Gas-phase photoemission study of 2-mercaptobenzoxazole

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 exp eriment and theory shows that only one of the two tautomers of 2-mercaptobe nzoxazole, the thione form, is present in the vapor phase. The equilibrium molecular geometry for both forms has been obtained at MP2 level, and the r esults show that the thione tautomer is the most stable one. Despite the la rge size of the molecule, vibrational structure has been clearly observed i n the photoelectron spectrum for, the lowest three electronic states; Carbo n Is and sulfur 2p photoelectron spectra of 2-mercaptobenzoxazole vapor hav e also been investigated. The experimental ionization potential values of t he seven chemically different carbon atoms have been obtained and found in good agreement with the results of ab initio calculations. High-resolution total-ion-yield spectra have been recorded in the gas phase near the carbon K-edge and sulfur L-2,L-3-edges. Experimental features due to the resonant decay of core-excited states have been observed and compared with the corr esponding near-edge X-ray-absorption fine structure (NEXAFS) spectra of mul tilayer 2-mercaptobenzoxazole. The experimental spectra have been also comp ared with theoretical NEXAFS spectra of the free molecule calculated by the ab initio STEX method. Differences between the gas-phase and the multilaye r 2-mercaptobenzoxazole have been observed and are discussed.