G. Van Lier

Acidity of substituted hydrofullerenes: Anab initio quantum-chemical study

Abstract A study is made of the acidity of a series of substituted hydrofullerenes, all resulting from the substitution of one hydrogen at the 1, 2-C60H2 hydrofullerene by a functional group (6, 6-ring fusion). Acidities are obtained via a previously set up correlation with calculated gas-phase deprotonation energies, and correlated with the properties of the functional group and the cage. All energy and property calculations have been performed atab initio HF/3–21G level on AMI fully optimised structures, for the acidic as well as for the conjugate base forms. Besides the deprotonation energy, ΔE, the charge on the acidic hydrogen and the electronic delocalisation, Δ, the global hardness and softness are also calculated for each system. Electronic delocalisation in the conjugate base plays an important role in the fairly high acidity of these systems. The interplay of group softness and electronegativity on the acidity sequence is shown, yielding a correlation with the charge acceptance of the functional group upon deprotonation.

A time dependent DFT study of the efficiency of polymers for organic photovoltaics at the interface with PCBM

The interface between donor and acceptor material in organic photovoltaics is of major importance for the functioning of such devices. In this work, the singlet excitation schemes of six polymers used in organic photovoltaics (P3HT, MDMO-PPV, PCDTBT, PCPDTBT, APFO3 and TBDTTPD) at the interface with a PCBM acceptor were studied using TDDFT in combination with the range-separated CAM-B3LYP exchange–correlation functional. By comparing with the excitations in the pure polymer and analyzing the excitation intensities and a measure for orbital overlap, it was possible to identify excitations as either excitation of the polymer or as a charge transfer between donor and acceptor. By combining orbital overlaps between the molecular orbitals involved in charge transfer and the intensity of the polymer excitation a broad correlation was seen with the record efficiencies found in the literature.

An ab initio quantum chemical study on the structure, stability and polymerization of C28 and its derivatives

Abstract STO-3G calculated reaction energies of the radical reactions producing C 28 H n ( n = 1, 2, 3, 4) from the unstable C 28 are used to discuss the stabilization of these derivatives. The influence of the nature of various groups X (X is F, OH, NH 2 , CI, CH 3 , i Pr, CN, CF 3 and t Bu) on the stabilization of C 28 is investigated using the isodesmic reactions in which one or more hydrogens of C 28 H 4 are substituted by X. In general, all our calculations predict that C 28 is stabilized by the addition of hard monovalent atoms (or groups) such as H and F on the four dangling bonds. When applying the Hard and Soft Acids and Bases (HSAB) principle to these isodesmic reactions, C 28 H 3 is predicted to be harder than CH 3 . The stability of C 28 H 3 and the localization of its unpaired electron suggest that this molecule may be used as a monomer in a radical polymerization. The stability analysis of the dimer, trimer, tetramer and pentamer (in linear or branched form) shows that these molecules are stable with strong C 28 C 28 bonds.

Ab initio study of the aromaticity of hydrogenated fullerenes

An analysis of the global and local aromaticity was made for a series of hydrogenated fullerenes of the type C60H2n (n=1–6) at the ab initio HF(Hartree-Fock)/3–21G level of theory, the isomers considered being obtained with an octahedral addition pattern. The relation between this addition pattern and the magnetic properties was established, showing low aromatic regions to be preferred for addition. These results show that local aromaticity, as shown in the nucleus-independent chemical shift, can be used to predict addition sites in these systems.

Ab initio study of the aromaticity of hydrogenated [70] and [76]-fullerenes

Abstract An analysis is made of the global and local aromaticity for a series of hydrofullerenes of the type C 70 H 2 and C 76 H 2 at ab initio HF/3-21G level of theory, as well as their deprotonated analogues. Additions at the hexagonal bonds of the pyracylenic units are considered. The relation between this addition pattern and the magnetic properties is established, showing low aromatic regions to be preferred for addition. These results show that local aromaticity, as shown by NICS, can be used to predict and interpret addition sites for these systems. As a probe of their reactivity, the acidity is predicted for the C 76 H 2 isomers and previously obtained results for C 70 H 2 are rationalised.

Acidity and delocalisation of C70-substituted hydrofullerenes: an ab initio quantum-chemical study

Abstract An ab initio HF/3-21G study is made of the acidity of a series of substituted [70]hydrofullerenes. All systems under consideration result from substituting one hydrogen atom in C70H2 by a functional group, for which a series of alkyl and fluorine- and chlorine-containing groups has been chosen. Acidities are obtained via a previously set up correlation with calculated gas-phase deprotonation energies. Electronic delocalisation in the conjugate base is seen to play an important role in the fairly high acidity of these systems. Also, the interplay of group hardness and electronegativity on the acidity sequence is shown.

A computational study on the role of noncovalent interactions in the stability of polymer/graphene nanocomposites

Understanding the interaction between graphene and polymers is of essential interest when designing novel nanocomposites with reinforced mechanical and electrical properties. In this computational study, the interaction of pristine graphene (PG) and graphene oxide (GO) with a series of functional groups, representative of the functionalised buildings blocks occurring in different polymers, and attached to aliphatic and aromatic chains, is analyzed using dispersion-corrected semi-empirical methods (PM6-D3H4X) and density functional theory calculations with empirical dispersion corrections. Functional groups include alkyl, hydroxyl, aldehyde, carboxyl, amino and nitro groups, and the binding energies of these groups with graphene derivatives (PG and GO) are determined. Nitro- and carbonyl groups display stronger interactions in both aliphatic and aromatic chains. The importance of dispersion-type and non-covalent interactions (NCI) in general, which typically, double the interaction energies, is revealed. The results are interpreted in an extensive NCI analysis in order to characterize the different types of NCI, providing a better understanding of the nature of the interaction (π–π stacking, CH–π bonding, H-bonding and lone pair–π interaction) at stake. In order to highlight the influence of polymer structure/conformation on top of that of their functional groups, the binding of three polymers, polyethylene (PE), polystyrene (PS) and polyvinylidene fluoride (PVDF), on pristine graphene is also investigated. Our calculations indicate that, although all polymers exhibit evident attractive interactions with the graphene sheet, the overall interaction is strongly influenced by the specific polymer structure. Thus, three main conformations of PVDF (the so-called α, β and γ, ε conformations) are analyzed and we find that, although the α-conformer with a trans-gauche-trans-gauche (TGTG’) conformation is the lowest energy conformer, the β-conformation of PVDF with the hydrogen atoms facing the graphene (“F-up”) has the strongest interaction with the graphene surface among the polymers under consideration. Taken together, our computational approach sheds light on the character and importance of non-covalent graphene-polymer functional group interactions combined with the structural/conformational properties of the polymer, which are at stake in the design of novel nanocomposites with reinforced mechanical and electrical properties.

Automated determination of chemical functionalisation addition routes based on magnetic susceptibility and nucleus independent chemical shifts

We present a modified version of our previously reported meta-code SACHA, for systematic analysis of chemical addition. The code automates the generation of structures, running of quantum chemical codes, and selection of preferential isomers based on chosen selection rules. While the selection rules for the previous version were based on the total system energy, predicting purely thermodynamic addition patterns, we examine here the possibility of using other system parameters, notably magnetic susceptibility as a descriptor of global aromaticity, and nucleus independent chemical shifts (NICS) as local aromaticity descriptor.