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

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 asymptotesmore » 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.« less

Authors:
 [1];  [1];  [1];  [2];  [2];  [2];  [3];  [4];  [5];  [5]
  1. Yale University
  2. BATTELLE (PACIFIC NW LAB)
  3. Ohio Wesleyan University
  4. Ohio State University
  5. University of Pittsburgh
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1571290
Report Number(s):
PNNL-SA-138980
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 116; Journal Issue: 30
Country of Publication:
United States
Language:
English

Citation Formats

Denton, Joanna, Kelleher, Patrick J., Johnson, Mark A., Baer, Marcel D., Kathmann, Shawn M., Mundy, Christopher J., Wellen Rudd, Bethany A., Allen, Heather C., Choi, Tae Hoon, and Jordan, Kenneth D. Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface. United States: N. p., 2019. Web. doi:10.1073/pnas.1818600116.
Denton, Joanna, Kelleher, Patrick J., Johnson, Mark A., Baer, Marcel D., Kathmann, Shawn M., Mundy, Christopher J., Wellen Rudd, Bethany A., Allen, Heather C., Choi, Tae Hoon, & Jordan, Kenneth D. Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface. United States. doi:10.1073/pnas.1818600116.
Denton, Joanna, Kelleher, Patrick J., Johnson, Mark A., Baer, Marcel D., Kathmann, Shawn M., Mundy, Christopher J., Wellen Rudd, Bethany A., Allen, Heather C., Choi, Tae Hoon, and Jordan, Kenneth D. Tue . "Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface". United States. doi:10.1073/pnas.1818600116.
@article{osti_1571290,
title = {Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface},
author = {Denton, Joanna and Kelleher, Patrick J. and Johnson, Mark A. and Baer, Marcel D. and Kathmann, Shawn M. and Mundy, Christopher J. and Wellen Rudd, Bethany A. and Allen, Heather C. and Choi, Tae Hoon and Jordan, Kenneth D.},
abstractNote = {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.},
doi = {10.1073/pnas.1818600116},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 30,
volume = 116,
place = {United States},
year = {2019},
month = {7}
}

Works referenced in this record:

The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors
journal, October 1970