Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy

Stiopkin, Igor V.; Weeraman, Champika; Pieniazek, Piotr A.; Shalhout, Fadel Y.; Skinner, James; Benderskii, Alexander V.

doi:10.1038/nature10173

Cited by 399 publications

(452 citation statements)

References 28 publications

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“…The vibrational signatures of an adsorbate in the mid-IR region are sensitive to the local environment, so VSFG spectroscopy can be used in unraveling the adsorbate-adsorbate and adsorbate-substrate interactions. [35][36][37][38][39][40] The input and output beam polarizations of VSFG can be independently controlled to produce information about the molecular orientation of the surface species. 41 The molecular structure and rotational dynamics of CH 3 -Si(111) surfaces has been elucidated using polarization-selected VSFG spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

et al. 2017

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Vibrational sum-frequency generation (VSFG) spectroscopy was used to investigate the orientation and azimuthal anisotropy of the C-H stretching modes for propynyl-terminated Si(111) surfaces, Si-C≡C-CH 3 . VSFG spectra revealed symmetric and asymmetric C-H stretching modes in addition to a Fermi resonance mode resulting from the interaction of the asymmetric C-H bending overtone with the symmetric C-H stretching vibration. The polarization dependence of the C-H stretching modes was consistent with the propynyl groups oriented such that the Si-C≡C bond is normal to the Si(111) surface. The azimuthal angle dependence of the resonant C-H stretching amplitude revealed no rotational anisotropy for the symmetric C-H stretching mode and a 3-fold rotational anisotropy for the asymmetric C-H stretching mode in registry with the 3-fold symmetric Si(111) substrate. The results are consistent with expectation that the C-H stretching modes of a -CH 3 group are decoupled from the Si substrate due to a -C≡C-spacer. In contrast, the methyl-terminated Si(111) surface, Si-CH 3 , was previously reported to have pronounced vibronic coupling of the methyl stretch modes to the electronic bath of bulk Si. Vacuum-annealing of propynyl-terminated Si(111) resulted in increased 3-fold azimuthal anisotropy for the symmetric stretch, suggesting that removal of

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Section: Introductionmentioning

confidence: 99%

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

et al. 2017

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“…Experimental proof for strengthened water H bonds and icebergs near purely hydrophobic solutes (alkanes and noble gases) would confirm unequivocally the classical view. Vibrational spectroscopy of the oxygen-hydrogen (O-H) stretching mode (ν OH ) is commonly used as the most reliable, sensitive, and model-free approach for determining relative strengths of H bonds (18)(19)(20)(21)(22)(23)(24). Any frequency downshift (redshift) of the ν OH mode, detected by comparing spectra of bulk water with those of the water molecules perturbed by solute, would indicate enhanced H-bond strength.…”

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confidence: 99%

Origin of hydrophobicity and enhanced water hydrogen bond strength near purely hydrophobic solutes

Grdadolnik

Merzel

Avbelj

2016

Proc. Natl. Acad. Sci. U.S.A.

197

145

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Hydrophobicity plays an important role in numerous physicochemical processes from the process of dissolution in water to protein folding, but its origin at the fundamental level is still unclear. The classical view of hydrophobic hydration is that, in the presence of a hydrophobic solute, water forms transient microscopic "icebergs" arising from strengthened water hydrogen bonding, but there is no experimental evidence for enhanced hydrogen bonding and/or icebergs in such solutions. Here, we have used the redshifts and line shapes of the isotopically decoupled IR oxygen-deuterium (O-D) stretching mode of HDO water near small purely hydrophobic solutes (methane, ethane, krypton, and xenon) to study hydrophobicity at the most fundamental level. We present unequivocal and model-free experimental proof for the presence of strengthened water hydrogen bonds near four hydrophobic solutes, matching those in ice and clathrates. The water molecules involved in the enhanced hydrogen bonds display extensive structural ordering resembling that in clathrates. The number of ice-like hydrogen bonds is 10-15 per methane molecule. Ab initio molecular dynamics simulations have confirmed that water molecules in the vicinity of methane form stronger, more numerous, and more tetrahedrally oriented hydrogen bonds than those in bulk water and that their mobility is restricted. We show the absence of intercalating water molecules that cause the electrostatic screening (shielding) of hydrogen bonds in bulk water as the critical element for the enhanced hydrogen bonding around a hydrophobic solute. Our results confirm the classical view of hydrophobic hydration.hydrophobic hydration | hydrogen bonding | IR spectroscopy | electrostatic screening | ab initio molecular dynamics D espite its great importance in numerous phenomena, the origin of hydrophobicity remains one of the most disputed topics in science (1-5). Experimental studies have shown that small purely hydrophobic solutes (alkanes and noble gases) in water increase the order (6, 7) and restrict the mobility (8) of neighboring water molecules. There are several opposing views on how to explain these data. The classical view is that a solute modifies water structure by forming transient, semiordered clathrate-like clusters ("icebergs"; used here only as a loose term) around it, arising from enhanced water hydrogen bonding (H bonding) (6, 7). This enhancement is brought about by either strengthening (9) or increasing the number of water to water H bonds (10). The classical view explains the characteristic changes in the thermodynamic variables of hydrophobic hydration (positive ΔG, ΔC p , negative ΔS, and ΔH) and the restricted mobility of water molecules observed by NMR (8). Neutron diffraction (11, 12) and extended X-ray absorption fine structure (EXAFS) (13) studies, however, show that the water molecules around small purely hydrophobic molecules do not differ significantly from those in pure liquid water. According to the dynamic view, the hydrophobic solute causes slowdown of t...

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“…Despite its importance, the characterization of acid-base chemistry at aqueous interfaces remains fraught with uncertainties (7)(8)(9)(10)(11). Basic questions linger about the thickness of interfacial layers (12), how acidity changes through the interfacial region (13), and the mechanistic differences between proton transfer (PT) across interfacial (IF) versus in bulk (B) water (10,14). Because aqueous surfaces are usually charged relative to the bulk liquid (15), the thermodynamic requirement of uniform electrochemical activity throughout (including the interfacial regions) implies that the chemical activity of protons (pH) in IF could be different from that in the B liquid.…”

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confidence: 99%

Brønsted basicity of the air–water interface

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

172

203

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Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H 2 O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH − ions are present therein. Thus, by detecting RCOO − we demonstrate the presence of OH − on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me) 3 N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH − densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH − gradients in the interfacial region. This fact could account for the weak OH − signals detected by surface-specific spectroscopies.gas-liquid reactions | surface potential | water surface acidity | interfacial proton transfer A cid-base chemistry at aqueous interfaces lies at the heart of major processes in chemistry and biology. Changes in the degree of dissociation of the acidic/basic residues upon translocation between aqueous and hydrophobic microenvironments orchestrate enzyme catalysis (1), drive proton/electron transport across biomembranes (2, 3), and mediate molecular recognition and self-assembly phenomena (4-6). Despite its importance, the characterization of acid-base chemistry at aqueous interfaces remains fraught with uncertainties (7-11). Basic questions linger about the thickness of interfacial layers (12), how acidity changes through the interfacial region (13), and the mechanistic differences between proton transfer (PT) across interfacial (IF) versus in bulk (B) water (10,14). Because aqueous surfaces are usually charged relative to the bulk liquid (15), the thermodynamic requirement of uniform electrochemical activity throughout (including the interfacial regions) implies that the chemical activity of protons (pH) in IF could be different from that in the B liquid. Reduced hydration of ionic species at the interface could force acids and bases toward their undissociated forms (16).These fundamental issues have been extensively investigated via electrostatic (17) and electrokinetic experiments (11), surface tension studies and analysis (1...

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Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy

Cited by 399 publications

References 28 publications

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

Origin of hydrophobicity and enhanced water hydrogen bond strength near purely hydrophobic solutes

Brønsted basicity of the air–water interface

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