Doubly charged ion mass spectra: III—N-alkanes

Doubly charged ion mass spectra have been obtained for 15 n-alkane hydrocarbons. Spectra were measured using a Nier-Johnson geometry Hitachi RMU-7L mass spectrometer operated at 1.6kV accelerating voltage. Fragment ions, which resulted from C? C bond rupture and extensive H loss, dominated the spectra. Molecular ions have not been observed. The most intense ions in the doubly charged ion mass spectra of n-alkanes were C₂H₄]²⁺, C₃H₂]²⁺, C₄H₃]²⁺, C₅H₂]²⁺, C₆H₆]²⁺, C₆H₈]²⁺, C₇H₆]²⁺, C₇H₈]²⁺, C₈H₆]²⁺ and C₈H₈]²⁺. Appearance energies for forming the prominent doubly charged fragment ions have been measured and range from 27.5 eV to energies greater than 60eV. A geometry optimized SCF approach has been used to compute the energies and structures of prominent ions in the doubly charged mass spectra.