The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid. Issue 24 (16th November 2017)

• Record Type: Journal Article
• Authors: Heringa, Maarten F.; Slowik, Jay G.; Goldmann, Maximilian; Signorell, Ruth; Hemberger, Patrick; Bodi, Andras
• Is Part Of: Chemphyschem. Volume 18:Issue 24(2017)
• Page Start: 3595
• Page End: 3604
• Publication Date: 2017-11-16
• Subjects: aerosol -- mass spectrometry -- photoelectron photoion coincidence spectroscopy -- reaction mechanism -- synchrotron radiation; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.201700983
• Languages: English
• ISSNs: 1439-4235

Abstract

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. Abstract : Easy way out ! The lowest‐energy fragmentation pathway of the oleic acid cation is shown to be water loss by the study of internal energy selected parent ions, instead of McLafferty rearrangement usually found in carboxylic acids. Calculations reveal that the positive charge is partially delocalized to the double bond, and the remote allylic hydrogens may transfer to the OH group to form the leaving water molecule more easily, yielding a cyclic ketone fragment ion.