Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation

Abstract

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)[middle dot]H(+) and CH2O[middle dot](C2H5OH)H(+), respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH(+), a new channel to generate (C2H5OH)[middle dot]CH2OH(+) is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected.

Internal energy selection in vacuum ultraviolet photoionization of ethanol and ethanol dimers

“…9 If ethanol ions were also made by the dissociative photoionization of clusters, they would lose some of the excess energy to the leaving neutral fragment and through kinetic energy release, meaning that they could only lose an H atom at higher photon energies than the monomer onset. Thus, contrary to the assumptions based on the HOMO-1 orbital character in the ethanol dimer, 14 the dissociative photoionization of the dimer or larger clusters does not appear to yield monomer cations in the energy range studied. Second, the sudden disappearance of the intact dimer ion signal at 9.72 eV tells us a similar story on dimer cations, namely that the dissociative photoionization of larger cluster ions does not yield ethanol dimer fragment ions.…”

“…This means that the electron collection efficiency decays reciprocally as 0.1 eV/E above a kinetic energy of 0.1 eV. The plotted curves are, thus, expected to agree with the photoionization efficiency (PIE) curves obtained in PIMS as reported by Li et al 14 close to the ionization onsets of the ions and slowly turn into PIE curve derivatives as the photon energy increases significantly above 0.1 eV from the onset. Protonated cluster signal has been observed up to n = 7, which mirror that of the protonated trimer only with a lower onset energy and are not plotted.…”

“…Li et al 14 suggest that C-C bond breaking in the Hacceptor yields (C 2 H 5 OH)(CH 2 OH) + , i.e., the higher energy CH 3 -loss product (see Table I and Figure 4(b)) from the à state of the dimer cation. Despite several attempts, no reaction path could be found corresponding to this process: C-O bond contraction scans between the acceptor α-C and the donor O actually yield the H-loss product, and the C-O bond is not formed in C-C bond elongation scans.…”

Communication: Protonation process of formic acid from the ionization and fragmentation of dimers induced by synchrotron radiation in the valence region

“…The ionization and fragmentation of small organic molecules, as well as their dimers and clusters, have been extensively studied in the valence and in the core level regions. Several experimental techniques, ionization sources, and quantum chemical calculations have been employed to obtain and interpret mass spectra of simple organic molecules from various functional groups such as carboxylic acids, 1-6 alcohols, 7,8 ethers, 9 amides, 10,11 and aldehydes. 12,13 There is an increasing interest in the dimers and small clusters of molecules involving multiple hydrogen bonds, because of their fundamental importance in the formation of biomolecules, 6 such as DNA base pairs.…”

Polarization Dependence in the Carbon K-Edge Photofragmentation of MAPDST Photoresist: An Experimental and Theoretical Study

“…The excellent properties of SR have encouraged researchers in the past to also look for selective bond dissociation in molecules and polymers. In particular, photon-stimulated ion desorption (PSID) studies combined with time-of-flight mass spectrometry (TOF-MS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to search for highly specific chemical bond scission using core-level photoexcitation. − The so-called “molecular knife or molecular scissor” effect was proposed where the localized energy in a particular core excitation would lead to selective molecular bond-breaking. , SR-selective photofragmentation was induced in the past using vacuum ultraviolet (VUV) , as well as inner shell excitations. − In several of those investigations, it was observed that some ions were selectively dissociated, but others were rapidly neutralized before fragmentation and desorption. , Recently, Nagaoka et al, in an experimental and computational study, have shown that for an effective molecular dissociation it is necessary to satisfy two conditions: the atomic sites to be distinguished should be connected through a chain of saturated bonds and located far from each other. In this way, the electron migration would be reduced.…”