Search

Density functional theory calculations with periodic boundary conditions are exploited to study the infrared spectrum of crystalline polyethylene. Spectral changes lead by the intermolecular packing in the orthorhombic three-dimensional crystal are discussed by means of a careful comparison with calculations carried out for an isolated polymer chain in the all-trans conformation, described as an ideal one-dimensional crystal. The results are analyzed in the framework of the "oligomer approach" through the modelling of the IR spectrum of n-alkanes of different lengths. The study demonstrates that a relevant absorption intensity modulation of CH2 deformation transitions takes place in the solid state. This finding suggests a new interpretation for the experimental evidences collected in the past by means of IR intensity measurement during thermal treatment. Moreover, the comparison between calculations for 3-D crystal and for the isolated polyethylene chain (1-D crystal) allows to put in evidence the effect of the local electric field on the computed infrared intensities. This observation provides guidelines for the comparison between infrared absorption intensities predicted for an isolated unit and for a molecule belonging to a crystal, through the introduction of suitable correction factors based on the refraction index of the material and depending on the dimensionality of such units (0D--molecule; 1D--polymer; 2D--slab). [ABSTRACT FROM AUTHOR]

Featured Application: In this research, a procedure to prepare a molecular imprinted polymer is presented for the selective extraction of phenolic acids from human urine samples. Studies for monitoring the bioavailability of dietary flavonoid compounds generate great interest. Among them, low-molecular-weight phenolic acids, secondary metabolites present in colonic catabolism and urinary excretion, have been proposed as biomarkers of polyphenol intake. Using 4-hydroxyphenylacetic acid as a template, a molecularly imprinted polymer (MIP) was synthesized for selective extraction of these hydroxylated metabolites from human urine samples and posterior analysis in an HPLC-DAD-MS system. Polymers were characterized by Scanning electron microscopy (SEM), Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), Brunauer-Emmett-Teller (BET) method, and binding experiments. MIP presents specific recognition ability for template and analogues molecules. This capacity of recognition and the pH dependence of the binding strength was also studied. The method was validated over a concentration range of 0.25–40 mg/L, r2 > 0.995. In the optimized conditions, the recovery value was 94% with RSD 1.2%. The Limit of Detection (LOD) and Limit of Quantification (LOQ) were 1.22 and 3.69 mg/L, respectively. In our knowledge, it is the first time that this methodology is applied to analyze urinary catabolites of the polyphenol compound and to provide a specific method and simple analysis alternative. The selective extraction of these metabolites improves the application and results obtained by other less sensitive analysis methods than the validation method. It also facilitates the development of new screening methods. [ABSTRACT FROM AUTHOR]

In previous studies [Puszyńska-Tuszkanow et al. Polyhedron, 2011, 30(12), 2016; Nogueira et al. J. Phys. Chem. A, 2014, 118(31), 5994; Nogueira et al., J. Mol. Struct., 2017, 1148, 111], two different polymorphs of 1-methylhydantoin (1-MH, C4H6N2O2) were identified (forms I and II) and characterized using infrared and Raman spectroscopies, as well as by X-ray diffraction. In this work, a new polymorph of the compound (form III) is described. The new polymorph was characterized spectroscopically and its structure was determined for the first time by single crystal X-ray diffraction. Very interestingly, the crystal of polymorph III was found to exhibit a high-Z′ (Z′ = 3) asymmetric unit and 12 molecules in the unit cell (Z = 12), which contrasts with the simpler crystal structures found previously for forms I and II (Z = 4; Z′ = 1). Besides, a thorough study of the polymorphism of 1-MH was performed with the help of state-of-the-art first principles fully periodic calculations of the structure, as well as infrared and Raman spectra of the different polymorphs of the compound. Marker-bands in the infrared and Raman spectra of the polymorphs are proposed for fast spectroscopic identification of the polymorphs. [ABSTRACT FROM AUTHOR]

The infrared (IR) spectrum of an adamantyl endcapped α, ω-polyyne (the hexayne, Ad-C12-Ad) is investigated both experimentally and computationally. A new IR band is observed upon UV photoexcitation of the compound (embedded in a poly methyl methacrylate matrix at 78 K), thus, revealing the existence of new photogenerated molecular structure trapped at low temperature. Complete reversibility is found, thus, demonstrating that the photoexcitation is responsible for the generation of metastable excited states of the molecule. Density functional theory and time dependent density functional theory calculations indicate that these metastable states result from the forbidden singlet (S1) or triplet (T1) excited states, and geometry optimizations of the polyyne trapped in either S1 and/or T1 states demonstrate that the carbon chain takes on a cumulenic structure. Comparison of the experimental and the computed IR spectra for the molecule trapped in the forbidden states confirms that the new IR features are clear markers of cumulenic species. The temperature and time dependent behavior of the new IR band is analyzed, while the experimentally determined value of the activation energy highlights the low stability of these molecular structures. [ABSTRACT FROM AUTHOR]

State-of-the-art computational methods in solid-statechemistrywere applied to predict the structural and spectroscopic propertiesof the α and γ crystalline polymorphs of nylon 6. Densityfunctional theory calculations augmented with an empirical dispersioncorrection (DFT-D) were used for the optimization of the two differentcrystal structures and of the isolated chains, characterized by adifferent regular conformation and described as one-dimensional infinitechains. The structural parameters of both crystalline polymorphs werecorrectly predicted, and new insight into the interplay of conformationaleffects, hydrogen bonding, and van der Waals interactions in affectingthe properties of the crystal structures of polyamides was obtained.The calculated infrared spectra were compared to experimental data;based on computed vibrational eigenvectors, assignment of the infraredabsorptions of the two nylon 6 polymorphs was carried out and criticallyanalyzed in light of previous investigations. On the basis of a comparisonof the computed and experimental IR spectra, a set of marker bandswas identified and proposed as a tool for detecting and quantifyingthe presence of a given polymorph in a real sample: several markerbands employed in the past were confirmed, whereas some of the previousassignments are criticized. In addition, some new marker bands areproposed. The results obtained demonstrate that accurate computationaltechniques are now affordable for polymers characterization, openingthe way to several applications of ab initio modeling to the studyof many families of polymeric materials. [ABSTRACT FROM AUTHOR]

Abstract: We present a computational study based on DFT simulations of the infrared spectra of several short alkyl chains carrying polar end groups. The work aims to provide guidelines for the detection of marker bands signalling the occurrence of specific intramolecular interactions between the polar head and CH2 groups at different distances. In particular, the CH stretching region is investigated and new features assigned to normal modes localized on the CH2 groups nearest to the electron-withdrawing atom are identified. The study has been extended also to the rationalization of the experimental IR features shown by a 1-Chloroeicosane (C20H41Cl) sample. [Copyright &y& Elsevier]

Abstract: Perfluoropolyether (PFPE) fluids constitute a class of polymers that fulfil a wide range of requirements for hi-tech applications, due to their pefluorinated backbone. For some of these applications they are requested to bear polar end groups, and the combination of a chemical inert backbone and a reactive end group can produce peculiar conformations and supramolecular structures. The molecular structure and the vibrational properties of the ethoxyl-terminated PFPE FLUOROLINK®E10H in solution are here investigated by means of IR spectroscopy. It is shown that the complex spectral features of the OH-stretching region cannot be explained without a thorough computational study, involving the investigation of the conformational space of an isolated model molecule by means of semiempirical AM1 calculations and DFT calculations. The most relevant conformers were singled out, showing a high degree of conformational disorder for FLUOROLINK®E10H. Furthermore, it is shown that intra- and intermolecular H-bonding affects significantly the molecular structure and the vibrational spectrum. Several interactions are shown to be relevant, such as OH…F interactions and complexes with residual water. Theoretical values of the absolute intensities of OH stretching IR bands, relevant for the analytical applications, are obtained. [Copyright &y& Elsevier]

The mechanism for the functionalization of graphene layers with pyrrole compounds was investigated. Liquid 1,2,5-trimethylpyrrole (TMP) was heated in air in the presence of a high surface area nanosized graphite (HSAG), at temperatures between 80 °C and 180 °C. After the thermal treatments solid and liquid samples, separated by centrifugation, were analysed by means of Raman, Fourier Transform Infrared (FT-IR) spectroscopy, X-Rays Photoelectron Spectroscopy (XPS) and 1H-Nuclear Magnetic Resonance (1H NMR) spectroscopy and High Resolution Transmission Electron Microscopy (HRTEM). FT-IR spectra were interpreted with the support of Density Functional Theory (DFT) quantum chemical modelling. Raman findings suggested that the bulk structure of HSAG remained substantially unaltered, without intercalation products. FT-IR and XPS spectra showed the presence of oxidized TMP derivatives on the solid adducts, in a much larger amount than in the liquid. For thermal treatments at T ≥ 150 °C, IR spectral features revealed not only the presence of oxidized products but also the reaction of intra-annular double bond of TMP with HSAG. XPS spectroscopy showed the increase of the ratio between C(sp2)N bonds involved in the aromatic system and C(sp3)N bonds, resulting from reaction of the pyrrole moiety, observed while increasing the temperature from 130 °C to 180 °C. All these findings, supported by modeling, led to hypothesize a cascade reaction involving a carbocatalyzed oxidation of the pyrrole compound followed by Diels-Alder cycloaddition. Graphene layers play a twofold role: at the early stages of the reaction, they behave as a catalyst for the oxidation of TMP and then they become the substrate for the cycloaddition reaction. Such sustainable functionalization, which does not produce by-products, allows us to use the pyrrole compounds for decorating sp2 carbon allotropes without altering their bulk structure and smooths the path for their wider application. [ABSTRACT FROM AUTHOR]

Abstract: The IR spectrum of Nylon 6,6 in the crystalline α phase is computed by means of dispersion-corrected density functional theory calculations carried out with periodic boundary conditions on the crystal. The results are carefully compared to experimental spectra through a detailed analysis of different frequency regions and focusing on the spectroscopic markers of crystallinity or regularity (i.e. of the regular conformation of the polymer chain). The previous assignments, based exclusively on experimental or semi-empirical investigations, are critically revised, demonstrating that state of the art computational methods in solid state chemistry can provide tools to obtain an unambiguous description of the vibrational properties of the crystalline phases of macromolecular materials. In particular, the ambiguities related to the assignment of some debated bands of crystallinity/regularity are solved. The structural and vibrational properties are interpreted on the basis of the peculiar intra and intermolecular interactions occurring in polyamides. [Copyright &y& Elsevier]