Raman Spectra Of Aqueous Solutions Research Articles

Lifting Hofmeister's Curse: Impact of Cations on Diffusion, Hydrogen Bonding, and Clustering of Water.

Water plays a role in the stability, reactivity, and dynamics of the solutes that it contains. The presence of ions alters this capacity by changing the dynamics and structure of water. However, our understanding of how and to what extent this occurs is still incomplete. Here, a study of the low-frequency Raman spectra of aqueous solutions of various cations by using optical Kerr-effect spectroscopy is presented. This technique allows for the measurement of the changes that ions cause in both the diffusive dynamics and the vibrations of the hydrogen-bond structure of water. It is found that when salts are added, some of the water molecules become part of the ion solvation layers, while the rest retain the same diffusional properties as those of pure water. The slowing of the dynamics of the water molecules in the solvation shell of each ion was found to depend on its charge density at infinite dilution conditions and on its position in the Hofmeister series at higher concentrations. It is also observed that all cations weaken the hydrogen-bond structure of the solution and that this weakening depends only on the size of the cation. Finally, evidence is found that ions tend to form amorphous aggregates, even at very dilute concentrations. This work provides a novel approach to water dynamics that can be used to better study the mechanisms of solute nucleation and crystallization, the structural stability of biomolecules, and the dynamic properties of complex solutions, such as water-in-salt electrolytes.

Machine Learning-Assisted Raman Spectroscopy for pH and Lactate Sensing in Body Fluids.

This study presents the combination of Raman spectroscopy with machine learning algorithms as a prospective diagnostic tool capable of detecting and monitoring relevant variations of pH and lactate as recognized biomarkers of several pathologies. The applicability of the method proposed here is tested both in vitro and ex vivo. In a first step, Raman spectra of aqueous solutions are evaluated for the identification of characteristic patterns resulting from changes in pH or in the concentration of lactate. The method is further validated with blood and plasma samples. Principal component analysis is used to highlight the relevant features that differentiate the Raman spectra regarding their pH and concentration of lactate. Partial least squares regression models are developed to capture and model the spectral variability of the Raman spectra. The performance of these predictive regression models is demonstrated by clinically accurate predictions of pH and lactate from unknown samples in the physiologically relevant range. These results prove the potential of our method to develop a noninvasive technology, based on Raman spectroscopy, for continuous monitoring of pH and lactate in vivo.

Attenuating the increased level of creatinine by N‐acetylcysteine: Raman spectroscopy and density functional theory‐based monitoring of in vitro complexation in aqueous solution

AbstractThe role of drug N‐acetylcysteine (NAC) in preventing contrast‐induced nephropathy (CIN) and kidney diseases has been investigated with the help of in vitro Raman spectroscopy and density functional theory (DFT). Renal dysfunction or kidney failure is diagnosed by an increase in serum creatinine (CRN). The exposure to contrast agents during angiography also causes an increase in serum CRN, a condition termed as CIN. NAC is given to such patients, as it is known to prevent the toxic effect of CRN, although its mechanism of action is not clearly known till date. In the present study, we have studied the interaction between CRN and NAC and tried to detect the formation of a stable complex between the two by analyzing the in vitro Raman spectra of aqueous solutions of CRN and NAC mixed in different molar ratios. From the Raman spectral analysis, it is observed that a stable complex is formed at 1:1 molar ratio of CRN and NAC. This complex has been synthesized in the laboratory, and upon drying, it is transparent, gel‐like in appearance, and slightly yellowish in color. The complex is hygroscopic and has much better water solubility than CRN. Fourier‐transform infrared (FT‐IR) and Raman spectral analyses of the synthesized complex show the structural changes taking place because of complexation and provide proof that the complex is stable at room temperature. DFT‐based studies on a number of plausible structures of the complex have also been done to determine the most stable structure of the complex, and the mechanism of its formation has been explored by transition‐state calculations. This study highlights the effective role of NAC in reducing the toxic effect of CRN as the water‐soluble complex of CRN, and NAC is likely to be removed through urine.

Quantum-mechanical calculation of the intensity distribution in Raman and resonance Raman spectra of a phenylalanine aqueous solution

Quantum-mechanical calculations of the intensity distribution in the resonance Raman spectrum of an aqueous solution of phenylalanine, excited by light with wavelengths of 193, 204, 218, and 235 nm, and in the nonresonant Raman spectrum excited at a wavelength of 488 nm have been performed. The calculation results are in satisfactory agreement with the experimental data in the literature. Spectral lines characteristic of phenylalanine are observed, which can be used as markers when analyzing the peptide structure. It is noted that the contribution of highly excited electronic states (spaced by less than 10 eV from the resonant lines) to the scattering tensor components must be taken into account. The important role of the Herzberg–Teller effect in the description of spectral intensity distribution is demonstrated.

Ratiometric detection of Raman hydration shell spectra

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. However the spectra of hydration shell are difficult to be obtained. Herein, a novel Raman ratio spectra, which is obtained through dividing the Raman spectra of aqueous solutions from the spectrum of water, was applied to deduce the spectra of hydration shell of organic (acetone-D6) and inorganic compounds (NaNO3, NaSCN, NaClO4, Na2SO4, NaCl) in water. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, the number of water molecules in the first hydration shell of free anions and acetone-D6, and the aggregation behavior of ions in the concentrated aqueous NaNO3. The number of water molecules in the hydration shell was supported by our molecular dynamic simulations. The Raman ratio spectra can be widely employed to obtain the spectra of the first hydration shell, and it is helpful to understand the micro-structure of aqueous solutions. Copyright © 2016 John Wiley & Sons, Ltd.

Pressure dependence of the Raman spectra of aqueous solutions and anthracene

The shifts of the main bands in the Raman spectra during isothermal pressurization up to 140 MPa for anthracene and 1 M solutions of NaClO4 and Na2WO4 are measured using an optical cell with sapphire windows. The resulting shifts in the maxima of the spectral bands (from 0.6 to 1.1 cm−1 per 100 MPa) are close to the published value for a 1 M Na2SO4 aqueous solution.

Structures of Vanadium(III)-Ethylenediamine-N,N′-diacetato-N,N′-di-3-propionato Complexes. Critical Role of pπ-dπ Donation in Determining the Coordination Number

Li[V(eddadp)]·3H(2)O (1a) and Cs[V(eddadp)]·2H(2)O (1b) were characterized by X-ray crystallography. 1a crystallizes in the monoclinic space group Cc with a = 11.467(7) Å, b = 13.398(8) Å, c = 12.529(8) Å, β = 114.85(4)°; V = 1746.7(2) Å(3), and Z = 4; 1b crystallizes in the monoclinic space group P2(1)/n with a = 10.265(5) Å, b = 11.673(6) Å, c = 15.507(8) Å, β = 104.29(2)°, V = 1800.6(2) Å(3), and Z = 4. The solution structure of 1 has been ascertained to be predominantly six-coordinated with a hexadentate eddadp which is based on a comparison of the electronic and Raman spectra of aqueous solutions of 1 with those in the solid state.

DFT studies of Al–O Raman vibrational frequencies for aquated aluminium species

Raman frequency calculations of Al ( H 2 O ) 6 3 + and Al ( OH ) 4 - species in different models were carried out with density functional theory (DFT). Gas-phase model (GP), polarizable continuum model (PCM), supermolecule model (SM) and supermolecule-polarizable continuum model (SM-PCM) were considered and frequencies over the wavenumber range from 0 to 1000 cm −1 were discussed. The calculated frequencies of the two species in SM-PCM model are in fair agreement with the observed frequencies in aqueous solution and the average deviation is 6 cm −1. Based on the results of Al ( H 2 O ) 6 3 + and Al ( OH ) 4 - , the DFT method and the SM-PCM model were used to study the Al–O Raman bands for some other monomeric and dimeric aluminiums. The monomeric aluminiums Al(OH) 2+, Al ( OH ) 2 + and Al ( OH ) 6 3 - as well as the dimer Al 2 O ( OH ) 6 2 - were included. As a result, we predict the main Al–O skeleton vibrations of Al ( OH ) ( H 2 O ) 5 2 + at 685 cm −1, trans- Al ( OH ) 2 ( H 2 O ) 4 + at 542 cm −1, Al ( OH ) 6 3 - at 510 cm −1 and Al 2 O ( OH ) 6 2 - at 510 and 699 cm −1 respectively in Raman spectra of aqueous solution. It can also be concluded that the calculated frequencies of Al 2 O ( OH ) 6 2 - are close to the experimental values and the existence of Al ( OH ) 6 3 - in aluminate solution cannot be ruled out.

Raman and Infrared Spectroscopic Investigation of Speciation in BeSO4(aq)

Measurements have been made of the Raman spectra of aqueous solutions of Be(ClO4)2, BeCl2, (NH4)2SO4 and BeSO4 to 50 cm−1. In some cases low concentrations (0.000770 mol⋅kg−1) have been used and two temperatures (23 and 40 °C) were studied. In BeSO4(aq), the ν 1- $\mathrm{SO}_{4}^{2-}$ mode at 980 cm−1 broadens with increasing concentration and shifts to higher wavenumbers. At the same time, a band at 1014 cm−1 is detectable with this mode being assigned to [BeOSO3], an inner-sphere complex (ISC). Confirmation of this assignment is provided by the simultaneous appearance of stretching bands for the $\mathrm{Be}^{2+}\mbox{-}\mathrm{OSO}_{3}^{2-}$ bond of the complex at 240 cm−1 and for the BeO4 skeleton mode of the [(H2O)3BeOSO3] unit at 498 cm−1. The ISC concentration increases with higher temperatures. The similarity of the $\nu_{1}\mbox{-}\mathrm{SO}_{4}^{2-}$ Raman bands for BeSO4 in H2O and D2O is further strong evidence for formation of an ISC. After subtraction of the ISC component at 1014 cm−1, the $\nu_{1}\mbox{-}\mathrm{SO}_{4}^{2-}$ band in BeSO4(aq) showed systematic differences from that in (NH4)2SO4(aq). This is consistent with a $\nu_{1}\mbox{-}\mathrm{SO}_{4}^{2-}$ mode at 982.7 cm−1 that can be assigned to the occurrence of an outer-sphere complex ion (OSCs). These observations are shown to be in agreement with results derived from previous relaxation measurements. Infrared spectroscopic data show features that are also consistent with a beryllium sulfato complex such as the appearance of a broad and weak $\nu_{1}\mbox{-}\mathrm{SO}_{4}^{2-}$ mode at ∼1014 cm−1, normally infrared forbidden, and a broad and asymmetric $\nu_{3}\mbox{-}\mathrm{SO}_{4}^{2-}$ band contour which could be fitted with four band components (including $\nu_{3}\mbox{-}\mathrm{SO}_{4}^{2-}(\mathrm{aq})$ ). The formation of ISCs in BeSO4(aq) is much more pronounced than in the similar MgSO4(aq) system studied recently.

Experimental study of the system H 3BO 3–NaF–SiO 2-H 2O at 350–800 °C and 1–2 kbar by the method of synthetic fluid inclusions

The phase state of fluid in the system H 3BO 3–NaF–SiO 2–H 2O was studied at 350–800 °C and 1–2 kbar by the method of synthetic fluid inclusions. The increase in the solubility of quartz and the high reciprocal solubility of H 3BO 3 and NaF in water fluid at high temperatures are due to the formation of complexes containing B, F, Si, and Na. At 800 °C and 2 kbar, both liquid and gas immiscible phases (viscous silicate-water-salt liquid and three water fluids with different contents of B and F) are dispersed within each other. The Raman spectra of aqueous solutions and viscous liquid show not only a peak of [B(OH) 3] 0 but also peaks of complexes [B(OH) 4] –, polyborates [B 4O 5(OH) 4] 2–, [B 3O 3(OH) 4] –, and [B 5O 6(OH) 4] –, and/or fluoroborates [B 3F 6O 3] 3–, [BF 2(OH) 2] –, [BF 3(OH)] –, and [BF 4] –. The high viscosity of nonfreezing fluid is due to the polymerization of complexes of polyborates and fluorine-substituted polyborates containing Si and Na. Solutions in fluid inclusions belong to P–Q type complicated by a metastable or stable immiscibility region. Metastable fluid equilibria transform into stable ones owing to the formation of new complexes at 800 ºC and 2 kbar as a result of the interaction of quartz with B-F-containing fluid. At high concentrations of F and B in natural fluids, complexes containing B, F, Si, and alkaline metals and silicate-water-salt dispersed phases might be produced and concentrate many elements, including ore-forming ones. Their transformation into vitreous masses or viscous liquids (gels, jellies) during cooling and the subsequent crystallization of these products at low temperatures (300–400 °C) should lead to the release of fluid enriched in the above elements.

Raman spectroscopy of pure H2O and NaCl-H2O containing synthetic fluid inclusions in quartz—a study of polarization effects

The O-H stretching region of the Raman spectrum of aqueous solutions (H2O-NaCl mixtures) is a set of poorly defined broad bands between 2,800cm−1 and 3,800cm−1. The morphology of this spectral region is systematically characterised by deconvolution, using three Gaussian–Lorentzian contributions. This method allows a purely geometrical description of the spectra of pure H2O as well as saline solutions at room temperatures. A pure H2O reference solution has peak positions at 3,223cm−1 (Peak1), 3,433cm−1 (Peak2) and 3,617cm−1 (Peak3). The morphology of the Raman bands is influenced by the presence of NaCl in the aqueous solution. The variation in Peak1 values is correlated with the salinity of the aqueous solutions according to the equation: $$\Delta \,v_1 = \,3222.8\,\, + \,\,1.69\,sal$$ where sal is the salinity (in mass%) and Δν1 is the relative wavenumber (cm−1) of Peak1. The contours of the Raman spectra of aqueous solutions in fluid inclusions in quartz are influenced by the birefringence of the host crystal, the orientation of the quartz-fluid interface, and the depth of the inclusion. The variation in Peak1 due to birefringence is 3,220cm−1 to 3,239cm−1 using a polarised laser beam. Reflection polarisation caused by the orientation of the quartz-fluid interface results in a variability of 14cm−1, independently from the orientation of the indicatrix. The effect of birefringence is reduced with increasing depth.

Probing water dynamics with OH −

Isotropic Raman spectra of aqueous solutions of LiOH, NaOH and KOH at concentrations ranging from high dilution to saturation have been measured and the frequency and width of the OH − stretching band have been analyzed. The dependence of the bandwidth on solute concentration suggests that the OH − vibration undergoes a transition from fast to slow modulation regimes as the solvent concentration decreases below the value of ∼20 water molecules per solute molecule. A correlation between this finding and structural modifications of the H-bond network of the solvent at similar concentrations is envisaged.

Raman analysis of formaldehyde aqueous solutions as a function of concentration

AbstractRaman spectra of aqueous solutions containing up to 40 mol% of formaldehyde were recorded in the 120–4000 cm−1 spectral range. The water vibrational modes are not too heavily modified by the presence of the polymerized species in the aqueous solutions. The intensity of the bands corresponding to the C–O and C–H vibrational modes depends on the water content. Two bands are found for the C–O symmetric and antisymmetric stretching modes. The first is related to the C–O–C groups formed within the oligomer chains as the condensation increases and the second to the C–OH groups located at the end of the oligomer chains. A change in the intensity evolution of the bands corresponding to the stretching modes of the C–O–C groups occurs above 15 mol% formaldehyde concentration, corresponding to the formation of the HO(CH2O)nH oligomers with n≥3. The intensity of the stretching modes due to the C–OH groups varies linearly with concentration. A linear variation of the band intensity of the CH modes as a function of formaldehyde content is also observed. Copyright © 2003 John Wiley & Sons, Ltd.

Raman spectroscopic investigation of speciation in MgSO4(aq)

Careful measurements have been made of the Raman spectra of aqueous solutions of Mg(ClO4)2, MgCl2, (NH4)2SO4 and MgSO4 down to 50 cm−1 and, in some cases, to extremely low concentrations (≥0.06 mmol kg−1) and high temperatures (≤200 °C). In MgSO4(aq), the well known asymmetry in the ν1-SO42− mode at ∼980 cm−1 that develops with increasing concentration has been assigned to a mode at 993 cm−1 associated with the formation of an MgOSO3 contact ion pair (CIP). Confirmation of this assignment is provided by the simultaneous and quantitative appearance of stretching modes for the Mg–OSO3 bond of the ligated SO42− at 245 cm−1 and for the (H2O)5MgOSO3 unit at 328 cm−1. The CIP becomes the dominant species at higher temperatures. Alternative explanations of the broadening of the ν1-SO42− mode are shown to be inconsistent with this and other Raman spectral evidence such as the similarity of the ν1-SO42− mode for MgSO4 in H2O and D2O. After subtraction of the CIP component at 993 cm−1, the ν1-SO42− band in MgSO4(aq) showed systematic differences from that in (NH4)2SO4(aq). This is consistent with a previously undetected ν1-SO42− mode at 982.2 cm−1 that can be assigned to the presence of solvent-shared ion pairs (SIPs). In solutions with high Mg2+/SO42− concentration ratios, a further ν1-SO42− mode was observed at 1005 cm−1, which has been tentatively assigned to a Mg2SO42+(aq) triple ion. All of these observations are shown to be in excellent agreement with recent dielectric relaxation spectroscopy measurements. In addition, the correct relationship between the Mg2+/SO42− association constant determined by Raman spectroscopic measurements and those obtained by other techniques is derived. It is shown that thermodynamic data measured by Raman spectroscopy for systems involving other (Raman-undetected) ion-pair types in addition to CIPs, cannot and should not be compared directly with those obtained by traditional techniques.

Raman Studies of the Solution Structure of Univalent Electrolytes in Water

This paper reports results of analysis of the OH stretch Raman spectra of aqueous solutions of electrolytes. Error analysis supports the view that the environment of the electrolyte in the liquid is that of a liquid crystalline hydrate. Fitting error analysis for a two-state model further suggests that the presence of the hydrated electrolyte perturbs the structure of the bulk water in the solution. This perturbation is consistent with the results of previous studies of the apparent density of water in aqueous solutions of electrolytes. The previous studies showed that electrolytes increase the hydrogen bond strength of the bulk water and thereby cause a shift in the water equilibrium, which results in a change of the apparent density of water in the solution.

Model of molecular reorientation in liquid water and anionic effect on Raman spectral data

New experimental data on OH and OD stretching vibrations were obtained from Raman studies on light and heavy water and some aqueous salt solutions. We estimate the previous results in this field under new aspects. The mechanism of defect formation was suggested as rotational reorientation of the liquid water molecules by analogy with the rotational isomerism when hindered rotation around single bonds takes place. This point of view is true for H 2O, HOD and D 2O molecules. On the basis of our Raman spectra of light, heavy liquid water, HOD and aqueous solutions of various salts we report on non homogeneity of the spatial H-bond network of liquid water. The reason for that is the presence of defects of the H-bond network from the different conformations of fully hydrogen-bonded five-molecule structural units found in liquid water. The presence of each of them depends on temperature, pressure and solution concentration. Each component of the stretching water band meeting a corresponding confirmation can be separated in the Raman spectra of aqueous solutions by selection of different alkali metal halides. The frequencies and intensities of these components practically do not depend on the nature of the cation but on the nature and size of the anion of the dissolved salt. Our result suggests water molecule mobility across the defects of the H-bond network and not by defects of the liquid water structure often reported in literature.

Raman, infrared and force field studies of K 212C 2O 4 · H 2O and K 213C 2O 4 · H 2O in the solid state and in aqueous solution, and of (NH 4) 212C 2O 4 · H 2O and (NH 4) 213C 2O 4 · H 2O in the solid state

The Raman and infrared spectra of solid K 2 12C 2O 4 · H 2O are reported together with, for the first time, the corresponding Raman and infrared spectra of solid K 2 13C 2O 4 · H 2O. Raman spectra of aqueous solutions of both isotopomers are also reported. In the solid state the oxalate anion is planar with D 2 h symmetry in this salt, whereas in aqueous solution the Raman spectra of the anion are best interpreted on the basis of D 2 d symmetry. The Raman spectra of solid (NH 4) 2 12C 2O 4 · H 2O and (NH 4) 2 13C 2O 4 · H 2O, in which the oxalate anion is twisted from planarity by 28° about the CC bond, have also been recorded. Several reassignments have been made. The harmonic force field for the oxalate anion in the D 2 h , D 2 and D 2 d geometries has been determined in part, and approximate values of key valence force constants determined. All the observed band wavenumbers and 12C/ 13C isotopic shifts are well reproduced by the force fields. The potential energy distribution of the totally symmetric normal modes of planar oxalate indicates that each mode consists of extensively mixed symmetry corrdinates and that the labels previously used for the bands seen here at 475 and 879 cm −1 would better be described as ν(CC) and δ s(CO 2), respectively, putting them in the same wavenumber order as ν(NN) and δ s(NO 2) for the isoelectronic and isostructural molecule N 2O 4. The stretching force constants of N 2O 4 and planar C 2O 4 2− are established to be in the order f NN< f CC and f NO> f CO, consistent with the known relative bond lengths.

Temperature Dependence of the Anomalous Conformational Behavior of a Short-Chain Poly(oxyethylene) in Water Studied by Raman Spectroscopy

Abstract The Raman spectra of aqueous solutions of CH3(OCH2CH2)3OCH3 were measured for mole fractions 0.002–0.06 at five temperatures between 5 and 45 °C. The population of the gauche conformation around the C–C bond shows its maximum at about 0.01 mole fraction. The appearance of the maximum is less prominent at higher temperatures. The observed conformational behavior is consistent with the results of molecular dynamics simulations.

Raman spectra of aqueous solutions of strong electrolytes: evolving-factor analysis

Ion association in aqueous solutions of varied concentrations of LiNO3, Mg(NO3)2, Co(NO3)2, Li2SO4 and MgSO4 was studied by means of Raman spectrometry assisted by principal-components (PCA) and evolving-factor (EFA) analyses. Formation of one Raman-active associated species, {M···L(R)} (M=Li+, Mg2+, Co2+; L=NO3−, SO42−), was detected at higher salt concentrations (≳2 mol dm−3 for LiNO3, ≳1 mol dm−3 for Co(NO3)2, ≳0.2 mol dm−3 for Li2SO4, ≳0.5 mol dm−3 for MgSO4). Spectral profiles of L(aq) and {M···L(R)} species were computed by means of EFA, as well as their equilibrium concentrations in each solution. Maximum fractions of anions engaged in {M···L(R)} species amounted (at highest salt concentrations) to ≈0.6 (LiNO3), ≈0.4 (Co(NO3)2), ≈0.60 (Li2SO4) and ≈0.3 (MgSO4). Since, for a given salt concentration, the eqilibrium concentration of Raman-active species, [M···L(R)], was always lower than the concentration of analogous UV-active species, [M···L(UV)] (reported in a previous paper) it is suggested that {M···L(UV)} could be a precursor of {M···L(R)}. In all instances, apparent stability constants of {M···L(R)} species, falling into the range of (0.01≲K1/dm3 mol−1≲1), have a progressively upward trend with increasing salt concentration.

Surface-Enhanced Raman Scattering Study of Quinoxaline-2,3-dithiol and Disodium Bis(quinoxaline-2,3-dithiolato)nickelate(II) Adsorbed on Silver Surface

Adsorption of quinoxaline-2,3-dithiol (H2QS2) and disodium bis(quinoxaline-2,3-dithiolato)nickelate(II) (Na2[Ni(QS2)2]) on a silver surface has been studied by SERRS as well as FT-SERS techniques. The surface Raman spectra of H2QS2 are very similar to the corresponding normal Raman spectra of aqueous solutions of disodium quinoxaline-2,3-dithiolate, suggesting that H2QS2 dissociates on a silver surface to the quinoxaline-2,3-dithiolate ion. Normal-mode analysis based on the results of ab initio calculation shows that in-plane modes are selectively enhanced, indicating a perpendicular orientation for the adsorbed dithiolate ion with respect to the surface. The FT-SERS spectra of H2QS2 are found to be very weak on colloidal silver at all stages of aggregation, whereas intense spectra are observed when adsorbed on chemically deposited silver films, indicating that the surface Raman spectrum is mainly enhanced through electromagnetic mechanism for near-infrared excitation. The vibrational frequencies of Na2[N...