/Vis absorption spectroscopy (UV/Vis), femtosecond transient absorption spectroscopy (fs-TA), nanosecond transient absorption spectroscopy (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3), as well as density functional theory (DFT) computations were employed to study the mechanisms and the intermediates in reactions of selected halogenated compounds, including trichloroethylene oxide (TCE oxide), halogenated benzophenones (4-FBP, 4-ClBP, 4-BrBP, 3-FBP, 33’-DFBP, 3-ClBP, 3-BrBP, 2-FBP, 2-ClBP, and 2-BrBP), and halogenated quinoline-based phototriggers (BHQ-OPh and BHQ-OAc). This study investigated the halogen substituent effect on the mechanisms of various water-involved reactions and the influences from the number of halogens present, the type of halogen and the substituent position of the halogen in the molecules of interest. The general mechanisms for the reactions of these halogenated compounds were summarized along with discussion of the driving forces from the substituted halogen. First, TCE oxide was hydrolyzed to release chloride ions one by one which led to a complicated water-catalyzed decomposition. To account for the dehalogenation and the formation of CO with three kinds of carboxylic acids (formic acid, glyoxylic acid, and dichloroacetic acid), the predominant decomposition pathways were examined by comparing the computed activation energies for the formation of different products. From these comparisons, the ring-opening reaction was identified as the rate-determining step, which is also supported by previous experimental observations reported in the literature. Based on all of these analyses, the mechanisms of the water-catalyzed decomposition reactions were determined and a water-assisted HCl elimination model has been proposed. Second, some halogen-substituted benzophenones demonstrated an efficiency for a photosubstitution reaction and the related photohydration reactions. Interestingly, the efficient photosubstitution reactions of 3-FBP and 33’-DFBP were dependent on the solution acidity and reached a maximum in 1 M HClO4 CH3CN/H2O (1/1) solution. Only the photohydration reaction took place for the 3-ClBP, 3-BrBP, 4-FBP, 4-ClBP, and 4-BrBP molecules. Nevertheless, no special photochemical reaction occurred for 2-FBP, 2-ClBP, and 2-BrBP. The mechanisms and intermediates were directly characterized by spectroscopic observations and rationalized by results from DFT computations. According to these results, the general mechanisms for the photosubstitution reaction and the related photohydration reactions of halogenated benzophenone derivatives were summarized. These results reveal that the efficiency in forming the corresponding hydroxy benzophenone is influenced by the solution acidity, substituent positions, and the character of the substituted halogens. The substituted halogen is the driving force of this photosubstitution reaction. This conclusion provides insight into several possible applications that are also briefly discussed in this thesis. Lastly, the BHQ-OPh system was found to undergo an extraordinary efficient excited-state proton transfer (ESPT) to initiate a dehalogenation reaction. The fs-TA and ns-TA spectra indicate clearly the interactions between four prototropic forms of BHQ-OPh, which were characterized by UV-Vis spectra under different pH values. These prototropic forms play important roles in inducing further dehalogenation, thus their structural configurations were also investigated by DFT computations. Besides, competing with the dehalogenation reaction, BHQ-OAc underwent another photodeprotection to release the OAc group. The comparison between BHQ-OPh and BHQ-OAc provides further information in understanding the mechanisms of dehalogenation reactions and photodeprotection reactions of these quinoline-based phototriggers.