Study Of Excited-state Dynamics And Photochemical Behavior Of Phenylurea Herbicide

Phenylurea herbicides,which are widely used in different crops, could inhibit the activity ofplant growth. This kind of compound has relatively stable chemical properties, and can exist inthe environment for a long time. It may pollute soil, surface water and groundwater, even mayhave a carcinogenic effect on people. In recent decades, the discussion about the degradationway of phenylurea herbicide has never stopped, and their photophysical and photochemicalprocess has received lots of attention. Main character of these compounds owns the benzene ringand urea. Monuron (MN) and Fenuron (FN) are representative species in phenylurea herbicidesfamily.In this paper, we use Time-dependent density functional (TD-DFT) calculations andResonance Raman spectroscopy to explore the excited-state structural dynamics information.Meanwhile, the CASSCF calculations are used to research the geometric structure of excited-state, and Resonance Raman(RR) spectroscopy analysis is used to reveal the excited-statemolecule of short-time dynamic information. Laser flash photolysis technique (LFP) combinedwith theoretical calculation are used to obtain the possible degradation mechanism. Thefollowing shows the main conclusions,(1) The electronic transition, geometric structure and vibrational frequencies of thecompound MN were calculated by TD-DFT. The strongest absorption band was assigned to theÏ€HOMOâ†'Ï€LUMO+1*transitions(S0-S2). We obtained four RR spectra of MN under differentwavelength in the solvent of methanol：water (1:10in volume), explored the excited-stateshort-term dynamic information in Franck-Condon region under different wavelengths. RRspectra results shows the Franck-Condon regional structural dynamics of MN has multipledimensions, including13(1600cm-1, Ring C=C strech),29(1183cm-1, Ring CH in-plane bend),34(1091cm-1, Ring deformation/Ring CH in-plane bend) and their fundamentals, overtonesand combination tones, in the239.5nm,245.9nm,252.7nm RR spectra. It suggested S2statestructural dynamics mainly along the ring C=C stretching vibration, ring C-H in-plane bend,ring deformation three reaction coordinates. CASSCF method calculation shows S1and S2stateexist conical intersection (S2/S1), which has great significance to S2state photolysis kinetics.(2) The electronic absorption spectra and four different wavelength RR spectra of FN have been Obtained, to explore excited-state short-term dynamic information in Franck-Condonregion. The geometric structure, vibrational frequencies and the electronic transition of thecompound were calculated by TD-DFT. The strongest absorption band was assigned to theÏ€HOMOâ†'Ï€LUMO+1*transitions(S0-S2). RR spectra showed the Franck-Condon regional structuraldynamics of FN has multiple dimensions, including14(1601cm-1, Ring C=C strech),30(1186cm-1, Ring CH in-plane bend),38(1014cm-1, Ring deformation/CH3rock) and their fundamentals,overtones and combination tone in the228.7nm,239.5nm,245.9nm. It suggested S2stateexcited-states of structural dynamics mainly along the ring C=C stretching vibration, ring C-Hin-plane bend, ring deformation three reaction coordinates.Compared the above two kinds of compounds, the presence of chlorine atom has littleinfluence on the main excited vibration such as: ring C=C stretching vibration, N-H in-planebend, ring CH in-plane bend, the difference only existed in MN25(1299cm-1, Ph-NH sicssor)and FN27(1307cm-1, Ring deformation), namely the chlorine atom existence changed ringdeformation vibration to Ph-NH sicssor.34ring deformation vibration of MN compared with38ring deformation vibration of FN is weakened. We can conclude that the chlorine atom atomicmass is relatively heavy, it will strongly influence ring deformation vibration.(3)Obtained transient absorption spectroscopy of benzophenone(BP) solution and BP/MNsolution. The transient particles in the spectra have been identified by theoretical calculation.Through analysing the change of the transient absorption spectra and the decay of dynamics, weconcluded the triplet BP successfully induced the degradation of MN, forming MN radicals(MN(-H)). This mechanism is used to explain the degradation of the structural stabilityphenylurea herbicide in the natural environment.