The valence electronic structure of 2-mercaptobenzoxazole has been studied experimentally by synchrotron radiation photoelectron spectroscopy (PES) in the gas phase, and theoretically at ab initio level. The comparison of experiment and theory shows that only one of the two tautomers of 2-mercaptobenzoxazole, the thione form, is present in the vapor phase. The equilibrium molecular geometry for both forms has been obtained at MP2 level, and the results show that the thione tautomer is the most stable one. Despite the large size of the molecule, vibrational structure has bee clearly observed in the photoelectron spectrum for the lowest three electronic states. Carbon 1s and sulfur 2p photoelectron spectra of 2-mercaptobenzoxazole vapor have also been investigated. The experimental ionization potential values of the seven chemically different carbon atoms been investigated. The experimental ionization potential values of the seven chemically different carbon atoms have been obtained and found in good agreement with the results of ab initio calculation. High-resolution total-ion-yield spectra have been recorded in the gas phase near the carbon K-edge and sulfur L_(2,3)-edge. Experimental features due to the resonant decay of core-excited states have been observed and compared with the corresponding near-edge X-ray-absorption fine structure (NEXAFS) spectra of multilayer 2-mercaptobenzoxazole. The experimental spectra have been also compared with theoretical NEXAFS spectra of the free molecule calculated by the ab initio STEX method. Differences between the gas-phase and the multilayer 2-mercaptobenzoxazole have been observed and are discussed.
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