Does ionized diacetylene have a positive proton affinity?

• Published in: International journal of mass spectrometry 2003-12, Vol.230 (2), p.113-121
• Main Authors: Schröder, Detlef, Loos, Jessica, Schwarz, Helmut, Thissen, Roland, Roithova, Jana, Herman, Zdenek
• Format: Article
• Language: English
• Subjects: Charge reversal; Charge stripping; Density functional theory; Dications; Proton affinities
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2003.08.011

Singly and doubly charged C 4H 3 +/2+ ions generated upon electron ionization (EI) of the neutral precursors 1,3-butadiene, benzene, and exo-methylene cyclopropane, respectively, are examined by sector-field mass-spectrometry. Charge stripping of the mass-selected monocations affords the corresponding dications and charge exchange of the C 4H 3 2+ dications allows for the reverse redox process. Refined analysis and additional MS/MS studies suggest that the monocations are mixtures of isomeric ions formed upon ionization, whereas only a single type of dication seems to be formed. As an average of energy-resolved measurements, a vertical ionization energy of IE v(C 4H 3 +)=16.5±0.4 eV is derived. In addition to the experimental work, density functional theory is used for a computational exploration of the mono- and dicationic species. The best theoretical estimates are IE a(C 4H 3 +)=16.33 eV and IE v(C 4H 3 +)=16.49 eV for the most stable isomer H 2CCCCH +. Combination of the experimental and theoretical findings leads to the conclusion that the diacetylene cation C 4H 2 + has indeed a positive proton affinity of PA(C 4H 2 +)=1.50±0.42 eV.