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Breaking inversion symmetry by protonation: Experimental and theoretical NEXAFS study of the diazynium ion, N2H+
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.ORCID iD: 0000-0003-4020-0923
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2021 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 23, no 32, p. 17166-17176Article in journal (Refereed) Published
Abstract [en]

As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N2H+. By ab initio calculations we show that the spectrum consists of a superposition of two nitrogen 1s absorption spectra, each including a π∗ band, and a nitrogen 1s to H+ charge transfer band followed by a weak irregular progression of high energy excitations. Calculations also show that, as an effect of symmetry breaking by protonation, the π∗ transitions are separated by 0.23 eV, only slightly exceeding the difference in the corresponding dark (symmetry forbidden) and bright (symmetry allowed) core excitations of neutral N2. By DFT and calculations and vibrational analysis, the complex π∗ excitation band of N2H+ is understood as due to the superposition of the significantly different vibrational progressions of excitations from terminal and central nitrogen atoms, both leading to bent final state geometries. We also show computationally that the electronic structure of the charge transfer excitation smoothly depends on the nitrogen-proton distance and that there is a clear extension of the spectra going from infinity to close nitrogen-proton distance where fine structures show some, although not fully detailed, similarities. An interesting feature of partial localization of the nitrogen core orbitals, with a strong, non-monotonous, variation with nitrogen-proton distance could be highlighted. Specific effects could be unraveled when comparing molecular cation NEXAFS spectra, as represented by recently recorded spectra of N2+ and CO+, and spectra of protonated molecules as represented here by the N2H+ ion. Both types containing rich physical effects not represented in NEXAFS of neutral molecules because of the positive charge, whereas protonation also breaks the symmetry. The effect of the protonation on dinitrogen can be separated in charge, which extends the high-energy part of the spectrum, and symmetry-breaking, which is most clearly seen in the low-energy π∗ transition.

Place, publisher, year, edition, pages
Royal Society of Chemistry (RSC) , 2021. Vol. 23, no 32, p. 17166-17176
Keywords [en]
Absorption spectroscopy, Calculations, Charge transfer, Electronic structure, Excited states, Ions, Molecules, Nitrogen, Ab initio calculations, Charge transfer bands, Charge-transfer excitations, Partial localization, Protonated molecules, Protonated species, Vibrational analysis, Vibrational progressions, Protonation
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-311201DOI: 10.1039/d1cp02002aISI: 000680883900001PubMedID: 34346432Scopus ID: 2-s2.0-85113469793OAI: oai:DiVA.org:kth-311201DiVA, id: diva2:1653783
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QC 20220425

Available from: 2022-04-25 Created: 2022-04-25 Last updated: 2022-06-25Bibliographically approved

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Couto, Rafael Carvalho

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