Unpolarized and Polarized Raman Spectroscopy of Nylon‑6 Polymorphs: A Quantum Chemical Approach

Exploiting the very recent potentialities of state-of-the-art quantum chemical simulations of crystalline solids, unpolarized Raman spectra of α and γ polymorphs of Nylon-6 obtained through periodic density functional theory calculations are presented for the first time. The computed spectra are compared with the experimental spectra reported in the literature and allow a detailed interpretation to be proposed of the patterns observed, identifying unambiguous Raman marker bands of the different phases. The calculations of single crystal directional intensities gave the further possibility to predict the polarization properties of the Raman spectra of these polymorphs: considering in particular the α phase, polarized Raman spectra have been computed and showed a very good agreement with measurements previously reported for uniaxially oriented samples

Crystal Structure and Vibrational Spectra of Poly(trimethylene terephthalate) from Periodic Density Functional Theory Calculations

The crystal structure and the IR spectrum of crystalline poly­(trimethylene terephthalate), PTT, have been investigated by means of periodic density functional theory calculations including Grimme’s correction for dispersion interactions. Both structural and spectroscopic results have been critically compared to the experimental data taken from the literature, showing very good agreement between theory and the experiments. The previous spectral assignments, based only on experimental investigations, have been revised, and further insights have been obtained. Furthermore, spectroscopic markers of crystallinity or regularity (i.e., of the regular conformation of the polymer chain) have been proposed. In addition to the analysis of the IR spectra, the effect of computational parameters on the crystal structure determination (basis sets and parameters for Grimme’s correction) have been analyzed. This work demonstrates that state-of-the-art computational methods can provide an unambiguous description of the structural and vibrational properties of crystalline polymers on the basis of the peculiar intra- and intermolecular interactions occurring in different macromolecular materials

Polymorphism of Poly(butylene terephthalate) Investigated by Means of Periodic Density Functional Theory Calculations

The conformation and solid state structure of the two α and β polymorphs of poly­(butylene terephthalate) are here studied by means of state-of-the-art first principles calculations, carried out both for the crystals and the infinite one-dimensional chain models. Focusing in details on the debated β form, induced by mechanical deformation, we verified the setting on of an all-trans conformation, as also supported by the simulation of the infrared spectra of the different polymorphs compared to the available experimental spectra. The transition from the α to the β form is also simulated by applying increasing strains to the infinite polymer chain: a peculiar evolution of the intramolecular structure is indeed predicted, showing a transition from the GTG′ conformation found for the α form to the TTT conformation of the strained β form

Ab Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs

State-of-the-art computational methods in solid-state chemistry were applied to predict the structural and spectroscopic properties of the α and γ crystalline polymorphs of nylon 6. Density functional theory calculations augmented with an empirical dispersion correction (DFT-D) were used for the optimization of the two different crystal structures and of the isolated chains, characterized by a different regular conformation and described as one-dimensional infinite chains. The structural parameters of both crystalline polymorphs were correctly predicted, and new insight into the interplay of conformational effects, hydrogen bonding, and van der Waals interactions in affecting the 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 infrared absorptions of the two nylon 6 polymorphs was carried out and critically analyzed in light of previous investigations. On the basis of a comparison of the computed and experimental IR spectra, a set of marker bands was identified and proposed as a tool for detecting and quantifying the presence of a given polymorph in a real sample: several marker bands employed in the past were confirmed, whereas some of the previous assignments are criticized. In addition, some new marker bands are proposed. The results obtained demonstrate that accurate computational techniques are now affordable for polymers characterization, opening the way to several applications of ab initio modeling to the study of many families of polymeric materials

π‑Conjugation and End Group Effects in Long Cumulenes: Raman Spectroscopy and DFT Calculations

We have investigated the structure and spectroscopic properties of cumulenic carbon chains, focusing on the peculiar π-conjugation properties and end-group effects that influence their behavior. With support from Density Functional Theory (DFT) calculations, we have analyzed the IR and Raman spectra of cumulenes characterized by different end-capping groups and we have related them to the bond length alternation (BLA) pattern and local spectroscopic parameters associated with the CC bonds along the sp-carbon chain. For cumulenes we observe a breakdown of the correlation existing in polyynes among frequencies, Raman intensities of the R line (longitudinal CC stretching modes), and BLA. While the low R line frequency and equalized CC bonds would indicate the “metallic” character of cumulenic species, we obtain an unusually strong Raman intensity, which is typical of bond-alternated (semiconductive) structures. DFT calculations reveal that this is a consequence of π-electron conjugation, which markedly extends from the sp-carbon chain to the aryl rings belonging to the end groups. These findings suggest the existence of a strong electronic, vibrational and structural coupling between sp-carbon chains and sp²-carbon species, which could play a key role in nanostructured sp/sp²-hybrid carbon materials (e.g., linear carbon chains coupled to graphene domains). Within this context, Raman spectroscopy is a valuable tool for the detailed characterization of the molecular properties of this kind of materials

Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons

Bottom-up synthesis of graphene nanoribbons (GNRs) has significantly advanced during the past decade, providing various GNR structures with tunable properties. The synthesis of chiral GNRs, however, has been underexplored and only limited to (3,1)-GNRs. We report herein the surface-assisted synthesis of the first heteroatom-doped chiral (4,1)-GNRs from the rationally designed precursor 6,16-dibromo-9,10,19,20-tetraoxa-9a,19a-diboratetra­benzo­[<i>a</i>,<i>f</i>,<i>j</i>,<i>o</i>]­perylene. The structure of the chiral GNRs has been verified by scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy in combination with theoretical modeling. Due to the presence of oxygen–boron–oxygen (OBO) segments on the edges, lateral self-assembly of the GNRs has been observed, realizing well-aligned GNR arrays with different modes of homochiral and heterochiral inter-ribbon assemblies