Coordination-Dependent Spectroscopic Signatures of Divalent Metal Ion Binding to Carboxylate Head Groups: H 2 - and He-Tagged Vibrational Spectra of M 2+ ·RCO 2 ¯ (M = Mg and Ca, R = -CD 3 , -CD 2 CD 3 ) Complexes.

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.