The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

Abstract

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.

Erythroxylum pungens Tropane Alkaloids: GC-MS Analysis and the Bioactive Potential of 3-(2-methylbutyryloxy)tropan-6,7-diol in Zebrafish (Danio rerio)

“…Usually, the substituent at C-3 is an ester, and some characteristics fragments, i.e., alpha cleavage, allow for recognizing the substituent [26,27]. However, CO 2 elimination, which is common to esters, may not be observed after the McLafferty rearrangement [28], and the 44 amu loss could be related to C6-C7 elimination if there is a hydroxyl group at this position. The loss for C6-C7 can have different mass losses, as mentioned before, and may be 60 amu for two hydroxyl groups or higher if there is a substituent attached to a hydroxyl [5].…”

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

“…demonstrated aerosol flash vaporization to be milder than bulk evaporation in a study of hypergolic ionic liquids . In the light of this result and further uses of aerosol flash vaporization, we wished to determine whether a lower final internal energy or the sudden vaporization plays a more important role in the mildness of the aerosol flash vaporization approach. Peak widths in the threshold photoelectron spectrum are in part determined by the rotational envelope and can be compared directly.…”

“…The experiments were conducted in the 8.7–15.5 eV energy range and repeated using two vaporization techniques. The experiments were first performed by vaporizing a thymine aerosol using a thermodesorber in the imaging photoelectron photoion coincidence (iPEPICO) endstation at the VUV beamline of the Swiss Light Source (SLS). A thymine aerosol of particle size 20–400 nm was generated by nebulization of an aqueous solution of thymine by nitrogen.…”

“…at the maximuma ttainablee nergy after three-photona bsorptionint heir experiment. [31] this result and further uses of aerosol flash vaporization, [39][40][41][42]59] we wished to determine whether al ower final internal energy or the sudden vaporization plays am ore important role in the mildnesso ft he aerosolf lash vaporization approach. Peak widths in the threshold photoelectron spectrum are in part determined by the rotational envelopea nd can be compared directly.F urthermore, the shape of the first crossover region in the breakdown diagram,H NCO loss, is in part determined by the internal energy distribution of the sample, broadened and blueshifted because of the metastable parent ion and the resulting kinetic shift.…”

VUV photoprocessing of oxygen-containing polycyclic aromatic hydrocarbons: Threshold photoelectron spectra

“…The two-ring compounds BF, DHF, and PH were introduced to the photon interaction region as an effusive beam at 300 K and photoionized by the incident monochromatic synchrotron radiation. The larger DBF, DBO, BNF, and cBNF were introduced by a thermodesorber, originally conceived to flash vaporize aerosol particles [22] and adapted to function as a heated solid and liquid sample source placed within 5 mm of the photon interaction region [23]. Desorption temperatures were 35 °C (DBF), 40 °C (DBO), 70 °C (BNF), and 60 °C (cBNF).…”