PNNL

Abstract

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information is used to clarify the experimental findings of surface sensitive spectroscopies that the CO stretching bands of long chain acids appear at very similar energies when the head group is deprotonated by high sub-phase pH or is exposed to relatively high concentrations of Ca2+ divalent metal ions (e.g., Ca2+ and Mg2+). To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2¯]+·(H2O)n=0-14 and RCO2¯·(H2O)n=0-14 cluster ions obtained using infrared photofragmentation mass spectrometry toward the features observed at the air-water interface. The carboxylate with R = -CD2CD3 was chosen in order to capture the longer chain behavior and to suppress band congestion that complicates the spectra of the light isotopologue. Surprisingly, not only does stepwise hydration of the RCO2¯ anion and the [Ca2+·RCO2¯]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases the responses of the two CO bands (?_s^COO and ?_as^COO) to hydration are opposite to each other! The result is that the two CO bands evolve toward their interfacial and bulk asymptotes from opposite directions. Theoretical simulations of the [Ca2+·RCO2¯]+·(H2O)n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the bulk value by n = 12, establishing that direct metal complexation can account for the interfacial and bulk behavior. We discuss these effects in the context of a qualitative model that invokes the water network-dependent local electric field along the C-C bond connecting the head group to the tail as the key parameter driving the observed trends. M.A.J. and H.C.A. thank the National Science Foundation through the NSF Center for Aerosol Impacts on Chemistry of the Environment (NSF-CAICE), CHE-1305427 for supporting this work. M.A.J. thanks the Air Force Office of Scientific Research for the cryogenic spectrometer used in the gas phase experiments. K.D.J. thanks the Department of Energy under grant no. DE-FG02-00ER15066. C.J. Mundy and S.M. Kathmann are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. M.D. Baer was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Biomolecular Materials Program at PNNL. PNNL is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract No. DE-AC05- 76RL01830. Computing resources for the calculation of harmonic spectra were allocated by PNNL’s Institutional Computing program.