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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 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 Ca 2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca 2+ ·RCO 2 ] + ·(H 2 O) n =0 to 12 and RCO 2 ·(H 2 O) n =0 to 14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO 2 anion and the [Ca 2+ ·RCO 2 ] + 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 [Ca 2+ ·RCO 2 ] + ·(H 2 O)more » 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 interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. 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.« less

Authors:
ORCiD logo; ; ORCiD logo; ; ; ; ; ; ;
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1531201
Grant/Contract Number:  
FG02-00ER15066
Resource Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America; Journal ID: ISSN 0027-8424
Publisher:
Proceedings of the National Academy of Sciences
Country of Publication:
United States
Language:
English

Citation Formats

Denton, Joanna K., 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 K., 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 K., 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. Fri . "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_1531201,
title = {Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air–water interface},
author = {Denton, Joanna K. 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 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 Ca 2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca 2+ ·RCO 2 − ] + ·(H 2 O) n =0 to 12 and RCO 2 − ·(H 2 O) n =0 to 14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO 2 − anion and the [Ca 2+ ·RCO 2 − ] + 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 [Ca 2+ ·RCO 2 − ] + ·(H 2 O) 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 interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. 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.},
doi = {10.1073/pnas.1818600116},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = ,
volume = ,
place = {United States},
year = {2019},
month = {7}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
DOI: 10.1073/pnas.1818600116

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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