Conformational landscape in chiral terpenes from vibrational spectroscopy and quantum chemical calculations: S-(+)-carvone

S-(+)-carvone (5-isopropenyl-2-methylcyclohex-2-en-1-one) is the primary component in the oil of caraway. Different experimental and theoretical works reveal that there are two possible conformers in which the isopropenyl group can be in equatorial or axial position. For each one, three rotamers were found theoretically, with the equatorial rotamers around 95% of the whole statistical population. In the current work, from a complete assignment of the IR and Raman spectra and the results obtained from the study of the VCD spectrum of the title compound, the three most stable rotamers have been detected experimentally in the liquid phase for the first time. The present work reveals that IR, Raman and VCD are helpful complementary techniques to characterize flexible systems, as terpenes, which present several conformers.     

Investigation of selectively substituted monosaccharides by the method of vibrational spectroscopy and X-ray analysis

The influence of steric factors on the formation of vibrational spectra of carbohydrates was illustrated by the example of selectively substituted monosaccharides. We consider compounds in which the difference and similarity of vibrational spectra are largely determined by the difference and similarity of the geometry of their molecular base (the molecule's skeleton). At the same time, in the case of selectively substituted nitrates of methyl-β- -glucopyranoside, the skeletal bases of which are identical, with differences in the localization site of volumetric substituents and their total number, or in rotational isomers (conformations) of the lateral nitrate groups, their Fourier Raman spectra are different over a wide spectral range. It is concluded that steric factors and the position of volumetric substituents of hydroxyls produce an exceptionally important effect on the formation of vibrational spectra of carbohydrates.     

Alkali metal intercalated titanate nanotubes: A vibrational spectroscopy study

Alkali metal (Li⁺, Na⁺, K⁺) intercalated titanate nanotubes have been studied by vibrational spectroscopy (Raman and FT-infrared), X-ray diffraction, and electron microscopy. The vibrational spectroscopic data shown that the most affected vibrational mode is that related to Ti-O bond whose oxygen is not shared among the TiO₆ units of the framework structure. A correlation between vibrational frequency shifts and intercalated metal was found, thus showing that vibrational spectroscopy is very useful for probing metal intercalated titanate nanotubes. Our results provide good evidences that the structure of titanate layers in titanate nanotube, a subject of long debate in the literature, is similar to trititanates (like Na₂Ti₃O₇).     

Solid state characterization of an antioxidant alkaloid boldine using vibrational spectroscopy and quantum chemical calculations

Boldo (Peumus boldus Mol.), a Chilean tree traditionally employed in folk medicine and recognized as a herbal remedy in a number of pharmacopoeias, mainly for the treatment of liver ailment, has recently been the subject of increasing attention. Boldine, in particular, the major and most characteristic alkaloid constituent of this plant species, now emerges as its most interesting active principle from the pharmacological viewpoint.In the present work the structural and spectral characteristics of boldine have been studied by methods of infrared, Raman spectroscopy and quantum chemistry. Electrostatic potential surface, optimized geometry, harmonic vibrational frequencies, infrared intensities and activities of Raman scattering were calculated by ab initio Hartree-Fock (HF) and density functional theory (DFT) employing B3LYP with complete relaxation in the potential energy surface using 6-311G(d,p) basis set. A complete vibrational assignment is provided for the observed Raman and infrared spectra of boldine molecule.     

Linear all-ortho oligomers of phenol–formaldehyde resins. I. Preparations,characterizations, and solution properties

Three kinds of all-ortho methylene-linked phenolic oligomers, i.e., 4-tert-butylphenol (BP), phenol (AO), and O-methylated BP (BPM), were prepared with good yields and their dilute solution properties were studied. In acetone, all of these oligomers are highly solvated and molecularly dispersed. In chloroform, however, AO and BP molecules strongly form hydrogen bonds among themselves. By intramolecular and intermolecular hydrogen-bondings, a large portion of the dimers and the trimers of AO and BP associate to form bimolecular aggregates in chloroform, assuming pseudo-cyclic conformations analogous to calix [4] arene and calix [6] arene, respectively. By intramolecular hydrogen-bonding, the tetramers and the hexamers also form pseudo-cyclic conformations by themselves in chloroform. © 1993 John Wiley & Sons, Inc.     

Understanding electrocatalysis at nanoscale electrodes and single atoms with operando vibrational spectroscopy

Electrocatalysis presents an environmentally benign route to carry out energy conversion reactions and organic transformations. The use of nanomaterials and single atoms as catalysts in electrochemical reactions has led to improvements in activity and selectivity over bulk electrocatalysts, however, optimal design of electrocatalytic reactions requires an in-depth understanding of the dynamic solid–liquid interface, reaction intermediates, and catalyst structural changes. Tracking bond breaking and formation events in real time is an important complement to catalyst characterization. This short review highlights advancements in fundamental understanding of electrocatalysis at nanoscale and single atom electrocatalysts obtained using operando vibrational spectroscopies. To push the boundaries of our fundamental understanding in electrocatalysis, we also emphasize a need to further develop operando vibrational techniques to reduce ensemble averaging with improvements in spatial and temporal resolution.     

玉米支链淀粉在单糖、寡糖水溶液中的粘度行为

通过乌氏粘度计和数字流变仪研究了玉米支链淀粉在单糖和寡糖溶液中的特性粘度和表观粘度.发现在稀溶液中,玉米支链淀粉的特性粘度随糖浓度的增加而降低;浓溶液中,玉米支链淀粉的糖溶液的表观粘度随糖浓度的增加而升高.小分子糖对支链淀粉在糖溶液中的特性粘度和表观粘度的影响由强到弱依次为蔗糖>麦芽糖>葡萄糖>半乳糖>果糖.     

Computationally-assisted approach to the vibrational spectra of molecular crystals: study of hydrogen-bonding and pseudo-polymorphism.

A new computationally-assisted methodology (PiMM), which accounts for the effects of intermolecular interactions in the crystal, is applied to the complete assignment of the Raman and infrared vibrational spectra of room temperature forms of crystalline caffeine, theobromine, and theophylline. The vibrational shifts due to crystal packing interactions are evaluated from ab initio calculations for a set of suitable molecular pairs, using the B3LYP/6-31G* approach.The proposed methodology provides an answer to the current demand for a reliable assignment of the vibrational spectra of these methyl-xanthines, and clarifies several misleading assignments. The most relevant intermolecular interactions in each system and their effect on the vibrational spectra are considered and discussed. Based on these results, significant insights are obtained for the structure of caffeine in the anhydrous form (stable at room temperature), for which no X-ray structure has been reported. A possible structure based on C((8))--H...N((9)) and C((1,3))--H...O intermolecular interactions is suggested.     

Studies of thiol additions of silane coupling agents by vibrational spectroscopy

Raman and infrared spectroscopies have been used to determine the addition reaction of mercaptopropyltrimethoxysilane (MPTMS) to allyltrimethoxysilane (ATMS) and 7-octenyltrichlorosilane based on the vibrational intensity variation of thiol and vinyl groups in the reaction mixtures. Due to the distinct and moderate intensity of Raman bands observed in the present experiment, the identification with Raman spectroscopic method is more sensitive than that with FTIR spectroscopy. In the presence of UV radiation, thiol addition reaction has been observed in the direct mixing samples of silanes. Hybrid sol-gels prepared with the use of MPTMS and ATMS as precursors in both acidic and basic conditions have revealed the progression of thiol addition under the UV radiation exposure. UV radiation is similarly effective to induce the thiol addition in the sol-gel coated aluminum tiles. Without UV radiation, the use of free radical initiator in the sol-gel samples might also help to induce the addition reaction.     

Structural changes in gluten protein structure after addition of emulsifier. A Raman spectroscopy study

Food protein product, gluten protein, was chemically modified by varying levels of sodium stearoyl lactylate (SSL); and the extent of modifications (secondary and tertiary structures) of this protein was analyzed by using Raman spectroscopy. Analysis of the Amide I band showed an increase in its intensity mainly after the addition of the 0.25% of SSL to wheat flour to produced modified gluten protein, pointing the formation of a more ordered structure. Side chain vibrations also confirmed the observed changes.     

Characterization of PVDF membranes by vibrational spectroscopy

In order to investigate the effectiveness of vibrational spectroscopy in the characterization of polymeric membranes, several poly(vinylidene fluoride) (PVDF) membranes with different porous structures were prepared by the phase inversion process using different casting solvents. An accurate analysis of the Fourier transform Raman (FT-Raman) and the Fourier transform infrared (FTIR) spectra was performed for each sample and the scanning electron microscopy (SEM) results were noted. To highlight the specific problems related to porosity and surface roughness in the acquisition of spectra by different sampling techniques, the attenuated total reflection (ATR) and photoacoustic spectroscopy (PAS) spectra were compared with corresponding spectra obtained from dense films. A detailed analysis of these spectra highlighted their ability in determining the differences in the polymer structure between the two membrane sides. This indicates that (considering the results given by all the different techniques) thorough qualitative membrane characterization can generally be achieved. Moreover, the good quality spectra of the PVDF membrane provide information on a portion of material which depends on its structure, highlighting the usefulness of FTIR-PAS in studying porous materials which, as a rule, give low quality infrared spectra when other sampling techniques are used. However, the complex and inhomogeneous structure of these materials can make quantitative analysis more, or less, difficult.     

A conformational study of hydroxylated isoflavones by vibrational spectroscopy coupled with DFT calculations

The conformational preferences of a series of hydroxylated isoflavones were studied by optical vibrational spectroscopy (FTIR and Raman) coupled with density functional theory (DFT) calculations. Special attention was paid to the effect of the hydroxyl substitution, due to the importance of this group in the biological activity of these systems. The isoflavones investigated – daidzein, genistein and formononetin – were shown to exist in distinct conformations in the solid state, namely regarding the orientation of the hydroxylic groups at C⁷ and within the catechol moiety, that are determinant factors for their conformational behaviour and antioxidant ability. In the light of the most stable conformers obtained for each molecule, a complete assignment of their experimental vibrational spectra was performed.     

Stoichiometry and conformation of the azacrown moiety in sodium complexes of azacrown ethers. A Raman/IR spectroscopic study. Part I: Complexes of 4,13-Diaza-18-crown-6

Three sodium complexes (bromide, iodide and thiocyanate) of 4,13-diaza-18-crown-6 were studied using Raman and IR spectroscopy and normal coordinate calculations to probe the stoichiometry of the complexes and the variation in the conformation of azacrown moiety on complex formation. complex formation is accompanied by characteristic shifts of the bands, especially of those in the 800–900 cm^–1 region. Complexes of both 11 and 21 stoichiometry were observed. Normal coordinate calculations showed the reduction of symmetry of azacrown moiety toC _i , in contrast to theC _2h symmetry known for the parent azacrown and potassium thiocyanate complex.     

Stoichiometry and conformation of the azacrown moiety in sodium complexes of azacrown ethers. a Raman/IR spectroscopic study. Part I: Complexes of 4,13-diaza-18-crown-6

Three sodium complexes (bromide, iodide an thiocyanate) of 4,13-diaza-18-crown-6 were studied using Raman and IR spectroscopy and normal coordinate calculations to probe the stoichiometry of the complexes and the variation in the conformation of azacrown moiety on complex formation. Complex formation is accompaniedby characteristic shifts of the bands, especially of those in the 800–900 cm^–1 region. Complexes of both 1: 1 and 2:1 stoichiometry were observed. Normal coordinate calculations showed the reduction of symmetry of azacrown moiety toC ₂ , in contrast to theC _2h symmetry known for the parent azacrown and potassium thiocyanate complex.     

Use of vibrational spectroscopy to study protein and DNA structure,hydration, and binding of biomolecules: A combined theoretical and experimental approach

We report on our work with vibrational absorption, vibrational circular dichroism, Raman scattering, Raman optical activity, and surface‐enhanced Raman spectroscopy to study protein and DNA structure, hydration, and the binding of ligands, drugs, pesticides, or herbicides via a combined theoretical and experimental approach. The systems we have studied systematically are the amino acids (L ‐alanine, L ‐tryptophan, and L ‐histidine), peptides (N‐4271 acetyl L ‐alanine N′‐methyl amide, N‐acetyl L ‐tryptophan N′‐methyl amide, N‐acetyl L ‐histidine N′‐methyl amide, L ‐alanyl L ‐alanine, tri‐L ‐serine, N‐acetyl L ‐alanine L ‐proline L ‐tyrosine N′‐methyl amide, Leu‐enkephalin, cyclo‐(gly‐L ‐pro)₃, N‐acetyl (L ‐alanine)_n N′‐methyl amide), 3‐methyl indole, and a variety of small molecules (dichlobenil and 2,6‐dochlorobenzamide) of relevance to the protein systems under study. We have used molecular mechanics, the SCC‐DFTB, SCC‐DFTB+disp, RHF, MP2, and DFT methodologies for the modeling studies with the goal of interpreting the experimentally measured vibrational spectra for these molecules to the greatest extent possible and to use this combined approach to understand the structure, function, and electronic properties of these molecules in their various environments. The application of these spectroscopies to biophysical and environmental assays is expanding, and therefore a thorough understanding of the phenomenon from a rigorous theoretical basis is required. In addition, we give some exciting and new preliminary results which allow us to extend our methods to even larger and more complex systems. The work presented here is the current state of the art to this ever and fast changing field of theoretical spectroscopic interpretation and use of VA, VCD, Raman, ROA, EA, and ECD spectroscopies. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Physicochemical Characterization of Two Polysulfon-Amides in Dilute Solution

Summary: As one class of high temperature resistant aromatic polymers, polysulfonamides (PSA) are widely used in industry, physicochemical properties and conformation in solution have not been disclosed yet. In this work, the dilute solution behaviour of poly(m-diphenylsulphone terephthal amide) (m-PSA) was investigated by laser light scattering (LLS) and viscometry. The results showed that the m-PSA exists as single chains both in dimethylsulfoxide (DMSO) and dimethylformamide (DMF). The value of radius of gyration to hydrodynamic radius (s/R_h) indicats that the m-PSA has a random coil conformation in solution. Both the persistence length a and the characteristic ratio C_∞ indicate that the m-PSA chain is flexible in solution. The Mark-Houwink equation of m-PSA in DMSO was determined as [η] = 1.62 × 10⁻⁵M, and the exponent of 0.87 supports the assumption that m-PSA forms random coils in DMSO-solution at 25 °C. It was found that the flexibility of the PSA was affected when 4,4'-diphenylsulphone terephthalamide monomer was incorporated in backbone. The PSA random copolymer (r-PSA) with a comonomer ratio of 3:1 (4,4′-diaminodiphenyl- sulfone to 3,3′-diaminodiphenylsulfone) also had a random coil conformation in DMF; the rigidity increased, while the mobility of the chain was maintained.     

Molecular and Conformational Properties of Dendronized Polymethacrylate in Organic Solvent

The conformational and dynamo-optical properties of a homologous series of polymethacrylate modified by first-generation Fréchet-type dendrons in the molecular mass range of 70–8800 kDa in organic solvents were studied by viscometry, isothermal diffusion, dynamic light scattering, and flow birefringence. The Mark-Kuhn-Houwink equations for this dendronized polymer in toluene and chloroform together with hydrodynamic diameter and Kuhn segment length in toluene are defined as well as the value of intrinsic anisotropy of polarizability for the monomer unit.     

Interpretation of vibrational and NMR spectra of allyl acrylate: An evidence for several conformers

The mid-IR, far-IR, and Raman spectra of allyl acrylate were measured and interpreted with support of the B3LYP/aug-cc-pVDZ calculated anharmonic vibrational spectra followed by the potential energy distribution analysis. The experimental ¹H and ¹³C NMR spectra of allyl acrylate dissolved in CDCl₃ or C₆D₆ were interpreted by means of the B3LYP/aug-cc-pVDZ-su2 calculated NMR chemical shifts and J(¹H,¹H) and J(¹H,¹³C) coupling constants. Exactly ten stable allyl acrylate conformers (five s-cis and five s-trans) were found after careful B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ scan of the conformational space. The experimental IR and Raman spectra are in good agreement with the theoretical spectra of the most stable conformers 1 with a presence of the second stable conformer 2, both exhibiting cis arrangement of the acrylic moiety. There are however two bands in the IR spectra, at ca. 1270 and 1260 cm⁻¹, that definitely indicate the conformers with trans arrangement of the acrylic moiety to be present in liquid allyl acrylate. The bands at ca. 2990 and 1650 cm⁻¹ are suggested to be due to Fermi resonances engaging CH and CC stretching vibrations, respectively. The careful inspection of the room temperature ¹H and ¹³C NMR spectra of allyl acrylate suggest that a dominating form of the allyl acrylate molecule in an inert solvent exhibits the cis conformation of the acrylic moiety and an extended allyl group.     

Vibrational CD spectroscopy as a powerful tool for stereochemical study of cyclophynes in solution

Stereochemical study of cyclophynes, which is otherwise rather difficult to perform, can be achieved by vibrational CD spectroscopy.     

Complementary optical and neutron vibrational spectroscopy study of bromanilic acid: 2,3,5,6-tetramethylpyrazine (1:1) cocrystal

Complementary structural and vibrational spectroscopy study of bromanilic acid:2,3,5,6-tetramethylpyrazine (BrA:TMP) 1:1 cocrystal is reported. The crystallographic structure was determined by means of single-crystal X-ray diffraction and can be described as a stacked net of hydrogen-bonded TMPH⁺⋯BrA⁻⋯BrA⁻⋯TMPH⁺ moieties. The structural analysis was supported by ¹³CP/MAS NMR study. The complementary vibrational analysis was performed by combining optical (infrared, Raman, terahertz) and inelastic neutron scattering spectroscopy with the state-of-the-art solid–state density functional theory (DFT) computations, which have proven to be superior to the hybrid cluster modeling approach. An excellent agreement between theoretical and experimental data was observed over the entire spectral range, allowing for deep understanding of the vibrational properties. While the primary hydrogen-bonding interactions are limited to the above quoted structural units, the system revealed very little dispersion of the phonon branches, manifested mainly in the intermolecular vibrations range. Moreover, the studied phase does not exhibit any mechanical instability, which could suggest a displacive structural transformation tendency.