Fragmentation of ionic and hydrogen-bonded molecules induced by synchrotron radiation
Date
2018-07-02
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Abstract
Selles töös uuriti eksperimentaalselt ja arvutuslikult fragmenteerumisprotsesse ioonsetes ja vesiniksidemetega seotud molekulides. Uurimisobjektiks oli neli proovi: ioonne vedelik 1-etüül-3-metüülimidasoolium tetrafluoroboraat (EMImBF4) ja vesiniksidemetega seotud molekulaarsed klastrid atsetamiidist (CH3CONH2), atsetamiid-2,2,2-d3-st (CD3CONH2) ja äädikhappest (CH3COOH).
Antud teadustöö esmärgiks oli uurida footoni energia mõju proovide stabiilsusele ja fragmenteerumismehhanismidele. Gaasfaasis olevate molekulide ioniseerimiseks kasutati sünkrotron- või gaaslahenduslambi kiirgust vaakumultravioleti piirkonnas. Kõiki proove uuriti massispektromeetria abil, ioonne vedelik oli täiendavalt uuritud fotoelektronspektroskoopia abil. Erinevate fragmenteerumiskanalite energeetiliste omaduste välja selgitamiseks mõõdeti ioonide osasaagised.
Näidati, et mõlemad uuritud ühenditüübid kipuvad ulatuslikult fragmenteeruma vaakumultravioletkiirguse mõjul. Erinevad fragmenteerumismehhanismid olid välja selgitatud ja mitmed sarnasused ioonse ja vesiniksidemetega seotud molekulaarsete süsteemide fragmenteerumisel välja toodud.
Olulise aspektina võib välja tuua, et erinevate fragmentatsioonikanalite esinemine ei sõltu mitte ainult footoni energiast, vaid on tugevalt mõjutatud ka uuritava molekuli klasterisatsiooni tingimustest. Äädikhappe klastrite korral erinevad klasterisatsiooni tingimused põhjustasid erinevusi massispektrites. Sellest võib järeldada, et fragmentatsiooni kanalid sõltuvad ka molekulide siseenergiast. Madalama siseenergiaga molekulaarsetes süsteemides on alla surutud aatomite ümberpaigutamisprotsessid, mille tulemusena võivad tekkida uued fragmendid, kuna aatomite liikumist süsteemis ei toimunud. Selline käitumine oli iseloomulik madalama siseenergiaga äädikhappe trimeerile, mis fragmenteerus enne prootoni ülekannet vesiniksidemete lõhkumisel, et moodustada dimeeri ioone. Kõrgematel siseenergiatel aga selline fragmentatsioonikanal puudus.
In this work, we investigated experimentally and computationally the fragmentation processes of ionic and hydrogen-bonded molecules following valence photoionization. Four samples were studied: ionic 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and hydrogen-bonded molecular clusters of acetamide (CH3CONH2), acetamide-2,2,2-d3 (CD3CONH2), and acetic acid (CH3COOH). The goal of the work was to investigate the influence of the photon energy on the stability of the samples and their fragmentation mechanisms. Tunable synchrotron radiation or gas discharge lamp radiation in the vacuum ultraviolet region was used to ionize the molecules in the gas phase. Clusters were studied by mass spectrometry, while ionic liquid was additionally studied by photoelectron spectroscopy. Partial ion yield technique was used to elucidate the energetics of various photofragmentation pathways. Both types of compounds were shown to be unstable toward near threshold ionization and therefore subjected to extensive fragmentation. Different fragmentation mechanisms were identified and common trends in dissociation behaviour of ionic and hydrogen-bonded compounds were observed. A significant finding is that not only photon energy influences the outcome of the valence ionization (at higher photon energy new fragmentation channels open up), but also the conditions at which the samples are introduced into the gas phase. In the case of acetic acid clusters, different clusterization conditions resulted in different mass spectra. A conclusion is drawn that photofragmentation channels of a molecule depend on its internal energy. Lower internal energy suppresses atomic rearrangements that might be required for a certain fragment formation and instead a new fragment is formed that does not require any rearrangement of the system. We observed such behaviour for acetic acid trimer that starts producing unprotonated dimers at stronger expansion conditions (lower internal energy), while there was no unprotonated dimers produced at weaker expansion conditions (higher internal energy).
In this work, we investigated experimentally and computationally the fragmentation processes of ionic and hydrogen-bonded molecules following valence photoionization. Four samples were studied: ionic 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and hydrogen-bonded molecular clusters of acetamide (CH3CONH2), acetamide-2,2,2-d3 (CD3CONH2), and acetic acid (CH3COOH). The goal of the work was to investigate the influence of the photon energy on the stability of the samples and their fragmentation mechanisms. Tunable synchrotron radiation or gas discharge lamp radiation in the vacuum ultraviolet region was used to ionize the molecules in the gas phase. Clusters were studied by mass spectrometry, while ionic liquid was additionally studied by photoelectron spectroscopy. Partial ion yield technique was used to elucidate the energetics of various photofragmentation pathways. Both types of compounds were shown to be unstable toward near threshold ionization and therefore subjected to extensive fragmentation. Different fragmentation mechanisms were identified and common trends in dissociation behaviour of ionic and hydrogen-bonded compounds were observed. A significant finding is that not only photon energy influences the outcome of the valence ionization (at higher photon energy new fragmentation channels open up), but also the conditions at which the samples are introduced into the gas phase. In the case of acetic acid clusters, different clusterization conditions resulted in different mass spectra. A conclusion is drawn that photofragmentation channels of a molecule depend on its internal energy. Lower internal energy suppresses atomic rearrangements that might be required for a certain fragment formation and instead a new fragment is formed that does not require any rearrangement of the system. We observed such behaviour for acetic acid trimer that starts producing unprotonated dimers at stronger expansion conditions (lower internal energy), while there was no unprotonated dimers produced at weaker expansion conditions (higher internal energy).
Description
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Keywords
hydrogen bond, ionic liquids, fragmentation, synchrothron radiation