Mechanism of vibrational energy dissipation of free OH groups at the air–water interface

Abstract

Interfaces of liquid water play a critical role in a wide variety of processes that occur in biology, a variety of technologies, and the environment. Many macroscopic observations clarify that the properties of liquid water interfaces significantly differ from those of the bulk liquid. In addition to interfacial molecular structure, knowledge of the rates and mechanisms of the relaxation of excess vibrational energy is indispensable to fully understand physical and chemical processes of water and aqueous solutions, such as chemical reaction rates and pathways, proton transfer, and hydrogen bond dynamics. Here we elucidate the rate and mechanism of vibrational energy dissipation of water molecules at the air-water interface using femtosecond two-color IR-pump/vibrational sum-frequency probe spectroscopy. Vibrational relaxation of nonhydrogen-bonded OH groups occurs at a subpicosecond timescale in a manner fundamentally different from hydrogenbonded OH groups in bulk, through two competing mechanisms: intramolecular energy transfer and ultrafast reorientational motion that leads to free OH groups becoming hydrogen bonded. Both pathways effectively lead to the transfer of the excited vibrational modes from free to hydrogen-bonded OH groups, from which relaxation readily occurs. Of the overall relaxation rate of interfacial free OH groups at the air-H 2 O interface, two-thirds are accounted for by intramolecular energy transfer, whereas the remaining one-third is dominated by the reorientational motion. These findings not only shed light on vibrational energy dynamics of interfacial water, but also contribute to our understanding of the impact of structural and vibrational dynamics on the vibrational sum-frequency line shapes of aqueous interfaces.sum-frequency generation | vibrational spectroscopy | energy relaxation | reorientation | isotopically diluted water

Changes in the hydrogen-bonding strength of internal water molecules and cysteine residues in the conductive state of channelrhodopsin-1

“…3(b)), done to attenuate the spectral contributions from water heating without the need of performing a baseline correction. The result clearly shows the presence of two positive bands at 3648 and 3623 cm −1 and an about twice as intense negative band at 3634 cm −1 , indicating that both a positive (at ∼3648 cm −1 ) and the negative band (at ∼3634 cm −1 ) are sensitive to 18 O labeling of water. In an alternative procedure, the broad spectral contributions of water heating was digitally removed from the lightinduced IR difference spectrum of CaChR1 as described in Sec.…”

“…16 In fact, vibrational sum-frequency spectroscopy, a surface-sensitive technique, has shown that the interface is particularly enriched in dangling O-H groups with a characteristic vibration at ∼3680 cm −1 . 17,18 This band is relatively narrow (∼30 cm −1 ) 17,18 with a line width partially limited by the fast reformation of H-bonds at the interface due to fast reorientation of interface water molecules. 18 Dangling O-D groups at the water-air interface oscillate at ∼2740 cm −1 and exhibit narrow bandwidths (11-14 cm −1 ).…”

“…17,18 This band is relatively narrow (∼30 cm −1 ) 17,18 with a line width partially limited by the fast reformation of H-bonds at the interface due to fast reorientation of interface water molecules. 18 Dangling O-D groups at the water-air interface oscillate at ∼2740 cm −1 and exhibit narrow bandwidths (11-14 cm −1 ). 19 Protonated water clusters (comprising 2-27 water molecules) have also been intensively studied by vibrational spectroscopy.…”

Untersuchung der Struktur und Dynamik von Wasser an der Wasser‐Luft‐Grenzfläche mittels oberflächenspezifischer Schwingungsspektroskopie

“…Falls eine freie OH-Gruppe H-Brücken bildet, verändert sich ihre Schwingungsfrequenz, jedoch wird die Anregung anfangs noch auf dieser OH-Gruppe verbleiben. Solche Experimente [93] enthüllten in der Tat, dass ein Austausch zwischen den beiden Untergruppen auf einer Zeitskala von ca. 1pss tattfindet, was mit der angenommenen Reorientierungsdynamik übereinstimmt.…”

“…[23,110] Daraus folgt, dass die Bildung einer Wasserstoffbrücke ein Wegf reier OH-Gruppen ist, schnell Energie abzugeben. Eine andere Mçglichkeit, welche auf vergleichbaren Zeitskalen stattfindet, ist ein beinahe resonanter Energietransfer von der freien OH-Gruppe zur anderen OH-Gruppe desselben Moleküls.B eide Relaxationsmechanismen -R eorientierung und Energietransfer -e rlauben es der freien OH-Gruppe,ü berschüssige Energie auf einer Zeitskala im Subpikosekundenbereich abzugeben, [93] was einige Grçßenordnungen schneller ist als beispielsweise für die nicht-H-gebundenen OH-Gruppen an einer Siliciumdioxidoberfläche. [11] Damit ermçglicht dieser Mechanismus der Wasseroberfläche die hocheffiziente Abgabe überschüs-siger Schwingungsenergie.…”