Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two‐Dimensional Heterodyne‐Detected VSFG Spectroscopy

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular‐level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate‐ion (TPB^?)/water and tetraphenylarsonium‐ion (TPA⁺)/water interfaces, which are clarified by two‐dimensional heterodyne‐detected vibrational sum‐frequency generation (2D HD‐VSFG) spectroscopy. The obtained 2D HD‐VSFG spectra of the anionic TPB^? interface reveal the existence of distinct π‐hydrogen bonded OH groups in addition to the usual hydrogen‐bonded OH groups, which are hidden in the steady‐state spectrum. In contrast, 2D HD‐VSFG spectra of the cationic TPA⁺ interface only show the presence of usual hydrogen‐bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.     

Ultrafast vibrational dynamics of water at a charged interface revealed by two-dimensional heterodyne-detected vibrational sum frequency generation

Two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy is performed for an aqueous interface for the first time. The 2D HD-VSFG spectra in the OH stretch region are obtained from a positively charged surfactant∕water interface with isotopically diluted water (HOD∕D(2)O) to reveal the femtosecond vibrational dynamics of water at the charged interface. The 2D HD-VSFG spectrum is diagonally elongated immediately after photoexcitation, clearly demonstrating inhomogeneity in the interfacial water. This elongation almost disappears at 300 fs owing to the spectral diffusion. Interestingly, the 2D HD-VSFG spectrum at the 0 fs shows an oppositely asymmetric shape to the corresponding 2D IR spectrum in bulk water: The bandwidth of the bleach signal gets narrower when the pump wavenumber becomes higher. This suggests that the dynamics and mechanism of the hydrogen bond rearrangement at the charged interface are significantly different from those in bulk water.     

Role of tetrapropylammonium cations in gel-phase silicalite synthesis

Silicalite synthesis from tetrapropylammonium (TPA⁺) sodium silicate gels was studied by X-ray diffraction, elemental analysis, ion exchange, ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. Based on this information we confirm a hydrogel—solid transformation mechanism for silicalite crystallization. The initial synthesis gel is a highly articulated silicate network containing pockets of water with solvated Na⁺ and TPA⁺ cations. As the silica condenses and becomes more hydrophobic, water and solvated cations are expelled. The condensed silicate gel then encapsulates the hydrophobic TPA⁺ cations in cages resembling the channel intersections of silicalite before X-ray crystalline silicalite is observed. Crystallization occurs within the gel via rearrangement of the TPA⁺-occluded silicate cages by the breaking and reformation of siloxane bonds into the more stable silicalite structure. Rates of nucleation and crystallization both increase with increasing TPA⁺ gel content. The amount of silicalite which forms is limited by the amount of TPA⁺, which must be present in the ratio of one TPA⁺ per channel intersection.     

Electrometric Behavior of Tetraphenylboron on Mercury

Electrometric behavior of tetraphenylboron (TPB) on mercury electrode was investigated, Potentiometric titration of Hg₂⁺⁺ with TPB⁻ showed that TPB⁻ reacted with Hg₂⁺⁺ with stoichiometric ratios (equivalent) of 1:3 and 1:8 (TPB: Hg₂⁺⁺) to form diphenylmercury, Hg(C₆H₅)₂, and phenylmercuric ion, C₆H₅—Hg⁺. Only 1:3 stoichiometry was found in the reverse titration. Potential response of TPB⁻ on mercury assumed the same result, and furthermore, if the concentration of TPB⁻ was around 10⁻³ M or higher, another stoichiometry of 1:1 with formation of Hg(I)-tetraphenylborate might be possible. Controlled potential oxidation of TPB⁻ on mercury gave Hg(C₆H₅)₂ and C₆H₅Hg⁺ which were identified by m. p. measurements and elementary analyses.     

Infrared and NMR Spectroscopic Fingerprints of the Asymmetric H7+O3 Complex in Solution

Infrared (IR) absorption in the 1000–3700 cm⁻¹ range and ¹H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H₇⁺O_3, in acetonitrile. The core H₇⁺O₃ motif persists in larger protonated water clusters in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible proton transport promoted by propagating the asymmetric H₇⁺O₃ structure in solution. The QM/MM calculations allow for the successful simulation of the measured IR absorption spectra of H₇⁺O₃ in the OH stretch region, which reaffirms the assignment of the H₇⁺O₃ spectra to a hybrid-complex structure: a protonated water dimer strongly hydrogen-bonded to a third water molecule with the proton exchanging between the two possible shared-proton Zundel-like centers. The H₇⁺O₃ structure lends itself to promoting irreversible proton transport in presence of even one additional water molecule. We demonstrate how continuously evolving H₇⁺O₃ structures may support proton transport within larger water solvates.     

Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air–Water Interface and Its Atmospheric Implications

The atmospheric role of photochemical processes involving NO₂ beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO₂^* beyond 420 nm reacts with water according to NO₂^*+H₂O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10⁻¹⁵–3.4×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO₂ adsorbed at the air–water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO₂ concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies.     

Membrane catalytic deprotonation effects

Membrane catalytic deprotonation of water (water splitting) has been studied on the base of a new model which suggests that water molecules are prepolarized by H⁺-affinited and OH⁻-affinited fixed charged groups of membrane before their dissociation is enhanced by electric field. Introducing some anion selective groups such as Mg(OH)₂·xH₂O or amine into a cation selective perfluorosulfonated membrane can initiate a dramatic water splitting effect and give rise to new high frequency peaks on the OH and OD stretching region of IR spectra. This supports the hypothesis that some water molecules were affected by the surrounding electrical field from the bipolar membrane-like structure. Perfluorocarboxylic membrane was also tested in a electrolytic cell and it causes H⁺ ion fluxes much larger than Nafion-type membrane. We classify the effect as membrane catalytic deprotonation of carboxylic acid group.     

Vibrational analysis of sodium α-, β- and γ-hydroxybutyrates. Inter- and intramolecular hydrogen bonds

The infrared and Raman spectra of sodium α-, β- and γ-hydroxybutyrates and their deuterated analogues are examined in the 4000-100 cm⁻¹ range and an assignment of the fundamental vibrations is given. Based on the localization of the asymmetric stretching vibrations ν_asOH and the out-of-plane vibration γOH, inter- and/or intramolecularly hydrogen-bonded forms are proposed: the low frequencies of ν_asOH (<3200 cm⁻¹) and high frequencies of γOH (≈800 cm⁻¹) argue in favour of the existence of intramolecular hydrogen bonding. Sodium α-hydroxybutyrate exhibits as a chelate ring with an intramolecular hydrogen bond between hydroxyl and carboxyl groups, whereas sodium, β-hydroxybutyrate has the two association forms with inter- and intramolecular hydrogen bonds. Sodium γ-hydroxybutyrate exists as a hydrogen-bonded polymer, with an intermolecular hydrogen bond between the hydroxyl groups and between the hydroxyl and carbonyl groups. At a crystallization temperature above 50°C, only the α- salt showed a structural change indicating the existence of intra- and intermolecular hydrogen bonds. This result is confirmed by differential scanning analysis.     

Vibrational circular dichroism in hydrogen bond systems: Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol

Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3- dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G⁻G⁺ conformation in the intramolecular hydrogen bond system

gives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as ΔH =−7.8 kJ mol⁻¹ and ΔS = −18 kJ K⁻¹ mol⁻¹.     

Electrochemical Investigation for Cu2+ Oscillatory Phenomena at the Liquid/Liquid Interface with a Specific Adsorption of Ion Pair Model

In this paper, a novel current oscillatory phenomenon for Cu²⁺ at the water/1,2‐dichloroethane interface is reported with cyclic voltammetry and potential‐step chronoamperometry. The small irregular current spikes were only observed near the site of the oxidation peak of CuCl₂^? and were mainly related to the Cu²⁺concentration in the aqueous phase. Our experimental results demonstrated that the current oscillation is caused by specific adsorption of ion pairs at the W/DCE interface between Cu²⁺ in the aqueous phase and TPB^? in the organic phase. Therefore, a specific adsorption of ion pair model has been formulated for the current oscillation at the liquid/liquid interface. The DFT calculation method was used to explain the mechanism of ion pair formation. The calculation results suggested that the TPB^?Cu²⁺TPB^? ion pair has the lowest‐energy state, thus providing qualitative support for the ion pair model. A probable mechanism for the observed current oscillation was also discussed in this paper. At the same time, a spectrophotometric experiment was performed to evidence a strong attractive interaction between Cu²⁺ and TPB^?.     

Early Stage Solvation of Protonated Methanol by Carbon Dioxide (cited: 1)

The solvation of protonated methanol by carbon dioxide has been studied via a cluster model. Quantum chemical calculations of the H⁺(CH₃OH)(CO₂)_n⁺(n=1-7) clusters indicate that the rst solvation shell of the OH groups is completed at n=3 or 4. Besides hydrogen-bond interaction, the C_CO2…O_CO2 intermolecular interaction is also responsible for the stabilization of the larger clusters. The transfer of the proton from methanol onto CO₂ with the formation of the OCOH⁺ moiety might be unfavorable in the early stage of solvation process. Simulated IR spectra reveal that vibrational frequencies of free O-H stretching, hydrogen-bonded O-H stretching, and O-C-O stretching of CO₂ unit a ord the sensitive probe for exploring the solvation of protonated methanol by carbon dioxide. IR spectra for the H⁺(CH₃OH)(CO₂)_n⁺(n=1-7) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes.     

Solvation of Calcium–Phosphate Headgroup Complexes at the DPPC/Aqueous Interface

The effects of sodium (Na⁺) and calcium (Ca²⁺) cations on model zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayers spread on metal chloride salt solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne‐detected (HD)‐VSFG spectroscopy. VSFG and HD‐VSFG spectra in the OH stretching region reveal cation‐specific effects on the interfacial water′s H‐bonding network, knowledge of which has been limited to date. It is found that low‐concentrated Ca²⁺ more strongly perturbs interfacial water organization relative to highly concentrated Na⁺. At higher Ca²⁺ concentrations, the water H‐bonding network at the DPPC/CaCl₂ interface reorganizes and the resulting spectrum closely follows that of the bare air/CaCl₂ interface up to ～3400 cm^?1. Most interesting is the appearance of a negative band at ～3450 cm^?1 in the DPPC/CaCl₂ Im χ_s⁽²⁾ spectra, likely arising from an asymmetric solvation of Ca²⁺–phosphate headgroup complexes. This gives rise to an electric field that orients the net OH transition moments of a subset of OH dipoles toward the bulk solution.     

Distribution of isomorphous substitutions in silicates by NMR spectroscopy

The use of spectroscopic methods permits in certain cases to investigate the relative distribution of cations and anions in silicate structures. In particular, in micas (phyllosilicates 2:1) with very heterogeneous composition, the use of multinuclear (²⁹Si, ²⁷Al, ¹H and ¹⁹F) NMR spectroscopy has demonstrated the existence of local ordering in both the tetrahedral and octahedral sheets.From the study of NMR spectra associated to OH⁻ and F⁻ ions we have concluded that: a) the Fe⁺² ions have not any preference to occupy M1 or M2 sites in the octahedral sheet of micas, b) the F⁻ ions are located in homogeneous domains composed only by F⁻ and Mg⁺² ions, and c) the Fe⁺² are in close association with OH⁻ ions. As the content in Fe⁺² and F⁻ increases Fe⁺² ions have a net tendency to cluster around the OH groups.The analysis of ²⁹Si and ²⁷Al NMR spectra showed that in the tetrahedral sheet of micas: a) each Al is surrounded by 3Si, and b) Si, Al distribution is more dispersed that imposed by Loewenstein's rule.     

Thermal analysis of fluorine containing gels and precursors of high silica zeolites

By means of simultaneous DTA-, TG- and DTG-technique, the silicate gels and the MFI crystals obtained from these gels have been investigated. The gels have been prepared in presence and in absence of tetrapropylammonium cation (TPA⁺) and with Li⁺, Na⁺, NH ₄ ⁺ and K⁺ fluorides. In absence of TPA⁺ no thermal effects have been observed in Li⁺- and Na⁺-gels. The effects observed in the NH ₄ ⁺ -gel stem from a decomposition and release of inorganic phases: SiF₄, NH₄F, NH₃. The DTA/DTG effects in the TPA⁺ containing gels and in the MFI crystals of monodisperse size are attributed to the decomposition of TPA⁺ cation. It can be concluded from these effects that the interaction between the gels and the TPA⁺ cation is rather weak. The interaction between TPA⁺ and MFI crystals obtained in fluoride medium is stronger than the interaction with crystals obtained from alkaline media. Similar thermal effects are obtained after grinding the long crystals to those having a large distribution of crystal sizes.

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Zusammenfassung Mittels simultaner DTA-, TG- und DTG-Techniken wurden Kieselgele und die MFI-Kristalle dieser Gele untersucht. Die Gele wurden in und ohne Gegenwart von Tetrapropylammoniumkationen (TPA⁺) und mit Li⁺-, Na⁺-, NH ₄ ^– und Kaliumfluoriden hergestellt. Bei Abwesenheit von TPA⁺ werden für Li⁺- und Na⁺-Gele keine thermischen Effekte beobachtet. Der bei NH ^4– -Gelen beobachtete Effekt ergibt sich aus der Zersetzung und Abspaltung von anorganischen Phasen: SiF₄, NH₄F, NH₃. Die DTA/DTG-Effekte in TPA⁺-haltigen Gelen und in den MFI-Kristallen monodispersen Ausmaes werden der Zersetzung des TPA⁺-Kations zugeschrieben. Daraus kann man schlieen, da die Wechselwirkung zwischen den Gelen und dem TPA⁺-Kation eher schwach ist. Die Wechselwirkung zwischen TPA⁺ und MFI-Kristallen aus fluoridischen Medien ist strker als die Wechselwirkung mit Kristallen aus alkalischen Medien. Ähnliche thermische Effekts erhlt man, nachdem man die langen Kristalle zerkleinert und eine breite Kristallgrenverteilung erhlt.

     

Isomer-Specific Vibrational Spectroscopy of Microhydrated Lithium Dichloride Anions: Spectral Fingerprint of Solvent-Shared Ion Pairs

The vibrational spectroscopy of lithium dichloride anions microhydrated with one to three water molecules, [LiCl₂(H₂O)_1–3]⁻, is studied in the OH stretching region (3800–2800 cm⁻¹) using isomer-specific IR/IR double-resonance population labelling experiments. The spectroscopic fingerprints of individual isomers can only be unambiguously assigned after anharmonic effects are considered, but then yield molecular level insight into the onset of salt dissolution in these gas phase model systems. Based on the extent of the observed frequency shifts Δν^OH of the hydrogen-bonded OH stretching oscillators solvent-shared ion pair motifs (<3200 cm⁻¹) can be distinguished from intact-core structures (>3200 cm⁻¹). The characteristic fingerprint of a water molecule trapped directly in-between two ions of opposite charge provides an alternative route to evaluate the extent of ion pairing in aqueous electrolyte solutions.     

Zeolite synthesis from tetraalkylammonium silicate gels

Siliceous zeolite synthesis gels containing tetraalkylammonium (TAA⁺) and sodium cations were studied using X-ray diffraction, elemental analysis, ion exchange, ²⁹Si magic-angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The TAA⁺ cations are encapsulated in silicate cages, and silicalite is formed via the rearrangement of these cages by the breaking and reformation of siloxane bonds. Tetrabutylammonium (TBA⁺) cations promote silicalite growth, but not as effectively as tetrapropylammonium (TPA⁺) because the larger TBA⁺ cations do not conform as well to the silicalite lattice, thus forming an intergrowth of the silicalite-1 and silicalite-2 structures. The time to nucleate silicalite is not affected by the TBA⁺ content of the gel, but the rate of silicalite crystal growth increases with increasing TBA⁺ in the gel. The TBA⁺ occupies all the channel intersections of the silicalite formed. Tetraethylammonium (TEA⁺) cations are encapsulated in silicate cages, but not to the same extent as TPA⁺ and TBA⁺, because TEA⁺ is not as hydrophobic. No silicalite forms in the TEA⁺ silicate gel. The addition of tripropylamine (TriPA) to a TPA⁺ silicate gel has no effect on the kinetics of silicalite formation. TriPA does not incorporate into the gel because it is neutral and, therefore, does not experience a coulombic attraction to the negatively charged surface of the gel.     

Ions at interfaces: the central role of hydration and hydrophobicity

It is increasingly being accepted that solvation properties of ions and interfaces (hydration of ions, hydrophobic or hydrophilic character of interfaces) play a fundamental role in ion-surface interaction in water. However, a fundamental understanding of the precise role of solvation in ionic specificity in colloidal systems is still missing, although important progress has been made over the last years. We present in this contribution experimental evidences (including also ions not usually included in specific ion studies) together with Molecular Dynamics (MD) simulations that highlight the importance of the hydration of ions and surfaces in order to understand the origin of ionic specificity. We first show that both surface polarity and ion hydration determine the sorting of ions according to their ability to induce specific effects (the so-called Hofmeister series). We extend these classical series by considering the addition of the inorganic anions IO₃⁻, BrO₃⁻ and ClO₃⁻, which present unusual properties as compared with the ions considered in classical Hofmeister series. We also consider big hydrophobic organic ions such as tetraphenylborate anion (Ph₄B⁻) and tetraphenylarsonium cation (Ph₄As⁺) that in the context of the Hofmeister series behave as super-chaotropes ions.     

Infrared spectroscopic studies of hydrogen-bonded complexes in cryogenic sulutions

We have measured the ν_s(OH) band parameters of the IR absorption spectra for a wide variety of hydrogen-bonded (HB) complexes of CH₃OH(D), CF₃CH₂OH, and (CF₃)₃COH(D) with some simplest representatives of various classes of bases in Xe and Kr in the temperature range 120–270 K. The ν_s(OH) absorption bands of the HB complexes in solution in atomic solvents have been demonstrated to be narrower by a factor of 2 to 4 than in molecular solvents at the same temperature. The fact that the ν_s(OH) bandwidths in Xe and in the gas phase at similar temperatures are practically the same indicates that these bandwidths are in both cases governed mainly by the contribution of “hot transitions” from a sequence of excited levels of the ν_β low-frequency bending mode of the hydrogen bond. The other characteristic features revealed for the complexes under study in liquid Xe and Kr at ν_s(OH) frequency shifts up to 500 cm⁻¹ include: (1) slight temperature dependence of the ν_s(OH) bandwidth (0.1–0.3 cm⁻¹/K), (2) almost “normal” isotope frequency ratio ν_s(OH)/ν_s(OD) (1.34–1.36) and (3) low ν_s(OH) temperature shift values (0.1–0.4 cm⁻¹/K).     

不同碱处理制备多级孔HZSM-5催化剂及噻吩烷基化性能研究

用Na₂CO₃、TPAOH和TPA⁺/CO₃^2-混合碱分别处理HZSM-5分子筛,采用FT-IR、XRD、XRF、N₂吸附脱附、SEM、NH₃-TPD及Py-FTIR表征手段对各类碱处理前后的HZSM-5分子筛进行表征。结果表明,3种类型的碱处理HZSM-5分子筛后,均能形成微孔-介孔多级孔道的HZSM-5(A)催化剂,并能调变催化剂的酸性,其中,TPA⁺/CO₃^2-混合碱处理得到的HZSM-5(TPA⁺/CO₃^2-)催化剂,比表面积最大,介孔数量最多。在小型固定床反应器上,考察了HZSM-5和HZSM-5(A)催化剂的噻吩烷基化性能,结果表明,HZSM-5(TPA⁺/CO₃^2-)催化剂因为具有适当的多级孔孔道和较多的B酸中心而表现出较高的噻吩转化率和1-己烯对噻吩的选择性。     

The use of variable temperature 13C solid-state MAS NMR and GIPAW DFT calculations to explore the dynamics of diethylcarbamazine citrate

Experimental ¹³C solid-state magic-angle spinning (MAS) Nuclear Magnetic Resonance (NMR) as well as Density-Functional Theory (DFT) gauge-including projector augmented wave (GIPAW) calculations were used to probe disorder and local mobility in diethylcarbamazine citrate, (DEC)⁺(citrate)⁻. This compound has been used as the first option drug for the treatment of filariasis, a disease endemic in tropical countries and caused by adult worms of Wuchereria bancrofti, which is transmitted by mosquitoes. We firstly present 2D ¹³C─¹H dipolar-coupling-mediated heteronuclear correlation spectra recorded at moderate spinning frequency, to explore the intermolecular interaction between DEC and citrate molecules. Secondly, we investigate the dynamic behavior of (DEC)⁺(citrate)⁻ by varying the temperature and correlating the experimental MAS NMR results with DFT GIPAW calculations that consider two (DEC)⁺ conformers (in a 70:30 ratio) for crystal structures determined at 293 and 235 K. Solid-state NMR provides insights on slow exchange dynamics revealing conformational changes involving particularly the DEC ethyl groups.