The collkional activation spectra of monosaccharide ions formed by [GI+, "a]' and [K]+ ion attachment under field desorption conditions are reported. It is shown that the elimination of the alkali ions is determined by the alkali ion affinities of the molecules (M) and competes with a fragmentation of M which is almost independent of the alkali ion attached. Correspondingly the alkali ion is predominantly retained in the fragment ions. The usefulness of this method for the differentiation of underhatized isomers is demonstrated.
hypochromism (i.e., association) in two pyrimidine-C3-pyrimidine series, upon increasing alkyl group size, but decreased quantum yields of dimerization. (a) Replacing the 5-methyl group in T-Qj-T by ethyl: K. Golankiewicz and L. Strekowski, Mol. Photochem., 4,189 (1972). (b) Increasing the size of the 3-alkyl group in bis-3-alkylated T-C3-T derivatives: K. Golankiewicz and A. Zasada-Parzynska, Bull. Acad. Pol. Scl., 22, 945 (1974).(35) The triplet 1,4 diradical which precedes dimer closes three times less efficiently for thymine than for uracil, a result attributed to the steric barrier of the 5-methyl group: P. J. Wagner and D. J. Bucheck, J. Am. Chem. Soc., 92, 181 (1970). (36) Triplet derived dimers are quenchable and several arguments make short-lived triplet aggregates unlikely precursors in the unquenchable dimerization. c (37) There is no quenching by 0.1 M pipery lene, the concentration of diene which is sufficient to trap all TMU triplets (cf. the 0isc data) and is sufficient to trap all DMT triplets.50 Very high diene concentrations do give some quenching as do high concentrations of vinyl acetate and tetramethylethylene. This may be due to small amounts of singlet quenching or result from an interference with association. DMT singlet dimerization can be completely eliminated by CCI4, presumably through a charge-transfer quenching mechanism: D.
Collisional activation spectra of quarternary ammonium cations produced by field desorption are reported. The principal fragmentation pathways of these ions are discussed. Furthermore, it is demonstrated that collisional activation in conjunction with field desorption is useful for the elucidation of the structure of complex organic cations.
Since no unimolecular fragmentation is observed with [M+Li]+ ions under normal operating conditions the collisional activation method was used to study the fragmentation behaviour of these ions. it was found that the liberation of the The ionization of organic molecules by alkali ion attachment leads to quasimolecular ions which are very stable with respect to monomolecular decomposition^.'-^ Methods by which an alkali ion attachment can be achieved are of particular interest in organic mass spectrometry for molecular weight determination and the analysis of mixtures. The stability of these ions has been attributed to the localization of the positive charge at the alkali atom and to a hi h Since the alkali ion affinities of organic molecules are often smaller (<2 eV4) than the bond energies in neutral molecules it has been suggested that the most probable decomposition reaction of a cationized molecule is the elimination of the alkali ion and not a fragmentation of the molecule. However, so far no information on the decomposition behaviour of these ions has been obtained because the mass spectra produced by [Li]+ surface ionization (s.i.) and field desorption (f.d.) do not show any unimolecular fragmentation.Only bimolecular reactions of halogenated compounds with alkali ions are known, leading predominantly to the elimination of a neutral salt m o l e c~l e .~ A similar type of reaction is assumed to occur on the surface of the salt layer under conditions of [Li]+ s.i. of halogenated compounds.6The unimolecular decomposition behaviour of ions formed by alkali ion attachment can be studied by collisional activation (~.a.).~*' For this purpose a [Li]' s.i. source has been connected with a c.a. mass spectrometer. Collisions of the [M+Li]' ions with a neutral target gas, e.g. helium, leads to an excitation of these ions which is sufficient to induce a decomposition.activation energy for a charge shift within the ion. Table 1 showing only the more intense fragment ions in the order of their relative intensities. In addition to these fragments a variety of smaller peaks were observed in the c.a. spectra. As an unambiguous mass assignment of these weak signals was difficult due to
It is shown that unpolar molecules can also be ionized by [Li]+ surface ionization. This is demonstrated with cyclohexane, neopentane and tetrachloromethane.The ionization of molecules by alkali ion attachment at the surface of a field anode covered with salts has been applied to a number of organic compounds.',' In [Li]' surface ionization (s.i.) a LiI layer is deposited onto a 10 p m tungsten wire and an electric field of about 106V cm-' is applied. The high field causes a charging of the salt layer which thus acts as an Since the thermal activation necessary for an attachment reaction decreases with increasing field strength the ionization of paraffins was attempted at higher field strengths (anode-cathode potential difference >16 kV) and with a higher loading of LiI on the tungsten wire than usual (layer thickness some 10 pni). The result of this experiment is shown in Fig. 1
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