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Shultz MJ, Bisson P, Wang J, Marmolejos J, Davies RG, Gubbins E, Xiong Z. High phase resolution: Probing interactions in complex interfaces with sum frequency generation. Biointerphases 2023; 18:058502. [PMID: 37902617 DOI: 10.1116/6.0002963] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2023] [Accepted: 09/20/2023] [Indexed: 10/31/2023] Open
Abstract
An often-quoted statement attributed to Wolfgang Pauli is that God made the bulk, but the surface was invented by the devil. Although humorous, the statement really reflects frustration in developing a detailed picture of a surface. In the last several decades, that frustration has begun to abate with numerous techniques providing clues to interactions and reactions at surfaces. Often these techniques require considerable prior knowledge. Complex mixtures on irregular or soft surfaces-complex interfaces-thus represent the last frontier. Two optical techniques: sum frequency generation (SFG) and second harmonic generation (SHG) are beginning to lift the veil on complex interfaces. Of these techniques, SFG with one excitation in the infrared has the potential to provide exquisite molecular- and moiety-specific vibrational data. This Perspective is intended both to aid newcomers in gaining traction in this field and to demonstrate the impact of high-phase resolution. It starts with a basic description of light-induced surface polarization that is at the heart of SFG. The sum frequency is generated when the input fields are sufficiently intense that the interaction is nonlinear. This nonlinearity represents a challenge for disentangling data to reveal the molecular-level picture. Three, high-phase-resolution methods that reveal interactions at the surface are described.
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Affiliation(s)
- Mary Jane Shultz
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
| | - Patrick Bisson
- Cambridge Polymer Group, Inc., 100 Trade Center Drive, Suite 200, Woburn, Massachusetts 01801
| | - Jing Wang
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
| | - Joam Marmolejos
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
| | - Rebecca G Davies
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
| | - Emma Gubbins
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
| | - Ziqing Xiong
- Laboratory for Water and Surface Studies, Tufts University, Pearson Laboratory, 62 Talbot Ave., Medford, Massachusetts 02155
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Rehl B, Li Z, Gibbs JM. Influence of High pH on the Organization of Acetonitrile at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:4445-4454. [PMID: 29580058 DOI: 10.1021/acs.langmuir.7b04289] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The acetonitrile-water mixture is one of the most commonly used solvents in hydrophilic interaction chromatography, which contains silica as the solid phase. As such, the silica/acetonitrile-water interface plays a large role in the separation of compounds. Varying the pH is one way to influence retention times, particularly of ionizable solutes, yet the influence of high pH is often unpredictable. To determine how the structure of this interface changes with pH, we utilized the surface specific technique sum frequency generation (SFG). Previous SFG studies at neutral pH have suggested the existence of acetonitrile bilayers at the aqueous silica interface even at low acetonitrile mole fractions. Here we find that the SFG signal from 2900 to 3040 cm-1 at the silica/acetonitrile-water interface increased as we adjusted the aqueous pH from near neutral to high values. This increase in signal was attributed to a greater amount of aligned water which is consistent with an increase in silica surface charge at high pH. In contrast, complementary measurements of the silica/acetonitrile-deuterium oxide interface revealed that the acetonitrile methyl mode nearly vanished as the aqueous pH was increased. This loss of methyl mode signal is indicative of a decrease in the number density of acetonitrile molecules at the interface, as orientation analysis indicates no significant change in the net orientation of the outer leaflet of the acetonitrile bilayer over the pH range studied.
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Affiliation(s)
- Benjamin Rehl
- Department of Chemistry , University of Alberta , Edmonton , Alberta T6G 2G2 , Canada
| | - Zhiguo Li
- Department of Chemistry , University of Alberta , Edmonton , Alberta T6G 2G2 , Canada
| | - Julianne M Gibbs
- Department of Chemistry , University of Alberta , Edmonton , Alberta T6G 2G2 , Canada
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Nojima Y, Suzuki Y, Takahashi M, Yamaguchi S. Proton Order toward the Surface of Ice I h Revealed by Heterodyne-Detected Sum Frequency Generation Spectroscopy. J Phys Chem Lett 2017; 8:5031-5034. [PMID: 28968104 DOI: 10.1021/acs.jpclett.7b02198] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Using heterodyne-detected sum frequency generation (HD-SFG) spectroscopy, we investigated surface proton order at the basal, primary prism, and secondary prism faces of single-crystalline ice Ih at ca. 130 K. The complex phase of the obtained spectra clearly indicates that second-order nonlinear polarization from which the HD-SFG signal arises is generated exclusively at the surfaces. This suggests surface proton ordering along the normal, whereas the bulk remains proton-disordered, as is well known for ice Ih. A strong positive peak observed in the HD-SFG spectra enables us to determine the "direction" of the surface proton order as "H-up", that is, the hydrogen atom of the OH group pointing away from the bulk, irrespective of the ice faces. Reliable HD-SFG measurements carried out in the present study have greatly advanced our understanding of surface structure of ice Ih.
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Affiliation(s)
- Yuki Nojima
- Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University , 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
| | - Yudai Suzuki
- Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University , 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
| | - Misato Takahashi
- Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University , 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
| | - Shoichi Yamaguchi
- Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University , 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
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Smit W, Tang F, Nagata Y, Sánchez MA, Hasegawa T, Backus EHG, Bonn M, Bakker HJ. Observation and Identification of a New OH Stretch Vibrational Band at the Surface of Ice. J Phys Chem Lett 2017; 8:3656-3660. [PMID: 28715224 PMCID: PMC5545757 DOI: 10.1021/acs.jpclett.7b01295] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
We study the signatures of the OH stretch vibrations at the basal surface of ice using heterodyne-detected sum-frequency generation and molecular dynamics simulations. At 150 K, we observe seven distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes at higher frequencies (∼3530 and ∼3700 cm-1). The band at ∼3530 cm-1 has not been reported previously. Using molecular dynamics simulations, we find that the ∼3530 cm-1 band contains contributions from OH stretch vibrations of both fully coordinated interfacial water molecules and water molecules with two donor and one acceptor hydrogen bond.
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Affiliation(s)
| | - Fujie Tang
- Max
Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
- International
Center for Quantum Materials, Peking University, 5 Yiheyuan Road, Haidian, Beijing 100871, China
| | - Yuki Nagata
- Max
Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
| | - M. Alejandra Sánchez
- Max
Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
| | - Taisuke Hasegawa
- Department
of Chemistry, Graduate School of Science, Kyoto University, Sakyoku, Kyoto 606-8502, Japan
| | - Ellen H. G. Backus
- Max
Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
| | - Mischa Bonn
- Max
Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
| | - Huib J. Bakker
- AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands
- E-mail:
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Ishiyama T, Morita A. Computational Analysis of Vibrational Sum Frequency Generation Spectroscopy. Annu Rev Phys Chem 2017; 68:355-377. [DOI: 10.1146/annurev-physchem-052516-044806] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
- Tatsuya Ishiyama
- Department of Applied Chemistry, Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan
| | - Akihiro Morita
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
- Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan
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Abstract
Ice is a fundamental solid with important environmental, biological, geological, and extraterrestrial impact. The stable form of ice at atmospheric pressure is hexagonal ice, Ih. Despite its prevalence, Ih remains an enigmatic solid, in part due to challenges in preparing samples for fundamental studies. Surfaces of ice present even greater challenges. Recently developed methods for preparation of large single-crystal samples make it possible to reproducibly prepare any chosen face to address numerous fundamental questions. This review describes preparation methods along with results that firmly establish the connection between the macroscopic structure (observed in snowflakes, microcrystallites, or etch pits) and the molecular-level configuration (detected with X-ray or electron scattering techniques). Selected results of probing interactions at the ice surface, including growth from the melt, surface vibrations, and characterization of the quasi-liquid layer, are discussed.
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Affiliation(s)
- Mary Jane Shultz
- Laboratory for Water and Surface Studies, Department of Chemistry, Tufts University, Medford, Massachusetts 02155
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Saito K, Peng Q, Qiao L, Wang L, Joutsuka T, Ishiyama T, Ye S, Morita A. Theoretical and experimental examination of SFG polarization analysis at acetonitrile–water solution surfaces. Phys Chem Chem Phys 2017; 19:8941-8961. [DOI: 10.1039/c6cp08856b] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Polarization analysis of SFG spectroscopy is thoroughly examined in collaboration of SFG measurements and MD simulations.
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Affiliation(s)
- Kengo Saito
- Department of Chemistry
- Graduate School of Science
- Tohoku University
- Aoba-ku
- Japan
| | - Qiling Peng
- Institute for Catalysis
- Hokkaido University
- Kita-ku
- Japan
| | - Lin Qiao
- Institute for Catalysis
- Hokkaido University
- Kita-ku
- Japan
| | - Lin Wang
- Department of Chemistry
- Graduate School of Science
- Tohoku University
- Aoba-ku
- Japan
| | - Tatsuya Joutsuka
- Department of Chemistry
- Graduate School of Science
- Tohoku University
- Aoba-ku
- Japan
| | - Tatsuya Ishiyama
- Department of Applied Chemistry
- Graduate School of Science and Engineering
- University of Toyama
- Toyama 930-8555
- Japan
| | - Shen Ye
- Institute for Catalysis
- Hokkaido University
- Kita-ku
- Japan
- Elements Strategy Initiative for Catalysts and Batteries (ESICB)
| | - Akihiro Morita
- Department of Chemistry
- Graduate School of Science
- Tohoku University
- Aoba-ku
- Japan
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Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice. Proc Natl Acad Sci U S A 2016; 114:227-232. [PMID: 27956637 DOI: 10.1073/pnas.1612893114] [Citation(s) in RCA: 77] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.
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Wan Q, Galli G. First-Principles Framework to Compute Sum-Frequency Generation Vibrational Spectra of Semiconductors and Insulators. PHYSICAL REVIEW LETTERS 2015; 115:246404. [PMID: 26705645 DOI: 10.1103/physrevlett.115.246404] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2015] [Indexed: 05/22/2023]
Abstract
We present a first-principles framework to compute sum-frequency generation (SFG) vibrational spectra of semiconductors and insulators. The method is based on density functional theory and the use of maximally localized Wannier functions to compute the response to electric fields, and it includes the effect of electric field gradients at surfaces. In addition, it includes quadrupole contributions to SFG spectra, thus enabling the verification of the dipole approximation, whose validity determines the surface specificity of SFG spectroscopy. We compute the SFG spectra of ice I_{h} basal surfaces and identify which spectra components are affected by bulk contributions. Our results are in good agreement with experiments at low temperature.
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Affiliation(s)
- Quan Wan
- Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, USA
| | - Giulia Galli
- Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, USA
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Shultz MJ, Bisson P, Vu TH. Insights into hydrogen bonding via ice interfaces and isolated water. J Chem Phys 2015; 141:18C521. [PMID: 25399186 DOI: 10.1063/1.4896603] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Water in a confined environment has a combination of fewer available configurations and restricted mobility. Both affect the spectroscopic signature. In this work, the spectroscopic signature of water in confined environments is discussed in the context of competing models for condensed water: (1) as a system of intramolecular coupled molecules or (2) as a network with intermolecular dipole-dipole coupled O-H stretches. Two distinct environments are used: the confined asymmetric environment at the ice surface and the near-isolated environment of water in an infrared transparent matrix. Both the spectroscopy and the environment are described followed by a perspective discussion of implications for the two competing models. Despite being a small molecule, water is relatively complex; perhaps not surprisingly the results support a model that blends inter- and intramolecular coupling. The frequency, and therefore the hydrogen-bond strength, appears to be a function of donor-acceptor interaction and of longer-range dipole-dipole alignment in the hydrogen-bonded network. The O-H dipole direction depends on the local environment and reflects intramolecular O-H stretch coupling.
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Affiliation(s)
- Mary Jane Shultz
- Laboratory for Water and Surface Studies, Chemistry Department, Tufts University, Medford, Massachusetts 02155, USA
| | - Patrick Bisson
- Laboratory for Water and Surface Studies, Chemistry Department, Tufts University, Medford, Massachusetts 02155, USA
| | - Tuan Hoang Vu
- Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA
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Shultz MJ, Bisson PJ, Brumberg A. Best face forward: crystal-face competition at the ice-water interface. J Phys Chem B 2014; 118:7972-80. [PMID: 24784996 DOI: 10.1021/jp500956w] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The ice-water interface plays an important role in determining the outcome of both biological and environmental processes. Under ambient pressure, the most stable form of ice is hexagonal ice (Ih). Experimentally probing the surface free energy between each of the major faces of Ih ice and the liquid is both experimentally and theoretically challenging. The basis for the challenge is the near-equality of the surface free energy for the major faces along with the tendency of water to supercool. As a result, morphology from crystallization initiated below 0 °C is kinetically controlled. The reported work circumvents supercooling consequences by providing a polycrystalline seed, followed by isothermal, equilibrium growth. Natural selection among seeded faces results in a single crystal. A record of the growth front is preserved in the frozen boule. Crystal orientation at the front is revealed by examining the boule cross section with two techniques: (1) viewing between crossed polarizers to locate the optical axis and (2) etching to distinguish the primary-prism face from the secondary-prism face. Results suggest that the most stable ice-water interface at 0 °C is the secondary-prism face, followed by the primary-prism face. The basal face that imparts the characteristic hexagonal shape to snowflakes is a distant third. The results contrast with those from freezing the vapor where the basal and primary-prism faces have comparable free energy followed by the secondary-prism face.
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Affiliation(s)
- Mary Jane Shultz
- Laboratory for Water and Surface Studies, Tufts University , 62 Talbot Avenue, Medford, Massachusetts 02155, United States
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Ishiyama T, Imamura T, Morita A. Theoretical Studies of Structures and Vibrational Sum Frequency Generation Spectra at Aqueous Interfaces. Chem Rev 2014; 114:8447-70. [DOI: 10.1021/cr4004133] [Citation(s) in RCA: 110] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- Tatsuya Ishiyama
- Department
of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Takako Imamura
- Department
of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Akihiro Morita
- Department
of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
- Elements
Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto 615-8520, Japan
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13
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Reprint of “Role of the fluidity of a liquid phase in determining the surface properties of the opposite phase”. J Mol Liq 2014. [DOI: 10.1016/j.molliq.2013.06.009] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Role of the fluidity of a liquid phase in determining the surface properties of the opposite phase at the liquid–liquid interface. J Mol Liq 2013. [DOI: 10.1016/j.molliq.2013.04.012] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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Bisson PJ, Shultz MJ. Hydrogen bonding in the prism face of ice I(h) via sum frequency vibrational spectroscopy. J Phys Chem A 2013; 117:6116-25. [PMID: 23451801 DOI: 10.1021/jp400129f] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
The prism face of single crystal ice I(h) has been studied using sum frequency vibrational spectroscopy focusing on identification of resonances in the hydrogen-bonded region. Several modes have been observed at about 3400 cm(-1); each mode is both polarization and orientation dependent. The polarization capabilities of sum frequency generation (SFG) are used in conjunction with the crystal orientation to characterize three vibrational modes. These modes are assigned to three-coordinated water molecules in the top-half bilayer having different bonding and orientation motifs.
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Affiliation(s)
- Patrick J Bisson
- Laboratory for Water and Surface Studies, Department of Chemistry, Pearson Laboratory, Tufts University, Medford, Massachusetts 02155, USA
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Ishiyama T, Takahashi H, Morita A. Origin of Vibrational Spectroscopic Response at Ice Surface. J Phys Chem Lett 2012; 3:3001-6. [PMID: 26292241 DOI: 10.1021/jz3012723] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Since the basal plane surface of ice was first observed by sum frequency generation, an extraordinarily intense band for the hydrogen(H)-bonded OH stretching vibration has been a matter of debate. We elucidate the remarkable spectral feature of the ice surface by quantum mechanics/molecular mechanics calculations. The intense H-bonded band is originated mostly from the "bilayer-stitching" modes of a few surface bilayers, through significant intermolecular charge transfer. The mechanism of enhanced signal is sensitive to the order of the tetrahedral ice structure, as the charge transfer is coupled to the vibrational delocalization.
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Affiliation(s)
- Tatsuya Ishiyama
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Hideaki Takahashi
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
| | - Akihiro Morita
- Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
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Barnett IL, Groenzin H, Shultz MJ. Hydrogen bonding in the hexagonal ice surface. J Phys Chem A 2010; 115:6039-45. [PMID: 21189006 DOI: 10.1021/jp110431j] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm(-1). On the basal face, the reddest resonance occurs at 3098 cm(-1); within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm(-1), occurs slightly blue-shifted at 3393 cm(-1) in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm(-1) mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm(-1) mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm(-1) mode is entirely longitudinal.
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Affiliation(s)
- Irene Li Barnett
- Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA
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