Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H2- and He-Tagged Vibrational Spectra of M2+·RCO2¯ (M = Mg and Ca, R = −CD3, −CD2CD3) Complexes

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• 作者：Joseph W. DePalmaPatrick J. Kelleher ; Laís C. Tavares ; Mark A. Johnson
• 刊名：The Journal of Physical Chemistry Letters
• 出版年：2017
• 出版时间：January 19, 2017
• 年：2017
• 卷：8
• 期：2
• 页码：484-488
• 全文大小：368K
• ISSN：1948-7185

文摘

We explore the intramolecular distortions present in divalent metal ion–carboxylate ion pairs using vibrational spectroscopy of the cryogenically cooled, mass-selected species isolated in the gas phase. The spectral signatures of the C–O stretching modes are identified using the perdeutero isotopologues of the acetate and propionate anions to avoid congestion arising from the CH₂ fundamentals. Both Ca²⁺ and Mg²⁺ are observed to bind in a symmetrical, so-called “bidentate” arrangement to the −CO₂¯ group. The very strong deformations of the head groups displayed by the binary complexes dramatically relax when either neutral water molecules or counterions are attached to the Mg²⁺RCO₂¯ cation. These results emphasize the critical role that local coordination plays when using the RCO₂¯ bands to deduce the metal ion complexation motif in condensed media.