Density Functional Theory and Mass Spectrometry of Phthalate Fragmentations Mechanisms: Modeling Hyperconjugated Carbocation and Radical Cation Complexes with Neutral Molecules

Abstract

This is the first ab initio study of the energetics of the fragmentation mechanisms of phthalate, by mass spectrometry, leading to protonated phthalic anhydride (m/z 149). Phthalates fragment by two major pathways; namely, the McLafferty+1 rearrangement and the loss of alkoxy. Both pathways involve a carbonyl oxygen attack to the ortho-carbonyl carbon leading to structures with tetrahedral carbon intermediates that eventually give m/z 149. These pathways were studied by collision induced dissociation (CID) using triple quadrupole mass spectrometry. The proposed McLafferty+1 pathway proceeds through a distonic M•+ , leading to the loss of an allylic-stabilized alkene radical. The McLafferty rearrangement step proceeds through a six-membered ring transition state with a small activation energy ranging 0.4-6.2 kcal/mol; the transfer of a second H from the distonic ion of the rearrangement step proceeds through a radical cation molecule complex. Based on quantum chemical modeling of the cation molecule complexes, two kinds of cation molecule complexes were identified as radical cation molecule complex and hyperconjugated cation molecule complex. This distinction is based on the cation and simplifies future modeling of similar complexes. Optimization of important fragments in these pathways showed cyclized and hydrogen-bonded structures to be favored. An exception was the optimized structure of the protonated phthalic anhydride (m/z 149) that showed a structure with an open anhydride ring.

Quantum chemical mass spectrometry:ab initioprediction of electron ionization mass spectra and identification of new fragmentation pathways

“…To measure the strength of the bond, the bond length or bond order was calculated by means of a QC method. Others limited themselves to the most intense peaks in the mass spectrum, as either the structures of the fragments associated with these peaks were determined or the associated fragmentation mechanisms were studied: to assign fragment structures to peaks, multiple possible structures (with the same m / z ) were geometry‐optimized and the one with the lowest energy was assumed to be the correct structure. Fragmentation pathways were elucidated by proposing several possibilities that may give rise to a certain peak: the calculated energy of the different intermediates or the activation energy associated with the pathway was then used to predict which pathway is most likely to occur.…”

Mass spectrometric characterization of halogenated flame retardants

“…An alternative fragmentation pathway competing with above reaction is responsible for the ion peak at m/z 383.8, shown in Scheme (c). It proceeds through a five‐membered cyclic oxonium structure, which is typically observed in the phthalate fragmentation, shown in Scheme (b) . This procedure is also accompanied by a [C–O] bond cleavage to lose a radical [C 8 H 17 O] • .…”

Tandem mass spectrometry and density functional theory of RDX fragmentation pathways: Role of ion–molecule complexes in loss of NO3 and lack of molecular ion peak

“…Formation of the anion–molecule complex of RDX followed by the dissociation of the NO 2 group can be used to explain the lack of the molecular ion peak in the RDX mass spectrum. It has been proposed that formation of ion‐molecule complexes usually takes place in the analyzer region of the mass spectrometer where the temperature is lower than the source temperature . The mechanism would also explain the lack of the RDX molecular anion peak in the full scan NCI mass spectrum or in other electron‐attachment approaches …”

“…It has been proposed that formation of ion-molecule complexes usually takes place in the analyzer region of the mass spectrometer where the temperature is lower than the source temperature. [30] The mechanism would also explain the lack of the RDX molecular anion peak in the full scan NCI mass spectrum or in other electron-attachment approaches. [8,9,13,14] Pathway for elimination of NO 2 In principle, all the RDX conformations are accessible under NCI-MS conditions because of the small difference in the relative energies.…”