Formation of [C18H36]+ ˙ and [C7H7O2]+ in the mass spectrum ofn-octadecyl benzoate

Electron-induced ionization of undeuterated and deuterated benzoic acid isopropyl esters and nicotinic acid isopropyl esters: Some implications for the mechanism of the McLafferty rearrangement

Electron ionization mass spectra, ionization, and appearance energies and bond energies (as dissociation energies) are reported for benzoic acid-1-methyl-ethyl ester (BAIPE), benzoic acid-1-deutero-1-methyl-ethyl ester (BAIPED1), benzoic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (BAIPED6) as well as nicotinic acid-1-methyl-ethyl ester (NAIPE), nicotinic acid-1-deutero-1-methyl-ethyl ester (NAIPED1), and nicotinic acid-2,2,2-trideutero-1-trideuteromethyl-ethyl ester (NAIPED6). Ionization energies of 9.39 eV for BAIPE, 9.40 eV for BAIPED1, 9.26 eV for BAIPED6 as well as 9.70 eV for NAIPE, 9.79 eV for NAIPED1, and 9.65 eV for NAIPED6 were determined. A gas-phase formation enthalpy of ΔHf0= (-4.10 ± 0.1) eV for BAIPE is calculated as well as ΔHf0= (-3.35 ± 0.1) eV for NAIPE. Molecular ions show two main fragmentation pathways. The first is a classical McLafferty rearrangement, characterized by the transfer of one γ-hydrogen atom from the isopropyl ester chain leading to the ions of the corresponding acid and neutral propene. The second is the double hydrogen transfer from the ester chain leading to the formation of the protonated acid and a C₃H₅√ allyl radical. For BAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of ≥98%, whereas the C-1-hydrogen is transferred with a probability of ≤2%. For NAIPE, both hydrogen atoms originate from the methyl groups of the aliphatic chain with a probability of 90%. Experimental proton affinities of PA = (8.75 ± 0.2) eV for benzoic acid and PA = (8.43 ± 0.2) eV for nicotinic acid are derived. For the protonation of the carbonyl group, B3LYP DFT calculations yielded PA = 8.66 eV for benzoic acid and PA = 8.41 eV for nicotinic acid. The overall fragmentation mechanism is explained with the initial formation of a 1,5-distonic ion by transfer of the first hydrogen. For the transfer of the second hydrogen, an intermediate ion/neutral complex is formulated.

The structure and mechanism of formation of C5H 9O (+) from ionized phytyl methyl ether

The recent proposal that ionized phytyl methyl ether [C16H33(CH3)C=CHCH2OCH 3 (+·) ] undergoes an allylic rearrangement to ionized isophytyl methyl ether [CH2=CHC(C16H33)(CH3)OCH 3 (+·) ] before elimination of an alkyl radical is discussed. Both literature precedent and new results in which the structure of the [M-C16H 33 (·) ](+) fragment ion is established by comparison of its collision-induced dissociation mass spectrum with the spectra of isomeric C5H9O(+) ions of known structure are inconsistent with this proposal. The forma Hon of CH3CH=CHCH=O(+)CH3 by loss of a γ-alkyl substituent without skeletal isomerization rather than CH2=CHC(CH3)=O(+)CH3 after allylic rearrangement is explained in terms of a mechanism that involves two 1,2-H shifts, followed by σ-cleavage of the resultant ionized enol ether, C16H33(CH3)CH-CH=CHOCH 3 (+·) .