Two-dimensional Raman Correlation Research Articles

Roles of hydrogen bonding interactions and hydrophobic effects on enhanced water structure in aqueous solutions of amphiphilic organic molecules

Insights into the solute-induced water structural transformations are essential to understand the role of water in biological and chemical reaction processes. Herein, the structural changes in water induced by amphiphilic organic molecules were investigated using concentration-dependent derivative Raman spectroscopy (DRS) combined with two-dimensional Raman correlation spectroscopy (2D Raman-COS). We shall restrict our attention in this work to binary mixtures of water with dimethyl sulfoxide (DMSO), acetone, and isopropanol (IPA), all of which have similar chemical structures. The spectral changes in O:H and OH stretching modes illustrate that the solute molecules induce an enhancement of the water structure in dilute solutions, where the enhanced degree of water structure is closely related to the size of the dipole moment of organic molecules. In addition, the transformations of solute-induced water-specific structures were evaluated by 2D Raman-COS, which shows that the strong hydrogen bond (H-bond) structure of water is more sensitive to organic molecules and induces a transition to the weak H-bond structure of water.

Roles of hydrogen bonding interactions and hydrophobic effects on enhanced water structure strength in aqueous alcohol solutions

The structure and dynamics of water in aqueous alcohol solutions were explored using two-dimensional Raman correlation spectroscopy (2D Raman-COS) combined with the density functional theory (DFT). The spectral changes in the H–O–H bending and O:H stretching modes demonstrated that ethanol and n-propanol induced an enhancement of the water structure compared to methanol. The extent of this effect was related to the length of the alkyl chain. Comparative studies with aqueous ethylene glycol solution revealed that an enhanced water structure stemmed mainly from hydrophobic effects rather than hydrogen bonding (H-bonding) interactions. Alcohol-induced water-specific structural transitions were further analyzed using 2D Raman-COS, which showed that the free OH and strong H-bond structure of water respond preferentially to changes in alcohol content, inducing a transition in the weak H-bond structure of water. In addition, the 2D Raman-COS results indicated that the CH3 stretching mode of alcohol responds preferentially to variations in water content compared to other C–H vibrational modes. Finally, the details of the alcohol-induced water structural transitions were calculated using DFT. The 2D Raman-COS combined with DFT calculations provided insight into alcohol-induced water structural transitions and can be easily extended to other studies of water-organic chemistry.

Hydrogen-Bond Dynamics and Water Structure in Aqueous Ethylene Glycol Solution via Two-Dimensional Raman Correlation Spectroscopy

The hydrogen-bond (H-bond) dynamics and water structural transitions in aqueous ethylene glycol (EG) solution were investigated on the basis of concentration- and temperature-dependent two-dimensional Raman correlation spectroscopy (2D Raman-COS). At room temperature, EG-induced enhancement of the water structure when the EG/water molar ratio is less than 1:28 resulted from the hydrophobic effect around the methylene groups of EG. The decrease in the temperature caused an enhancement of the Raman peak at about 3200 cm-1, representing an increase in the orderliness of water molecules. Further analysis of the water-specific structures by 2D Raman-COS reveals that the strong H-bond structure preferentially responds to external perturbations and induces a weak H-bond structural transition in water. Finally, EG-induced water structural transitions were calculated by the density functional theory (DFT). Hopefully, 2D Raman-COS combined with DFT calculations would advance the study of solute-induced water structural transitions in water-organic chemistry.

Monitoring Deuterium Uptake in Single Bacterial Cells via Two-Dimensional Raman Correlation Spectroscopy

Raman-stable isotope labeling using heavy water (Raman-D2O) is attracting great interest as a fast technique with various applications ranging from the identification of pathogens in medical samples to the determination of microbial activity in the environment. Despite its widespread applications, little is known about the fundamental processes of hydrogen-deuterium (H/D) exchange, which are crucial for understanding molecular interactions in microorganisms. By combining two-dimensional (2D) correlation spectroscopy and Raman deuterium labeling, we have investigated H/D exchange in bacterial cells under time dependence. Most C-H stretching signals decreased in intensity over time, prior to the formation of the C-D stretching vibration signals. The intensity of the C-D signal gradually increased over time, and the shape of the C-D signal was more uniform after longer incubation times. Deuterium uptake showed high variability between the bacterial genera and mainly led to an observable labeling of methylene and methyl groups. Thus, the C-D signal encompassed a combination of symmetric and antisymmetric CD2 and CD3 stretching vibrations, depending on the bacterial genera. The present study allowed for the determination of the sequential order of deuterium incorporation into the functional groups of proteins, lipids, and nucleic acids and hence understanding the process of biomolecule synthesis and the growth strategies of different bacterial taxa. We present the combination of Raman-D2O labeling and 2D correlation spectroscopy as a promising approach to gain a fundamental understanding of molecular interactions in biological systems.

Investigation of the Brill transition in nylon 6,6 by Raman, THz-Raman, and two-dimensional correlation spectroscopy

The Brill transition is a phase transition process in polyamides related with structural changes between the hydrogen bonds of the lateral functional groups (CO) and (NH). In this study, we have used the potential of Raman spectroscopy for exploring this phase transition in polyamide 6,6 (nylon 6,6), due to the sensitivity of this spectroscopic technique to small intermolecular changes affecting vibrational properties of relevant functional groups. During a step by step heating and cooling process of the sample we collected Raman spectra allowing us from two-dimensional Raman correlation spectroscopy to identify which spectral regions suffered the largest influence during the Brill transition, and from Terahertz Stokes and anti-Stokes Raman spectroscopy to obtain complementary information, e.g. on the temperature of the sample. This allowed us to grasp signatures of the Brill transition from peak parameters of vibrational modes associated with (CC) skeletal stretches and (CNH) bending, and to verify the Brill transition temperature at around 160°C, as well as the reversibility of this phase transition.

Raman Identification of Multiple Melting Peaks of Polyethylene

Multiple melting peaks of high-density polyethylene produced by complex thermal pretreatment are investigated with variable temperature Raman spectroscopy and differential scanning calorimetry (DSC). We examine the origin of the observed multiple melting peaks with three possible model mechanisms: melting–recrystallization, premelting via mesophases, and melting of multithickness lamellae. We quantitatively analyze Raman spectral change associated with each melting peak with respect to temperature. The Raman spectral change is represented by a least squares moving-window (LSMW) method, which is effective in suppression of high-frequency derivative noise. The conformational similarity among multiple melting peaks strongly suggests that dominant melting occurs from the crystalline phase directly to the amorphous phase at all transitions. The observed multiple melting transitions are consistent with one-step melting of multithickness lamellae as the dominant origin. However, two-dimensional Raman correlation...

Reaction at the Electrolyte–Electrode Interface in a Li‐Ion Battery Studied by In Situ Raman Spectroscopy

Monitoring the reaction on the electrolyte–electrode interface using in situ Raman spectroscopy during the second charging process: the mechanism of the surface reaction during the second charging process by two-dimensional Raman correlation spectroscopy is discussed in the manuscript.

Two-dimensional Raman correlation spectroscopy reveals molecular structural changes during temperature-induced self-healing in polymers based on the Diels-Alder reaction.

The thermally healable polymer P(LMA-co-FMA-co-MIMA) has been studied by temperature-dependent FT-Raman spectroscopy, two-dimensional Raman correlation analysis and density functional theory (DFT) calculations. To the best of our knowledge this study reports for the first time on the investigation of a self-healing polymer by means of two-dimensional correlation techniques. The synchronous correlation spectrum reveals that the spectrally overlapping C[double bond, length as m-dash]C stretching vibrations at 1501, 1575, 1585 and 1600 cm(-1) are perfect marker bands to monitor the healing process which is based on a Diels-Alder reaction of furan and maleimide. The comparison between experimental and calculated Raman spectra as well as their correlation spectra showed a good agreement between experiment and theory. The data presented within this study nicely demonstrate that Raman correlation analysis combined with a band assignment based on DFT calculations presents a powerful tool to study the healing process of self-healing polymers.

Energy funneling and macromolecular conformational dynamics: a 2D Raman correlation study of PEG melting

Cascading energy landscapes through funneling has been postulated as a mechanistic route for achieving lowest energy configuration of a macromolecular system (such as, proteins and polymers). In particular, understanding molecular mechanism of melting and crystallization of polymers is a challenging fundamental question. The structural modifications that lead to melting of PEG are investigated here. Specific Raman bands corresponding to different configurations of the PEG chain have been identified and the molecular structural dynamics of PEG melting has been addressed using Raman spectroscopy, two-dimensional Raman correlation and DFT calculations.

Probing local structure of sub and supercritical CO 2 by using two-dimensional Raman correlation spectroscopy

Using two-dimensional correlation spectroscopy and curve fitting procedure, we analyzed the hypothesis that each of the two Raman Fermi dyad modes of CO 2 may be resolved in two contributions. The contribution to the highest frequency side of each vibration dyad mode of CO 2 is associated with molecules experiencing a low local density and the contribution to the lowest frequency side is associated with molecules experiencing a high local density. The Raman spectra were recorded along the liquid–gas coexistence curve of CO 2 in the temperature range between 250 K and 303 K, and along the critical isochore between 306 K and 320 K. The two-dimensional synchronous and asynchronous contour maps obtained by processing the dynamic spectra revealed that these two contributions undergo band shift, band width changes and intensity variation. Particularly, it was shown that the temperature affects in an asymmetric way the high and low frequency sides of the Fermi dyad centered at 1385 cm − 1 . The Fermi dyads were also decomposed into two Lorentzian components. The effect of temperature on the position, the width and the intensity of each of these components was quantified. The results show a systematic red shift of the two Fermi dyads along the coexistence curve and almost no dependence along the critical isochore. More interestingly, in the case of the Fermi dyad centered at 1280 cm − 1 , the width of these two contributions increases along the coexistence curve and remains almost constant along the critical isochore. This behavior was traced back to the increase of the heterogeneous distributions of CO 2 molecules and indicates that there are more different environment of individual CO 2 molecules when approaching the critical temperature. Finally, our results demonstrate the complementarities between two-dimensional spectroscopy and curve fitting procedure in order to analyze the Raman Fermi dyads and to characterize the changes in the local structure of sub and supercritical CO 2.

PH-induced conformational transitions in α-lactalbumin investigated with two-dimensional Raman correlation variance plots and moving windows

Raman spectroscopy is being increasingly applied to the investigation of conformational transitions in polypeptides and proteins, and in particular protein unfolding, due to its ability to monitor changes in secondary structure. Here we focus on the application of two different two-dimensional correlation methods, autocorrelation and moving windows techniques, to investigate the pH-induced conformational transitions in α-lactalbumin monitored with Raman scattering. From our results we have identified three distinct stages in the conformational transition occurring at ranges ∼pH 6.5–4.6, ∼pH 4.6–3.6 and ∼pH 3.6–1.8, suggesting the existence of three possible conformational species. The first two transition phases the conformational changes are predominantly occurring in the backbone secondary structure, while in the final transition phase changes are occurring in both secondary structure and side chain residue solvent exposure.

Microstructures formation in a seemingly ideal homogeneous mixture of ethanol and methanol: An experimental evidence and two-dimensional correlation spectroscopy approach

An anomalous solution behavior at the molecular scale was observed for macroscopically homogeneous mixtures of methanol and ethanol. Two-dimensional Raman correlation spectroscopy was used to elucidate the possible existence of microstructures formed in the mixture. The result suggests that separate methanol and ethanol clusters are formed without heterohydrogen bonding between different alcohol species. Supramolecular structures seem to be formed by the interaction of such clusters with each other through cohesion and dispersion forces, but not through direct hydrogen bonding connections.

Two-dimensional Raman correlation analysis of benzophenone

Effect of varying excitation wavelength on the Raman spectral intensity of benzophenone molecule has been examined. The overlapped bands are resolved using two-dimensional correlation method. The symmetric and asymmetric pair associated with a particular type of vibration is highly asynchronously correlated. Evidently no corresponding cross peaks are present in the synchronous spectra. This finding indicates that the succession of change of amplitude of symmetric and asymmetric vibrations related with that specific mode is not simultaneous but completely chronological. Interaction of C O group with the surroundings may be responsible for the appearance of a relatively lower wavenumber peak correspond to new type of C O stretching vibration.

Two-Dimensional Raman Correlation Spectroscopy Study of an Emulsion Copolymerization Reaction Process

The emulsion copolymerization of styrene and 1,3-butadiene using an oligomeric nonionic surfactant as an emulsifier to make a styrene-butadiene rubber (SBR) copolymer latex was monitored by real-time in situ Raman spectroscopy. Time-resolved Raman spectra collected during the early stage of the polymerization reaction were subjected to a series of data analysis techniques, including two-dimensional (2D) correlation spectroscopy, multivariate self-modeling curve resolution (SMCR), and kernel analysis, to elucidate the fine details of the complex reaction process. Generalized 2D correlation analysis of time-resolved Raman spectra readily identified the characteristic Raman scattering bands for the monomers and copolymer. Cross-peaks appearing in 2D Raman correlation spectra showed that the decrease in the spectral intensity of Raman bands assignable to 1,3-butadiene occurs before the band intensity changes for styrene or SBR copolymer. The positions of asynchronous cross-peaks were used to identify a spectral region with the most distinct pattern of intensity variations, which in turn could be used as the starting point for the alternating least squares iteration of the SMCR analysis. SMCR analysis of the time-resolved Raman spectra generated a set of estimated pure component spectra and concentration profiles of styrene, 1,3-butadiene, and SBR copolymer without requiring independently measured calibration data. The estimated concentration profiles of monomers and copolymer indicated that the reaction of 1,3-butadiene started before the consumption of styrene and production of SBR copolymer. Kernel analysis of the estimated concentration profiles provided a succinct measure of the similarity and dissimilarity of the concentration changes of monomers and copolymer.

Characterization of a series of sodium molybdate structures by two‐dimensional Raman correlation analysis

AbstractThis paper demonstrates the use of two‐dimensional (2D) correlation spectroscopy in the characterization of the sodium molybdate structures that include and are intermediate between the hexagonal and orthorhombic forms; this series of sodium molybdates is produced by varying the preparation conditions, particularly the acid concentration and the heat‐treatment temperature. In order to verify the assignments of the vibrational modes of hexagonal sodium molybdate, 2D correlation analysis was applied to the acid concentration‐dependent Raman spectra. We can confirm from 2D Raman correlation analysis that this series of preparation conditions results in a series of sodium molybdates including and intermediate between the hexagonal and orthorhombic forms. Furthermore, 2D Raman correlation analysis elucidates the sequence of the structural transformation of hexagonal sodium molybdate into the orthorhombic form. With progression within the series from the hexagonal to the orthorhombic structures, the intensities of the oxygen–molybdenum stretching bands in sodium molybdate change in the following sequence: 973 cm−1 (terminal OMo in the hexagonal structure, decrease in intensity) → 996 cm−1 (terminal OMo in the orthorhombic structure, increase in intensity) → (897, 881) cm−1 (corner OMo2 in the hexagonal structure, decrease in intensity) → 664 cm−1 (edge OMo3 in the orthorhombic structure, increase in intensity) → 820 cm−1 (corner OMo2 in the orthorhombic structure, increase in intensity) → 692 cm−1 (edge OMo3 in the hexagonal structure, decrease in intensity). This sequence implies that the changes in the corner OMo2 and edge OMo3 structures along the series of sodium molybdates are strongly correlated. Copyright © 2003 John Wiley & Sons, Ltd.

Two-Dimensional Raman Correlation Spectroscopy Study of the Pathway for the Thermal Imidization of Poly(amic acid)

The pathway producing imide ring closure during the thermal imidization of poly(amic acid) (PAA) was investigated in detail using a new analytical method, two-dimensional (2D) Raman correlation spectroscopy. The signs of the cross peaks in synchronous spectra provided evidence of the thermal imidization of PAA into PI as the heating temperature increased. The signs of the cross peaks in asynchronous spectra suggested that the imide-related modes changed prior to the amide or carboxylic mode, which indicates that cyclization occurred before the amide proton was abstracted.