Molecular Vibrations of an Oxygen-Evolving Complex and Its Synthetic Mimic

Bio-inspired catalysis for artificial photosynthesis has been widely studied for decades, in particular, with the purpose of using bio-disposable and non-toxic metals as building blocks. The characterisation of such catalysts has been achieved by using different kinds of spectroscopic methods, from X-ray crystallography to NMR spectroscopy. An artificial Mn₄CaO₄ cubane cluster with dangling Mn₄ was synthesised in 2015 [Zhang et al. Science 2015 , 348, 690–693]; this cluster showed many structural similarities to that of the natural oxygen-evolving complex. An accurate structural and spectroscopic comparison between the natural and artificial systems is highly relevant to understand the catalytic mechanism. Among data from different techniques, the differential FTIR spectra (S_n+1−S_n) of photosystem II are still lacking a complete interpretation. The availability of IR data of the artificial cluster offers a unique opportunity to assign absolute absorption spectra on a well-defined and easier to interpret analogous moiety. The present work aims to investigate the novel inorganic compound as a model system for an oxygen-evolving complex through measurement of its spectroscopic properties. The experimental results are compared with calculations by using a variety of theoretical methods (normal mode analysis, effective normal mode analysis) in the S₁ state. We underline the similarities and the differences in the computational spectra based on atomistic models of Mn₄CaO₅ and Mn₄CaO₄ complexes.     

IR,Raman, and UV/Vis Spectra of Corannulene for Use in Possible Interstellar Identification

The spectroscopic characterization of corannulene (C₂₀H₁₀) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography‐mass spectrometry (GC‐MS). During a high‐performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time‐dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.     

Subsystem‐Based Theoretical Spectroscopy of Biomolecules and Biomolecular Assemblies

The absorption properties of chromophores in biomolecular systems are subject to several fine‐tuning mechanisms. Specific interactions with the surrounding protein environment often lead to significant changes in the excitation energies, but bulk dielectric effects can also play an important role. Moreover, strong excitonic interactions can occur in systems with several chromophores at close distances. For interpretation purposes, it is often desirable to distinguish different types of environmental effects, such as geometrical, electrostatic, polarization, and response (or differential polarization) effects. Methods that can be applied for theoretical analyses of such effects are reviewed herein, ranging from continuum and point‐charge models to explicit quantum chemical subsystem methods for environmental effects. Connections to physical model theories are also outlined. Prototypical applications to optical spectra and excited states of fluorescent proteins, biomolecular photoreceptors, and photosynthetic protein complexes are discussed.     

Far‐Infrared Spectra of Yttrium‐Doped Gold Clusters AunY (n=1–9)

The geometric, spectroscopic, and electronic properties of neutral yttrium‐doped gold clusters Au_nY (n=1–9) are studied by far‐infrared multiple photon dissociation (FIR‐MPD) spectroscopy and quantum chemical calculations. Comparison of the observed and calculated vibrational spectra allows the structures of the isomers present in the molecular beam to be determined. Most of the isomers for which the IR spectra agree best with experiment are calculated to be the energetically most stable ones. Attachment of xenon to the Au_nY cluster can cause changes in the IR spectra, which involve band shifts and band splittings. In some cases symmetry changes, as a result of the attachment of xenon atoms, were also observed. All the Au_nY clusters considered prefer a low spin state. In contrast to pure gold clusters, which exhibit exclusively planar lowest‐energy structures for small sizes, several of the studied species are three‐dimensional. This is particularly the case for Au₄Y and Au₉Y, while for some other sizes (n=5, 8) the 3D structures have an energy similar to that of their 2D counterparts. Several of the lowest‐energy structures are quasi‐2D, that is, slightly distorted from planar shapes. For all the studied species the Y atom prefers high coordination, which is different from other metal dopants in gold clusters.     

Hydrogen Bonding in Fluorinated Amides: FTIR,Two Dimensional Correlation Spectroscopy and DFT Calculations

Fluorinated macromers with amidic functional groups are used as additives in several high tech applications. We show here how aggregation phenomena related to hydrogen bonding are one of the key factor determining their chemical/physical and macroscopic properties. IR spectra are analyzed depending on different external parameters such as the concentration of amide groups and temperature. The experimental findings have been interpreted by means of DFT (Density Functional Theory) calculations on suitable molecular models. Moreover, 2D correlation spectroscopy has been applied to different sets of data, considering concentration and temperature as perturbing variables. The two dimensional correlation approaches confirmed the computational results and give an overall interpretation of the effects due to concentration and temperature.     

FT Raman and DFT Study on a Series of All‐anti Oligothienoacenes End‐Capped with Triisopropylsilyl Groups

Herein, we study the π‐conjugational properties of a homologous series of all‐anti oligothienoacenes containing four to eight fused thiophene rings by means of FT Raman spectroscopy and DFT calculations. The theoretical analysis of the spectroscopic data provides evidence that selective enhancement of a very limited number of Raman scatterings is related to the occurrence in these oligothienoacenes of strong vibronic coupling between collective ν(C?C) stretching modes in the 1600–1300 cm^?1 region and the HOMO/LUMO frontier orbitals (HOMO=highest occupied molecular orbital; LUMO=lowest unoccupied molecular orbital). The correlation of the Raman spectroscopic data and theoretical results for these all‐anti oligothienoacenes with those previously collected for a number of all‐syn oligothienohelicenes gives further support to the expectation that cross‐conjugation is dominant in heterohelicenes. Fully planar all‐anti oligothienoacenes display linear π conjugation which seemingly does not reach saturation with increasing number of annulated thiophene rings in the oligomeric chain at least up to the octamer.     

Spectroscopy of Dibenzorubicene: Experimental Data for a Search in Interstellar Spectra

We report on the characterization of dibenzo[cde,opq]rubicene (C₃₀H₁₄). The molecule was studied in solution at room temperature with absorption spectroscopy in the visible (vis) and ultraviolet (UV) wavelength ranges, and with emission spectroscopy. The infrared (IR), visible, ultraviolet, and vacuum ultraviolet (VUV) absorption spectra of a thin film were measured also at room temperature. In addition, the UV/vis absorption spectrum was measured at cryogenic temperatures using the matrix isolation spectroscopy technique. The interpretation of spectra was supported by theoretical calculations based on semiempirical and ab initio models, as well as on density functional theory. Finally, the results of the laboratory study were compared with interstellar spectra.     

Vibrational spectral fingerprinting for chemical recognition of biominerals

Pathologies associated with calcified tissue, such as osteoporosis, demand in vivo and/or in situ spectroscopic analysis to assess the role of chemical substitutions in the inorganic component. High energy X-ray or NMR spectroscopies are impractical or damaging in biomedical conditions. Low energy spectroscopies, such as IR and Raman techniques, are often the best alternative. In apatite biominerals, the vibrational signatures of the phosphate group are generally used as fingerprint of the materials although they provide only limited information. Here, we have used first principles calculations to unravel the complexity of the complete vibrational spectra of apatites. We determined the spectroscopic features of all the phonon modes of fluoroapatite, hydroxy-apatite, and carbonated fluoroapatite beyond the analysis of the phosphate groups, focusing on the effect of local corrections induced by the crystalline environment and the specific mineral composition. This provides a clear and unique reference to discriminate structural and chemical variations in biominerals, opening the way to a widespread application of non-invasive spectroscopies for in vivo diagnostics, and biomedical analysis.     

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.     

Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization

To develop new and flexible Cu^I containing luminescent substances, we extend our previous investigations on two metal-centered species to four metal-centered complexes. These complexes could be a basis for designing new organic light-emitting diode (OLED) relevant species. Both the synthesis and in-depth spectroscopic analysis, combined with high-level theoretical calculations are presented on a series of tetranuclear Cu^I complexes with a halide containing Cu₄X₄ core (X=iodide, bromide or chloride) and two 2-(diphenylphosphino)pyridine bridging ligands with a methyl group in para (4-Me) or ortho (6-Me) position of the pyridine ring. The structure of the electronic ground state is characterized by X-ray diffraction, NMR, and IR spectroscopy with the support of theoretical calculations. In contrast to the para system, the complexes with ortho-substituted bridging ligands show a remarkable and reversible temperature-dependent dual phosphorescence. Here, we combine for the first time the luminescence thermochromism with time-resolved FTIR spectroscopy. Thus, we receive experimental data on the structures of the two triplet states involved in the luminescence thermochromism. The transient IR spectra of the underlying triplet metal/halide-to-ligand charge transfer (³M /XLCT) and cluster-centered (³CC) states were obtained and interpreted by comparison with calculated vibrational spectra. The systematic and significant dependence of the bridging halides was analyzed.     

Boronate Ligands in Materials: Determining Their Local Environment by Using a Combination of IR/Solid‐State NMR Spectroscopies and DFT Calculations

Boronic acids (R‐B(OH)₂) are a family of molecules that have found a large number of applications in materials science. In contrast, boronate anions (R‐B(OH)₃^?) have hardly been used so far for the preparation of novel materials. Here, a new crystalline phase involving a boronate ligand is described, Ca[C₄H₉‐B(OH)₃]₂, which is then used as a basis for the establishment of the spectroscopic signatures of boronates in the solid state. The phase was characterized by IR and multinuclear solid‐state NMR spectroscopy (¹H, ¹³C, ¹¹B and ⁴³Ca), and then modeled by periodic DFT calculations. Anharmonic OH vibration frequencies were calculated as well as NMR parameters (by using the Gauge Including Projector Augmented Wave—GIPAW—method). These data allow relationships between the geometry around the OH groups in boronates and the IR and ¹H NMR spectroscopic data to be established, which will be key to the future interpretation of the spectra of more complex organic–inorganic materials containing boronate building blocks.     

Self‐Assembly Structures of 1H‐Indazoles in the Solution and Solid Phases: A Vibrational (IR,FIR, Raman,and VCD) Spectroscopy and Computational Study

1H‐indazoles are good candidates for studying the phenomena of molecular association and spontaneous resolution of chiral compounds. Thus, because the 1H‐indazoles can crystallize as dimers, trimers, or catemers, depending on their structure and the phase that they are in, the difficulty in the experimental analysis of the structure of the family of 1H‐indazoles becomes clear. This difficulty leads us to contemplate several questions: How can we determine the presence of different structures of a given molecular species if they change according to the phase? Could these different structures be present in the same phase simultaneously? How can they be determined? To shed light on these questions, we outline a very complete strategy by using various vibrational spectroscopic techniques that are sensitive (VCD) and insensitive (IR, FIR, and Raman) towards the chirality, together with quantum chemical calculations.     

Conformational Preferences of a β‐Octapeptide as Function of Solvent and Force‐Field Parameters

The ability to design properly folded β‐peptides with specific biological activities requires detailed insight into the relationship between the amino acid sequence and the secondary and/or tertiary structure of the peptide. One of the most frequently used spectroscopic techniques for resolving the structure of a biomolecule is NMR spectroscopy. Because only signal intensities and frequencies are recorded in the experiment, a conformational interpretation of the recorded data is not straightforward, especially for flexible molecules. The occurrence of conformational and/or time averaging, and the limited amount and accuracy of experimental data hamper the precise conformational determination of a biomolecule. In addition, the relation between experimental observables with the underlying conformational ensemble is often only approximately known, thereby aggravating the difficulty of structure determination of biomolecules. The problematic aspects of structure refinement based on NMR nuclear Overhauser effect (NOE) intensities and ³J‐coupling data are illustrated by simulating a β‐octapeptide in explicit MeOH and H₂O as solvents using three different force fields. NMR Data indicated that this peptide would fold into a 3₁₄‐helix in MeOH and into a hairpin in H₂O. Our analysis focused on the conformational space visited by the peptide, on structural properties of the peptide, and on agreement of the MD trajectories with available NMR data. We conclude that 1) although the 3₁₄‐helical structure is present when the peptide is solvated in MeOH, it is not the only relevant conformation, and that 2) the NMR data set available for the peptide, when solvated in H₂O, does not provide sufficient information to derive a single secondary structure, but rather a multitude of folds that fulfill the NOE data set.     

Half-Sandwich Manganese Complex Bearing Fused Oxazoline-NHC Ligand: Conformational Analysis and Evaluation in Asymmetric Ketone Hydrosilylation

The first manganese complex bearing a chiral N-heterocyclic carbene (NHC) ligand was prepared and studied by spectroscopic methods and X-ray diffraction. While IR spectroscopy revealed the existence of two isomers in solution with distinct ν_CO band patterns, DFT calculations indicated that these isomers correspond to rotamers around the Mn−NHC bond and their different spectroscopic properties were rationalized by the occurrence of attractive π(C=C)⋅⋅⋅π*(C≡O) or σ(C−H)⋅⋅⋅π*(C≡O) intramolecular interligand interactions. The evaluation of this complex in catalytic hydrosilylation of acetophenone using Ph₂SiH₂ under UV irradiation led to the formation of the corresponding (R)-alcohol with low enantioselectivity.     

Optical Spectroscopy and Theoretical Studies in Calixarene Chemistry

This review summarizes different applications of optical spectroscopic methods in calixarene chemistry including UV/Vis spectrometry, vibrational spectroscopic techniques (FTIR and Raman spectroscopy), luminescence spectroscopy (including fluorescence and phosphorescence), ellipsometry and various optical microscopic methods. Moreover, the results of theoretical studies (AM1, PM3, DFT, ab initio, etc.) are summarized based on selected papers in the field of conformational studies, thermodynamics and complexation features. About 300 references are processed systematically from the results reported mainly in the recent years with emphasis on the potential of practical application of these molecules.     

Chemical Imaging of Monolayers on Metal Surfaces: Applications in Corrosion,Catalysis, and Self‐Assembled Monolayers

In situ techniques are indispensable to understanding many topics in surface chemistry. As a consequence, several spectroscopic methods have been developed to provide molecular‐level information that only spectroscopy can supply. However, as important as this information is, it is just as critical to realize that nearly all surfaces under investigation have spatial heterogeneities of the order of nanometers to millimeters; thus, spatial analysis is very important to the overall interpretation. This Minireview focuses on a few of the recent developments in spectroscopic techniques that can provide spatial, spectroscopic, and in situ information. These techniques include photo‐electron microscopy, infrared and Raman imaging, and nonlinear optical imaging vibrational spectroscopy as applied to topics in corrosion, catalysis and self‐assembled monolayers.     

The Spectroscopic Characterization of Halogenated Pollutants through the Interplay between Theory and Experiment: Application to R1122

In the last decade, halogenated ethenes have seen an increasing interest for different applications; in particular, in refrigeration, air-conditioning and heat pumping. At the same time, their adverse effects as atmospheric pollutants require environmental monitoring, especially by remote sensing spectroscopic techniques. For this purpose, an accurate characterization of the spectroscopic fingerprint—in particular, those of relevance for rotational–vibrational spectroscopy—of the target molecules is strongly needed. This work provides an integrated computational–theoretical investigation on R1122 (2-Chloro-1,1-difluoro-ethylene, ClHC=CF2), a compound widely employed as a key intermediate in different chemical processes. State-of-the-art quantum chemical calculations relying on CCSD(T)-based composite schemes and hybrid CCSD(T)/DFT approaches are used to obtain an accurate prediction of the structural, rotational and vibrational spectroscopic properties. In addition, the equilibrium geometry is obtained by exploiting the semi-experimental method. The theoretical predictions are used to guide the analysis of the experimentally recorded gas-phase infrared spectrum, which is assigned in the 400–6500 cm−1 region. Furthermore, absorption cross sections are accurately determined over the same spectral range. Finally, by using the obtained spectroscopic data, a first estimate of the global warming potential of R1122 vibrational spectra is obtained.     

Understanding J‐Modulation during Spatial Encoding for Sensitivity‐Optimized Ultrafast NMR Spectroscopy

Ultrafast (UF) NMR spectroscopy is an approach that yields 2D spectra in a single scan. This methodology has become a powerful analytical tool that is used in a large array of applications. However, UF NMR spectroscopy still suffers from an intrinsic low sensitivity, and from the need to compromise between sensitivity, spectral width, and resolution. In particular, the modulation of signal intensities by the spin–spin J‐coupling interaction (J‐modulation) impacts significantly on the intensities of the spectral peaks. This effect can lead to large sensitivity losses and even to missing spectral peaks, depending on the nature of the spin system. Herein, a general simulation package (Spinach) is used to describe J‐modulation effects in UF experiments. The results from simulations match with experimental data and the results of product operator calculations. Several methods are proposed to optimize the sensitivity in UF COSY spectra. The potential and drawbacks of the different strategies are also discussed. These approaches provide a way to adjust the sensitivity of UF experiments for a large range of applications.     

Recent developments in two-dimensional (2D) correlation spectroscopy

Recent noteworthy developments in the field of two-dimensional(2D) correlation spectroscopy are reviewed.2D correlation spectroscopy has become a very popular tool due to its versatility and relative ease of use.The technique utilizes a spectroscopic or other analytical probe from a number of selections for a broad range of sample systems by employing different types of external perturbations to induce systematic variations in intensities of spectra.Such spectral intensity variations are then converted into2 D spectra by a form of correlation analysis for subsequent interpretation.Many different types of 2D correlation approaches have been proposed.In particular,2D hetero-correlation and multiple perturbation correlation analyses,including orthogonal sample design scheme,are discussed in this review.Additional references to other important developments in the field of 2D correlation spectroscopy,such as projection correlation and codistribution analysis,were also provided.