Multiphase‐Separated Polyurethanes Studied by Micro‐Raman Spectroscopy

Micro‐Raman spectroscopy has been used to investigate the chemical micro‐heterogeneity of multiphase‐separated poly(ether urethanes) (PETU). Analysis of PETU cross‐sections by means of micro‐Raman spectroscopy revealed the nearly complete absence of soft segments in AI aggregates (called globules). These aggregates are in the order of a few micrometers in size. The composition of the matrix and the AII aggregates (spherulites) was comparable.

Example of an AFM image (sample 706, scan size 25 μm, converted to monochromatic image).     

Visualization of Vibrational Modes in Real Space by Tip‐Enhanced Non‐Resonant Raman Spectroscopy

We present a general theory to model the spatially resolved non‐resonant Raman images of molecules. It is predicted that the vibrational motions of different Raman modes can be fully visualized in real space by tip‐enhanced non‐resonant Raman scattering. As an example, the non‐resonant Raman images of water clusters were simulated by combining the new theory and first‐principles calculations. Each individual normal mode gives rise its own distinct Raman image, which resembles the expected vibrational motions of the atoms very well. The characteristics of intermolecular vibrations in supermolecules could also be identified. The effects of the spatial distribution of the plasmon as well as nonlinear scattering processes were also addressed. Our study not only suggests a feasible approach to spatially visualize vibrational modes, but also provides new insights in the field of nonlinear plasmonic spectroscopy.     

Distance‐dependent Enhancement in Raman Spectroscopy Probed by Conjugated Molecules with Different Molecular Lengths

In this study, the distance‐dependent enhancement effect in surface‐enhanced Raman scattering (SERS) was explored with molecules bearing different lengths of conjugated double bonds. These conjugated molecules were synthesized utilizing the diazotization‐coupling reaction allowing a thio group on one end and a nitro group on the other end. The thiol group allows the probed molecule to chemisorb on the surface of silver nanoparticles (AgNPs). The opposite end of each molecule contains a nitro group, which gives an intense SERS signal to show a fair and accurate comparison of the effect of chain length. The obtained SERS intensities were correlated with the chain lengths of these synthesized molecules, which ranged from 0.6 to 2.0 nm between the nitro and thiol groups. Based on these results, the electromagnetic field effect was mainly responsible for the signal enhancements in SERS measurements. Also, the obtained signals were exponentially decayed due to the distances of the surface of AgNPs. Based on the SERS intensities of the conjugated molecules, the contribution of CT effect to SERS for these examined molecules were limited.     

Noble‐Metal‐Free Materials for Surface‐Enhanced Raman Spectroscopy Detection

Surface‐enhanced Raman spectroscopy (SERS) is an attractive tool for the sensing of molecules in the fields of chemical and biochemical analysis as it enables the sensitive detection of molecular fingerprint information even at the single‐molecule level. In addition to traditional coinage metals in SERS analysis, recent research on noble‐metal‐free materials has also yielded highly sensitive SERS activity. This Minireview presents the recent development of noble‐metal‐free materials as SERS substrates and their potential applications, especially semiconductors and emerging graphene‐based nanostructures. Rather than providing an exhaustive review of this field, possible contributions from semiconductor substrates, characteristics of graphene enhanced Raman scattering, as well as effect factors such as surface plasmon resonance, structure and defects of the nanostructures that are considered essential for SERS activity are emphasized. The intention is to illustrate, through these examples, that the promise of noble‐metal‐free materials for enhancing detection sensitivity can further fuel the development of SERS‐related applications.     

Self‐Assembled Au Nanoparticles as Substrates for Surface‐Enhanced Vibrational Spectroscopy: Optimization and Electrochemical Stability

Three‐dimensional nanostructured metallic substrates for enhanced vibrational spectroscopy are fabricated by self‐assembly. Nanostructures consisting of one to 20 depositions of 13 nm‐diameter Au nanoparticles (NPs) on Au films are prepared and characterized by means of AFM and UV/Vis reflection–absorption spectroscopy. Surface‐enhanced polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) is observed from Au NPs modified by the probe molecule 4‐hydroxythiophenol. The limitation of this kind of substrate for surface‐enhanced PM‐IRRAS is discussed. The surface‐enhanced Raman scattering (SERS) from the same probe molecule is also observed and the effect of the number of Au‐NP depositions on the SERS efficiency is studied. The SERS signal from the probe molecule maximizes after 11 Au‐NP depositions, and the absolute SERS intensities from different batches are reproducible within 20 %. In situ electrochemical SERS measurements show that these substrates are stable within the potential window between ?800 and +200 mV (vs. Ag/AgCl/sat. Cl^?).     

Accelerating Diffusion‐Ordered NMR Spectroscopy by Joint Sparse Sampling of Diffusion and Time Dimensions

Diffusion‐ordered multidimensional NMR spectroscopy is a valuable technique for the analysis of complex chemical mixtures. However, this method is very time‐consuming because of the costly sampling of a multidimensional signal. Various sparse sampling techniques have been proposed to accelerate such measurements, but they have always been limited to frequency dimensions of NMR spectra. It is now revealed how sparse sampling can be extended to diffusion dimensions.     

Infrared and Raman Spectroscopy of Methylcyanodiacetylene (CH3C5N)

A spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH₃C₅N). Gas‐phase, cryogenic matrix‐isolated, and pure solid‐phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas‐phase IR absorption bands were revisited and extended.     

Vibrational Property of α-Borophene Determined by Tip-Enhanced Raman Spectroscopy

We report a Raman characterization of the α borophene polymorph by scanning tunneling microscopy combined with tip-enhanced Raman spectroscopy. A series of Raman peaks were discovered, which can be well related with the phonon modes calculated based on an asymmetric buckled α structure. The unusual enhancement of high-frequency Raman peaks in TERS spectra of α borophene is found and associated with its unique buckling when landed on the Ag(111) surface. Our paper demonstrates the advantages of TERS, namely high spatial resolution and selective enhancement rule, in studying the local vibrational properties of materials in nanoscale.     

Prediction of Raman Optical Activity Spectra of Chiral 3‐Acetylcamphorato‐Cobalt Complexes

We examine calculated vibrational Raman optical activity (ROA) spectra of octahedral cobalt complexes containing different combinations of acetylacetonato and 3‐acetylcamphorato ligands. Starting from the Δ‐tris(acetylacetonato)cobalt(III) complex, the ROA spectra of isomers generated by successive replacement of acetylacetonato ligands by chiral (+)‐ or (?)‐3‐acetylcamphorato ligands are investigated. In this way, it is possible to assess the influence of the degree of ligand substitution, ligand chirality, and geometrical isomerism on the ROA spectra. In addition, the effect of the Λ‐configuration is studied. It is found that the ROA spectra contain features that make it possible to identify each of the isomers, demonstrating the great sensitivity of ROA spectroscopy to the chiral nature of the various complexes.     

A Vibrational Raman Optical Activity Study of 1,1′‐Binaphthyl Derivatives

The Raman polarized and vibrational Raman optical activity (VROA) backward spectra are simulated for a series of 2,2′‐substituted 1,1′‐binaphthyl compounds presenting a variety of torsion angles between the two naphthalene rings. The substitution prevents free rotation along this torsion angle and the chirality of these compounds is thus called atropisomerism. However, the rotation is not completely frozen so that two different conformations, namely cisoid and transoid, are found and their Raman and VROA signatures are studied. As expected, the Raman spectra are not very sensitive whereas the VROA spectra present more complex patterns, which evolve as a function of the torsion angle between the two naphthalene groups. In particular, our analysis shows that some modes can be used as a probe for the determination of the torsion angle of these molecules in solution. The contributions of both invariants to the VROA backward intensity are also assessed.     

Formation of High‐Valent Iron–Oxo Species in Superoxide Reductase: Characterization by Resonance Raman Spectroscopy

Superoxide reductase (SOR), a non‐heme mononuclear iron protein that is involved in superoxide detoxification in microorganisms, can be used as an unprecedented model to study the mechanisms of O₂ activation and of the formation of high‐valent iron–oxo species in metalloenzymes. By using resonance Raman spectroscopy, it was shown that the mutation of two residues in the second coordination sphere of the SOR iron active site, K₄₈ and I₁₁₈, led to the formation of a high‐valent iron–oxo species when the mutant proteins were reacted with H₂O₂. These data demonstrate that these residues in the second coordination sphere tightly control the evolution and the cleavage of the O? O bond of the ferric iron hydroperoxide intermediate that is formed in the SOR active site.     

Characterization of Gasoline by Raman Spectroscopy with Chemometric Analysis

A rapid Raman spectroscopy protocol is reported to classify gasoline according to its distributor and to identify and quantify common adulterants. Gasoline from three distributors was collected from 19 stations in São Paulo, Brazil. Principal component analysis (PCA) showed specific clusters for each distributor, and partial least squares discriminant analysis (PLS-DA) correctly identified the origin of the samples. To evaluate the technique for the identification and quantification of the adulterants, authentic samples from each distributor were fortified at levels from 2.5 up to 25.0% (v/v) using ethanol, methanol, toluene, and turpentine to obtain 120 altered samples. PCA showed clear separation among the samples with the adulterants and PLS-DA precisely identified the adulterants (478 in 480 predictions by cross-validation), irrespective of the distributor and the concentration. One classification model was used to characterize all distributors. To quantify the adulterants, 36 multivariate calibration models were constructed using partial least squares (PLS), interval PLS, and PLS genetic algorithm for each distributor and for each adulterant. Cross-validation errors of less than 5.0% were obtained for all adulterants regardless of the distributor. Raman spectroscopy and multivariate analysis were shown to be powerful for rapid and inexpensive for the characterization of gasoline origin and the identification and quantification of common adulterants.     

Intensity‐Carrying Modes in Raman and Raman Optical Activity Spectroscopy

We describe a quantum‐chemical approach for the determination of modes with maximum Raman and Raman optical activity (ROA) intensity by maximizing the intensities with respect to the Raman and Raman optical activity intensity, respectively, which is shown to lead to eigenvalue equations. The intensity‐carrying modes are in general hypothetical modes and do not directly correspond to a certain normal mode in the spectrum. However, they provide information about those molecular distortions leading to intense bands in the spectrum. Modes with maximum Raman intensity are presented for propane‐1,3‐dione, propane‐1,3‐dionate, and Λ‐tris(propane‐1,3‐dionato)cobalt(III). Moreover, the mode with highest ROA intensity is examined for this chiral cobalt complex and also for the (chiral) amino acid L ‐tryptophan. The Raman and ROA high‐intensity modes are an optimal starting guess for intensity‐tracking calculations, in which selectively normal modes with high Raman or ROA intensity are converged. We present the first Raman and ROA intensity‐tracking calculations. These reveal a high potential for large molecules, for which the selective calculation of normal modes with high intensity is desirable in view of the large computational effort required for the calculation of Raman and ROA polarizability property tensors.     

Simultaneous Quantification of Three Polymorphic Forms of Carbamazepine using Raman Spectroscopy and Multivariate Calibration

Carbamazepine is a pharmaceutical product used to treat epilepsy and bipolar disorder. Some active pharmaceutical ingredients, such as carbamazepine, present polymorphism that may alter the bioavailability. Consequently, the determination of different polymorphic forms has become important for the pharmaceutical industry. In this work, polymorphic forms were synthesized and characterized by differential scanning calorimetry and X-ray diffraction. Raman spectroscopy was used to quantify mixtures of the three common polymorphic forms of carbamazepine. A ternary mixture design was used to create the calibration set of ten samples and six levels of concentration for each polymorph. Partial least squares was performed to build the prediction models. Ten spectra were obtained to obtain representative Raman spectra of the mixtures. The calibration models were built using the average spectra, and an external set of samples was used to evaluate the models. The partial least squares model gave a root mean square error of prediction of 6.2% for carbamazepine I, 6.8% for carbamazepine III, and 11.6% for carbamazepine dihydrate. The results showed that good results were obtained for the solid state characterization of the mixtures of polymorphs using a fast strategy for simultaneous analysis.     

Length‐Scale‐Dependent Phase Transformation of LiFePO4: An In situ and Operando Study Using Micro‐Raman Spectroscopy and XRD

The charge and discharge of lithium ion batteries are often accompanied by electrochemically driven phase‐transformation processes. In this work, two in situ and operando methods, that is, micro‐Raman spectroscopy and X‐ray diffraction (XRD), have been combined to study the phase‐transformation process in LiFePO₄ at two distinct length scales, namely, particle‐level scale (～1 μm) and macroscopic scale (～several cm). In situ Raman studies revealed a discrete mode of phase transformation at the particle level. Besides, the preferred electrochemical transport network, particularly the carbon content, was found to govern the sequence of phase transformation among particles. In contrast, at the macroscopic level, studies conducted at four different discharge rates showed a continuous but delayed phase transformation. These findings uncovered the intricate phase transformation in LiFePO₄ and potentially offer valuable insights into optimizing the length‐scale‐dependent properties of battery materials.     

Spectroscopy of Single Dibenzoterrylene Molecules in para‐Dichlorobenzene

We study single dibenzoterrylene (DBT) molecules embedded in 1,4‐dichlorobenzene (para‐dichlorobenzene, pDCB) at 1.2 K. Due to the relatively low melting point of pDCB (53 °C), this host‐guest system can be easily prepared from the molten phase. Narrow linewidths, stable molecular lines and high saturation count rates of single DBT molecules were observed. For this reason, we consider this host‐guest system a promising candidate for the study of interactions of single molecules with other small objects such as waveguides or nanoparticles.     

Spin Crossover in Nitrito‐Myoglobin as Revealed by Resonance Raman Spectroscopy

The myoglobin (Mb) heme Fe‐O‐N=O and heme Fe‐O‐N=O/2‐nitrovinyl species have been characterized by resonance Raman spectroscopy. In the heme Fe‐O‐N=O species, the bound nitrite ligand is removed by solvent exchange, thus reforming metmyoglobin (metMb). The high‐spin heme Fe‐O‐N=O unit is converted into a low‐spin heme Fe‐O‐N=O/2‐nitrovinyl species that can be reversibly switched between a low‐ and a high‐spin state without removing the bound nitrite ligand, as observed in the case of the heme Fe‐O‐N=O species. This spin‐state change is likely to be accompanied by a general structural rearrangement in the protein‐binding pocket. This example is the first of a globin protein that can reversibly change its metal spin state through an internal perturbation. These findings provide a basis for understanding the structure–function relationship of the spin cross found in other metalloenzymes and Fe^III–porphyrin complexes.