Metastable transitions in the mass spectra of fluoroaromatic compounds

Short communications

Metastable transitions in the mass spectra of fluoro omatic compounds In the course of identifying certain aromatic fluorine compounds we have had occasion to record the mass spectra of several wellcharacterized compounds. A number of peaks, corresponding to interesting and hitherto unreported "metastable'" transitions, were observed in these spectra. The normal mass spectra of fiuoroaromatic compounds are well documented and have been presented in a review 1. Cohen and Massey 2 have reported a metastable transition corresponding to the elimination of CF3 in the mass spectra of perfluoropolypbenyls. They suggest that at least part of the CFs is lost in one step, the trifluoromethyl group presumably being formed by migration of neighbouring fuorine atoms to a carbon atom. We have been interested mainly in hexa-, penta and tetra- fluorobenzene,s and have observed a number of diffuse peaks corresponding to metastable transition elimination of not only CFa, but also CF~ and F. The base peak in the mass spectra of these compounds is always that due to the parent ion. The next largest peak in the spectra corresponds to the loss of CF.~ from the parent. Since a metastable transition occurs corresponding to the loss of CFa, as demonstrated by the diffuse peak at M'~=73.59, representing the reaction

impact. The following diagram represents the most likely fragmentation pathways resulting in the major peaks occurring in the mass spectrum of hexafluorobenzcne. Several of the processes arc confirmed by the observed metastable transitions, shown in the usual way, ie with the superscript t. Pentafluorobenzene exhibits a spectrum with similar features under the same conditions. A diffuse peak corresponding to the metastable transition elimination of CF3 from the parent is again observed but the intensity is lower by a factor of ten compared with hexafluorobenzene. The parent ion also loses CF2 and then fluorine to yield the second most abundant ion, CsHF= +, in the spectrum. Both these processes are confirmed by the presence of metastable peaks.

C6F6+

ie C6F 6 + -+ CsF,, + + CF2 °, M * = 99.44. (186) (136) (50) However, mesatable transitions corresponding to the loss of fluorine from the parent ion minus CF2,

C6F,+--*CsF3 + + F °, M * = 100.65 (136)

(117)

(19)

~-

C6F

+

-CF 2

C5F3+

C 6 F 6 + ~ C s F a + + CF3 °, (186) (117) (69) then at least part of the CFa must be lost in one step 2. Also observed is a metastable transition corresponding to the loss of CF~ from the parent ion,

-P

osF A n interesting situation occurs in the case of 1, 2, 4, 5-tetrafluorobenzene. Although in this molecule no three fluorine atoms are on adjacent carbons, we do observe a metastable peak corresponding to elimination of CF3 from the parent ion. Cohen and Massey = have suggested that, in fluoroaromatic compounds, there is migration of fluorine in the formation of CFs. However, in this case, one of the fluorine atoms must migrate past the hydrogen on an adjacent carbon atom. Further studies of the mass spectra of other tetrafuoro- and trifluoro-benzenes may provide more information on this migration process. F G Drake.smith and M R Jones

C 6 F s + -+ C s F 3 + + C F z °, M * = 81.97,

Electricity Council Research Centre Capenhurst Chester

also occur. Thus, it would appear that the stability of the daughter ion, ie the CsFs+ ion in the case of bexafluorobenzcne, as well as the stability of the neutral fragment, plays an important role in determining the decomposition paths of the ions formed on electron

References x j R Majer, Advances in Fluorine Chemistry, Part 2, Butterworth, London, 1961. i S C Cohen and A G Massey, J Organometal Chem, I0, 1967, 471,

and, elimination of CF, from the parent ion minus fluorine,

Vacuum/volume 19/number 1. Pergamon Press Ltd/Printed in Great Britain

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