Mass spectrometry of substituted benzils

dc.contributor.authorO'Toole, Joseph Herberten
dc.description.abstractThe positive and negative ion mess spectra and appearance potentials were determined for the following compounds: benzil, 4-methylbenzil, 4- methoxybenzil, 4-methylmercaptobenzil, 4-dimethylaminobenzil, 4-chlorobenzil, 4—nitrobenzil, 4,4'-dimethylbenzil, 4,4'-dimethoxybenzil, 4,4'- bis(methylmercapto) benzil, 4,4'-bis(dimethylamino)benzil, 4,4'-dichlorobenzil, and 4,4‘-dinitrobenzil. Metastable data were used to establish that the fragmentation mechanism for positive ions involves cleavage between the carbonyls followed by loss of CQ. Calculated appearance potentials are compared wilh those generated experimentally. The appearance potentials of the substituted benzoyl ions exhibit substituent effects correlatable with the Hammett σ, but no substituent effect is demonstrated for the unsubstituted benzoyl. Loss of CO by the benzoyl ion does not appear to involve substituent effects but required 2.5-4.0 eV excess energy above that required to form the precursor. The nitro-substituted benzils fragment by two competing mechanisms, that described supra and the loss of NO⋅ following rearrangement of the substituent. The base peak in all negative ion spectra at 50 eV was the parent-molecule jon. Benzil, the methyl-substituted benzils, and the dimethyl-amino- substituted benzils form no daughter ions. The methoxy-substituted benzils and the nethyimercapto-substituted benzils formed a daughter jon by loss of CH₃⋅ from the parent, The echloro-substituted benzils fragment to produce Cl⁻. The nitro-substituted benzils lose NO⋅, apparently following a rearrangement Similar to that observed for positive ions, and also fragment to lose NO⁻ in low abundance. Appearance potential data indicate that all parent-molecule ions are formed by electron capture, and all fragments by dissociative-attachment. The electron-withdrawing substituents cause a broadening of the ionization efficiency curve for the parent-molecule ion; all other substituents have no effect on the curves' shape. Daughter ion ionization efficiency curves are broader than the parent-molecule i.e. curves, and the i.e. curves for the daughter ions from the methoxy-substituted benzils appear to be conmposits of at least two curves. NO⁻ and Cl⁻ are of low abundance, with onset at 2.2 eV and 5.5 eV respectively above SF¯₆. The most abundant negative daughter ions are resonance stabilized; Cl⁻ and NO⁻ are formed by electron pairing at an electronegative site. Negative ions are not formed with sufficient excess energy to fragment by the same mechanism as positive ions, with the exception of the anomalous nitro-substituted mechanism. Polarographic half-wave potentials were compared to negative ion mass spectrometric data. Benzils with electron-donating substituents fit a plot of E<sub>1/2</sub> vs.o. Substituent effects for these benzils are additive, as indicated by comparison of Ag for one and two substituents to that for benzil. Chloro-substituted and nitro-substituted benziils have second waves at only 0.3 to 0.4 v more negative than the first wave, in contrast to 0.6 to 0.8 v differences for the other benzils. Loss of a substituent, as observed in the negative ion mass spectra, is suggested as an explanation E<sub>3/4</sub> - E<sub>1/4</sub> values were calculated and indicate that waves in the region 1.0 to 1.4 v are probably reversible.en
dc.description.degreePh. D.en
dc.format.extent171 leavesen
dc.publisherVirginia Techen
dc.relation.isformatofOCLC# 21992766en
dc.rightsIn Copyrighten
dc.subject.lccLD5655.V856 1975.O855en
dc.subject.lcshMass spectrometryen
dc.titleMass spectrometry of substituted benzilsen
dc.type.dcmitypeTexten Polytechnic Institute and State Universityen D.en


Original bundle
Now showing 1 - 1 of 1
Thumbnail Image
6.74 MB
Adobe Portable Document Format