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Title: Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air–water interface

Journal Article · · Proceedings of the National Academy of Sciences of the United States of America
ORCiD logo [1];  [1]; ORCiD logo [1];  [2];  [2];  [3];  [4];  [5];  [6];  [7]
  1. Yale Univ., New Haven, CT (United States). Sterling Chemistry Lab.
  2. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Physical Sciences Div.
  3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Physical Sciences Div.; Univ. of Washington, Seattle, WA (United States). Dept. of Chemical Engineering
  4. The Ohio State Univ., Columbus, OH (United States). Dept. of Chemistry and Biochemistry; Ohio Wesleyan Univ., Delaware, OH (United States). Dept. of Chemistry
  5. The Ohio State Univ., Columbus, OH (United States). Dept. of Chemistry and Biochemistry
  6. Univ. of Pittsburgh, PA (United States). Dept. of Chemistry
  7. Univ. of Pittsburgh, PA (United States). Dept. of Chemistry and Petroleum Engineering

We exploit here 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 clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2-]+·(H2O)n=0to12and RCO2-·(H2O)n=0to14cluster ions toward the features observed at the air–water interface. 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 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2-]+·(H2O)nclusters 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 interfacial value byn= 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. Furthermore, we discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Research Organization:
Univ. of Pittsburgh, PA (United States); Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division; National Science Foundation (NSF); US Air Force Office of Scientific Research (AFOSR); Univ. of California, Berkeley, CA (United States)
Grant/Contract Number:
FG02-00ER15066; CHE-1801971; FA9550-17-1-0267; AC05-76RL01830; AC02-05CH11231; FG02-06ER15800
OSTI ID:
1531201
Alternate ID(s):
OSTI ID: 1598780
Journal Information:
Proceedings of the National Academy of Sciences of the United States of America, Vol. 116, Issue 30; Related Information: Supplementary Information forMolecular-level origin of the carboxylate head group response to divalent metalion complexation at the air-water interfaceJoanna K. Denton, Patrick J. Kelleher, Mark A. Johnson, Marcel D. Baer, Shawn M. Kathmann,Christopher J. Mundy, Bethany A. Wellen Rudd, Heather C. Allen, Tae Hoon Choi, andKenneth D. Jordan; ISSN 0027-8424
Publisher:
National Academy of SciencesCopyright Statement
Country of Publication:
United States
Language:
English

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Cited By (1)

Photochemical aging of atmospherically reactive organic compounds involving brown carbon at the air–aqueous interface journal January 2019


Figures / Tables (24)