JMSSJ On-line Abstracts, Vol.46, No.6 (1998)

Unimolecular Dissociation of Gaseous Ions in EI Mass Spectrometry.

Takae Takeuchi* (Department of Chemistry, Faculty of Science, Nara Women's University, Kitauoyanishi-machi, Nara 630-8506, Japan)

J. Mass Spectrom. Soc. Jpn., 46(6), 477-482 (in Japanese), 1998

The fragmentation mechanisms of gaseous ions were studied experimentally and theoretically. In order to contribute to construction of fragmentation theory in mass spectra, the principle was examined using three separate approaches. Firstly, energy dependence on the fragmentation of molecular ions was investigated using the threshold electron-photoion coincidence (TEPICO) technique. Secondary, fragmentation mechanisms of molecular ions (n-butane, 1-propanol, n-propyl amine, and cycloglycylglycine cation radicals and a series of organosilicon cation radicals) in the low energy region were studied using ab initio molecular orbital (MO) methods. Finally, the dissociation (by hydrogen loss) of the first excited state (A²B₁) of the formaldehyde cation as a function of its vibrational excitation was analyzed. Its mechanism involved an electronic predissociation by the ground X²B₂ state in the tunneling regime. A statistical treatment for nonadiabatic interactions accounted for the low value of the rate constant as well as for a large isotope effect that persists over an energy interval of ca. 0.7 eV.

Stable Cluster Ions in Carboxylic acid-water Binary Systems and their Reactivity with Ammonia Studied by Liquid Ionization Mass Spectrometry.

Masahiko Tsuchiya,*# Shiho Teshima, Atsushi Shigihara, Tsuneo Hirano (Yokohama National University, Faculty of Engineering, Tokiwadai, Hodogaya-ku, Yokohama, 240 Japan, #Present address: 4-37-27, Kugayama, Suginami-ku, Tokyo 168 Japan)

J. Mass Spectrom. Soc. Jpn., 46(6), 483-489, 1998

Intense peaks of cluster ions, (RCOOH)₅(H₂O)H⁺, have been observed for monocarboxylic acid-water binary systems and a stable ring-type structure has been suggested. The stability and the structure of such cluster ions in the acetic acid-water mixture were investigated by measuring reactions between binary cluster ions and ammonia by liquid ionization (LPI) mass spectrometry and MO calculation. The LPI mass spectra showed again the special stablity of (CH₃COOH)₅(H₃O)⁺ ion in the substitution reactions with ammonia. This substitution reaction seemed to require a certain threshold concentration of ammonia. The results of theoretical calculation with the MNDO-PM3 method revealed that the ring-type structure assumed was more stable than well known catamer-type structures. The results of calculation agreed well with the results obtained from LPI mass spectra. Differences between water and ammonia as a component of clusters are also described.

Electron Impact Mass Spectrometry of Steroidal Dithiolanes.

Gerardo M. Caballero, Eduardo G. Gros* (Departamento de Quimica Organica, and LANAIS-EMAR (FCEyN-CONICET), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina, e-mail: gros@qo.fcen.uba.ar)

J. Mass Spectrom Soc. Jpn., 46(6), 490-494, 1998

The EI mass spectrometry data of several steroid dithiolanes are reported. The main fragments were characterized by high resolution mass spectrometric measurements and by linked scan experiments. According to the proposed structures, tentative mechanisms for the formation of important ions are postulated and discussed.

Collisionally activated dissociation of 13-membered peptides, ustiloxins and phomopsins.

Naoko Morisaki,* Yukiko Koiso, Yukiko Mitsui, Yukiko Yamashita, Hisayoshi Kobayashi, Ryuichi Shirai, Yuichi Hashimoto, Shigeo Iwasaki (Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan)

J. Mass Spectrom. Soc. Jpn., 46(6), 495-501, 1998

Collisionally activated dissociation (CAD) of [M+H]⁺ of ustiloxins and phomopsins was examined. Fragments of ustiloxins were formed by the cleavage of O(1)-C(2) and the peptide chain to form "a", "b", "x", and "y" series ions, and of the C(9)-C(10) bond to form ions "M-Bz" and "b-Bz". Phomopsins gave the "b" and "y" series ions. The difference may be at least partly explained by different hydrogen bond-forming characteristics of ustiloxins and phomopsins.