6,808 research outputs found
Can a whole nanoparticle accurately describe a single C60 fullerene when it comes to weak electrostatic interactions?
No full texttechnologically-relevant processes, from printing and powder coating to the removal of fine dust from coal-fired power plant emissions. C60 fullerene, owing to its peculiar mechanical and electronic properties, has been oftentimes used as a model nanoparticle in the study of e.g. cluster conductivity and Coulomb fission, allowing for a deeper understanding on e.g. cluster polarizability and charge transfer among spherical or nearly-spherical particles.
Several classical models describing the interaction between two spherical particles or a spherical particle and a point charge can be found in literature, involving either metallic or dielectric particles [1]. However, attaching macroscopic properties, such as dielectric or metallic character, to a nanometer-sized object is not straightforward. The C60 case has in fact spawned what is now an on-going debate on the matter [2-6].
In this talk, new insights from DFT simulation of neutral C60 molecules interacting with selected electron donors and acceptors are presented, and compared to both literature model interactions (in which point charges are preferred to chemically meaningful ligands) and classical model interactions.
[1] E.B. Lindgren et al. Phys. Chem. Chem. Phys. 18 (2016) 5883.
[2] A.J. Stace et al. Phys. Chem. Chem. Phys. 13 (2011) 18339.
[3] H. Zettergren et al. Phys. Chem. Chem. Phys. 14 (2012) 16360.
[4] G. Raggi et al. Phys. Chem. Chem. Phys. 15 (2013) 20115.
[5] H. Zettergren et al. Phys. Chem. Chem. Phys. 16 (2014) 14969.
[6] F. Lindén et al. J. Chem. Phys. 145 (2016) 194307
On the inter-ring torsion potential of regioregular P3HT: a first principles reexamination with explicit side chains
No full textPoly(3-alkylthiophene) is a family of conjugated semicrystalline polymers for organic electronic applications. Crucial for the fine-tuning of such systems is a detailed understanding of the correlation between molecular structure/morphology and electronic properties. However, a series of a priori assumptions is commonly made in order to deduce macromolecular-scale geometric and energetic features from those of rather small homologous molecular systems. Alkyl side chains are routinely shortened (if not systematically removed) during such high-accuracy ab initio calculations in order to reduce their conformational space. We will show through first principles calculations on a monosubstituted bithiophene molecule how a full-length alkyl fragment can influence both side chain energetics and backbone flexibility in alkylthiophene-based polymers and copolymers. Folded side chains, characterized by a gauche arrangement of the second torsion angle from the ring, are found to be substantially favoured over extended ones, thanks to a network of CH–π hydrogen-bond-like interactions with both aromatic rings. Trans-planar (conjugated) arrangements of limit-ordered crystalline models, and cisoid sequences suitable for the investigation of chain-folding phenomena, are also discussed in detail
Exploring short intramolecular interactions in alkylaromatic substrates
No full textFrom proteins and peptides to semiconducting polymers, aliphatic chains on aromatic groups are recurring motifs in macromolecules from very diverse application fields. Fields in which molecular folding and packing determine the macroscopic physical properties that make such advanced materials appealing in the first place. Within each macromolecule, the intrinsic structure of each unit defines how it interacts with its neighbours, ultimately opening up or denying certain backbone conformations. This eventually also determines how macromolecules interact with each other. This account deals specifically with the conformational problem of many common alkylaromatic units, examining the features of an intramolecular interaction involving a side chain with as few as three methylene groups. A set of 23 model compounds featuring an intramolecular interaction between an aliphatic X-H (X = C, N, O, and S) bond and an aromatic ring was considered. Quantitative computational analysis was made possible, thanks to complete basis set extrapolated CCSD(T) calculations and NCI topological analysis, the latter of which revealed an elaborate network of dispersive and steric interactions leading to somewhat unintuitive and unexpected results, such as the higher energetic stability of certain twisted conformational isomers over those with extended side chains. Vicinal covalent effects from polarizing groups and various heteroatoms, along with the occurrence of non-dispersive phenomena, were also investigated. The conclusions drawn from the investigation include a comprehensive set of guidelines intended to aid in the prediction of the most stable conformation for this class of building blocks. Our findings affect a variety of different research fields, including the tailoring of functional materials for organic electronics and photovoltaics, with insights into a rational treatment of conformational disorder, and the study of protein- and peptide-folding preferences, putting an emphasis on peculiar interactions between the backbone and aromatic residues
Unusual crystallization mode of tubular helical 2,3-exo-disyndiotactic polynorbornene: a combined X ray diffraction / molecular modelling study
No full textThis contribution reports the study of the peculiar crystalline structure of the 2,3-exo-disyndiotactic polynorbornene (dsPNB) obtained by polymerization with a TiCl4 based catalysts. Using X-ray diffraction data (Fig. 1a) combined with molecular modelling studies (including the use of molecular mechanics and dynamics together with quantum mechanical methods), the stereochemistry was proved with stereoregular chains adopting an unusual tubular helical conformation in the crystalline state [1,2]. Interestingly, the estimated coherence length along the backbone axis is much higher than the value predicted by the average degree of polymerisation thus suggesting that a sequence of oligomers can packs along the chain axis. The resulting helices three dimensional assembly generates an empty accessible core, in which guest molecules (e.g. toluene, see Fig. 1b) or chain terminals can be easily hosted. A guest like I2 influences substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces guest molecules inside dsPNB channels [2]. Molecular modelling confirms the stability of empty and host-guest crystalline structures. MD simulations rationalises the guest exchange mechanism and the role of chain tails in the crystalline structure. In particular, it has been demonstrated that chain ends of dsPNB has a sufficient mobility to “fill” the empty cavities of the structure even at room temperature [3]
Nucleophilicity and electrophilicity of the C(sp(3))-H bond: methane and ethane binary complexes with iodine
No full textThe occurrence of stable van der Waals complexes of small saturated hydrocarbons with molecular iodine is assessed in order to investigate the ability of sp(3)-hybridized carbon atoms to act as either electron donors or electron acceptors depending on the ligand orientation. Systematic ab initio potential energy surface exploration of methane-I2 and ethane-I2 model dimers was followed by thorough characterization. Despite modest evidence of whole-adduct polarization, the resulting interactions feature a dominant dispersive character. The noncovalent interaction descriptors employed comprise NBO, AIM, NCI, and source function analyses. The relevance of bonding C-H orbitals in donor-acceptor interactions involving saturated hydrocarbons is highlighted. The results here presented corroborate existing literature regarding the electrophilicity of the aliphatic C-H group, and also indicate that the nucleophilic character of C(sp(3)) shares a dependence on electron withdrawing/donating substituents similar to that extensively documented for σ-holes. Indeed, the sole difference between the two, aside from the nucleophilicity/electrophilicity switch, seems to lie in their directionality. Nucleophilic sites on C(sp(3)) are not limited to the outermost region of C along a covalent bond axis, but can also engage electrophiles via the bifurcation plane of a CH2 unit. Since valence electrons on these carbon atoms are engaged in covalent bonds, they can only interact with polarizing ligands via the electron density accumulation/depletion in the four corresponding σ orbitals. These, however, do not seem to interact individually with the accompanying electrophile. Source function and NCI results suggest instead that nucleophilic carbon centres participate in the noncovalent bond themselves by drawing electron density from their shared electron pairs
The structure of 2,3-exo-disyndiotactic polynorbornene: a combined X-ray diffraction / molecular modelling study
No full textThe present communication reports studies of modulations of the crystalline structure of 2,3-exo-disyndiotactic polynorbornene (dsPNB) obtained by catalysts based on TiCl4. Using X-ray patterns analysis (see Fig. 1a) combined with molecular modelling methods, the 2,3-exodisyndiotactic
stereochemistry por this polymer has been established,
with stereoregular chains adopting an unusual tubular helical
conformation in the crystalline state [1]. The helices’ packing leaves empty accessible channels, in which guest molecules can be hosted (see the case of toluene in Fig. 1b). Modulations are apparent as different guests alter the diffraction patterns of dsPNB. For example, a
guest like I2 influences very substantially the relative intensities while leaving peak positions invariant as expected for a polymer structure in which I2 replaces toluene inside the dsPNB channels [2]. Molecular modelling, including molecular mechanics and dynamics and quantum mechanics calculations specific for solid state phases may help to evaluate relative stabilities of of different host/guest crystals. Developments can be envisaged both in a fundamental perspective and
for applications in sensing and in recognition/separation technologies
2,3-EXO-DISYNDIOTACTIC POLYNORBORNENE: A CRYSTALLINE POLYMER WITH TUBULAR HELICAL MOLECULAR STRUCTURE
No full textThe present communication reports the study of the crystalline structure of the 2,3-exodisyndiotactic
polynorbornene obtained by catalysts based on TiCl4. Using molecular modelling
methods combined with powder X-ray diffraction patterns analysis the 2,3-exo-disyndiotactic
stereochemistry was proved with stereoregular polynorbornene (dsPNB) chains adopting an
unusual conformation in the crystalline state. The helices packing leave an empty accessible
tubular channel at the core, in which guest molecules like toluene can be hosted. Molecular
modelling, including also quantum mechanics calculations specific for solid state phases,
demonstrated the stability of empty and host guest crystalline structures
A Molecular Modelling Study of 2,3-exo-disyndiotactic Polynorbornene
No full textThe present contribution reports a detailed molecular modelling study of the peculiar crystalline structure of the 2,3-exo-disyndiotactic polynorbornene (dsPNB) obtained by polymerization with a TiCl4 based catalysts. Molecular mechanics and dynamics have been used together with quantum mechanical methods specific for solid crystalline phases. Using the low energy models obtained and data from X-ray diffraction experiments, the stereochemistry was proved revealing macromolecular chains, in the crystalline state, with an unusual tubular helical (12 units) conformation (Figure 1) [1]. Interestingly, the estimated coherence length (obtained by X-ray diffraction analysis) along the backbone axis is higher than the value predicted by the average degree of polymerisation of the system thus suggesting that a single chain is plausibly a sequence of oligomers. Moreover, the hexagonal packing of the helices (Figure 1) generates an empty accessible core in which guest molecules (toluene or I2 molecules) and also chain terminals can be easily hosted by reversible processes. Experimental study demonstrated that a guest like I2 influences very substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces molecular guests inside dsPNB channels [2]. Molecular modelling methods confirmed the stability of empty and host-guest crystalline structures. In particular, molecular dynamics simulations rationalises both the guest exchange mechanism and the key role of chain tails in the crystalline structure [3]
Thermodynamics of aqueous perfluorooctanoic acid (PFOA) and 4,8-dioxa-3H-perfluorononanoic acid (DONA) from DFT calculations: Insights into degradation initiation
No full textModern fluorosurfactants introduced during and after perfluoroalkyl carboxylates/sulfonates phase-out present chemical features designed to facilitate abatement, hence reducing persistence. However, the implications of such features on environmental partitioning and stability are yet to be fully appreciated, partly due to experimental difficulties inherent to the handling of their (diluted) aqueous solutions. In this work, rigorous quantum chemistry calculations were carried out in order to provide theoretical insights into the thermodynamics of hydroperfluorosurfactants in aqueous medium. Estimates of acid dissociation constant (pKa), standard reduction potential (E0), and bond dissociation enthalpy (BDE) and free energy (BDFE) were computed for perfluorooctanoic acid (PFOA), 4,8-dioxa-3H-perfluorononanoic acid (DONA) and their anionic forms via ensemble averaging at density functional theory level with implicit solvent models. A ‹pKa› in the neighborhood of zero and a E0of about 2.2 V were obtained for PFOA. Predictions for the acidic function of DONA compare well with PFOA's, with a pKaof 0.8–1.5 and a E0of 2.07–2.15 V. Deprotonation thus represents the dominant phenomenon at environmental conditions. Calculations indicate that H-abstraction of the aliphatic proton of DONA by a hydroxyl radical is the thermodynamically favored reaction path in oxidative media, whereas hydrolysis is not a realistic scenario due to the high dissociation constant. Short intramolecular interactions available to the peculiar hydrophobic tail of DONA were also reviewed, and the relevance of the full conformational space of the fluorinated side chain discussed
Unusual crystallization mode of tubular helical 2,3- exo-disyndiotactic polynorbornene
No full textThe present contribution reports the study of the peculiar crystalline structure of the 2,3-exo-disyndiotactic polynorbornene (dsPNB) obtained by polymerization with a TiCl4 based catalysts. Using X-ray diffraction measurements (Fig. 1a) combined with molecular modelling approaches (including molecular mechanics and dynamics together with quantum mechanical methods), the stereochemistry was proved with stereoregular chains adopting an unusual tubular helical conformation in the crystalline state [1]. Interestingly, the estimated coherence length along the backbone axis is much higher than the value predicted by the average degree of polymerisation suggesting that what is modelled as a single chain is plausibly a sequence of oligomers. Moreover, the helices packing generates an empty accessible core, in which guest molecules (e.g. toluene) and chain terminals can be easily hosted (Fig. 1b). A guest like I2 influences very substantially the relative intensities of the diffraction patterns while leaving peak positions unchanged as expected for a unique polymer conformation in which I2 replaces toluene or other hydrocarbon guests inside dsPNB channels [2]. Molecular modelling confirms the stability of empty and host-guest crystalline structures. In particular, MD simulations rationalises both the guest exchange mechanism and the role of chain tails in the crystalline structure
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