Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation
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J. Am. Soc. Mass Spectrom. (2019) DOI: 10.1007/s13361-019-02312-5
FOCUS: HONORING HELMUT SCHWARZ ʻ S ELECTION TO THE NATIONAL ACADEMY OF SCIENCES: RESEARCH ARTICLE
Modulation of Gas-Phase Lithium Cation Basicities by Microsolvation Konrad Koszinowski, Thomas Auth Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstr. 2, 37077, Göttingen, Germany
Abstract. In contrast to the extensive knowledge of lithium cation affinities and basicities, Li(MeCN)(Me2O) the thermochemistry of microsolvated lithium + cations is much less explored. Here, we deterLi(Me2O)2 mine the relative stabilities of Li(A,B)n+ com+ plexes, n = 2 and 3, by monitoring their gasLi(MeCN)2 phase reactions with A and B substrate molecules, A/B = Me2O, Et2O, tetrahydrofuran, and 0 200 400 600 800 1000 MeCN, in a three-dimensional quadrupole-ion reaction time t (ms) trap mass spectrometer. Kinetic analysis of the observed ligand displacement reactions affords equilibrium constants, which are then converted into Gibbs reaction energies. In addition, we use high-level quantum chemical calculations to predict the structures and sequential ligand dissociation energies of the homoleptic Li(A)n+ complexes, n = 1–3. As expected, the ligands dissociate more easily from complexes in higher coordination states. However, the very nature of the ligand also matters. Ligands with different steric demands can, thus, invert their relative Li+ affinities depending on the coordination state of the metal center. This finding shows that microsolvation of Li+ can result in specific effects, which are not recognized if the analysis takes into account only simple lithium cation affinities and basicities. Keywords: Ion-molecule reactions, Lithium, Quantum chemical calculations +
Received: 28 February 2019/Revised: 12 August 2019/Accepted: 12 August 2019
Introduction he smallest alkali metal cation Li+ plays pivotally important roles in organic and inorganic chemistry, biochemistry, and energy storage alike [1–3]. Despite the extreme diversity of these roles, a common feature of lithium chemistry is given by the strong tendency of Li+ to interact with Lewis bases, thereby tuning their reactivity or stabilizing supramolecular structures [4–7]. For any understanding of these interactions, their strength must be known. For this reason, there is a high interest in gas-phase lithium cation basicities (LCB), as reflected in numerous experimental and theoretical studies aiming at the determination of these values for a wide range
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Electronic supplementary material The online version of this article (https:// doi.org/10.1007/s13361-019-02312-5) contains supplementary material, which is available to authorized users. Correspondence to: Konrad Koszinowski; e-mail: [email protected]–goettingen.de
of compounds [8–16]. The LCB of a given molecule A and the related lithium-ion affinity (LCA) [17] are defined as the standard Gibbs reaction energy and the standard reaction enthalpy, respectively, of the dissociation of the complex Li(A)+, Eq. (1
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