A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation (shown as figure 1) were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and nitrogen in pyrimidine and purine as well as the corresponding ionic fragments following core excitation.   The photon energies were in the range 284 � 297 eV for the carbon K-edge and 396.2 � 418.1 eV for the nitrogen K-edge of pyrimidine. The photon energies were in the range 284.4 � 300.5 eV for the carbon K-edge and 397.3 � 414.2 eV for the nitrogen K-edge of purine. The total ion yield, ion intensity for each ionic fragment and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Time-dependent density-functional theory (TD-DFT) were utilized for the quantum-chemical calculations and assignments. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region, but is clear for some products at the nitrogen K-edge. It's worthy to notice that one ionic product with m/z = 93 is specifically formed following some carbon atoms of purine were core excited to p*.

Core excitation and following specific dissociations of pyrimidine and purine

248th National Meeting (2014)

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