Computations on FullerenesCharacterization, Reactivity and Growth

Resumen

This Thesis titled ‘Computations on Fullerenes: Characterization, Reactivity and Growth’ has been performed in the Quantum Chemistry group of Universitat Rovira i Virgili at Tarragona by Laura Abella and supervised by Dr. Antonio Rodríguez and Prof. Josep M. Poblet. The Thesis is divided in seven chapters; a brief introduction about the world of fullerenes, the main goals of the projects, a short explanation of the computational methods used, the growth of endohedral metallofullerenes, the characterization of oxide clusterfullerenes family, the formation of chlorofullerenes and the concluding remarks. Fullerenes are young molecules that consist of closed carbon cages formed by only hexagons and twelve pentagons, like a soccer ball. To introduce to the reader them, the first chapter, named Getting into the World of Fullerenes, highlights their fundamental features and one of the biggest headaches for the scientific area of these molecules; their formation. Thus, the chapter contains a brief description related to fullerenes: their discovery, different types, the most common properties, some relevant applications, the most common hypothetical models of fullerene formation mechanisms and isomerization paths. Most part of our research has been carried out in collaboration with different experimental groups, therefore we aimed to understand and rationalize their experiments on fullerenes. We mainly focused on the characterization and formation of fullerenes previously detected in experiments. The formation of fullerenes has become one of the main interests in the scientific community since their discovery. Thus, the experimental detection of new families of endohedral metallofullerenes let us the change of study and analyse their growth. Until now, although many hypothetical models have been proposed, the fullerene formation mechanism is still a mystery. In case of the detection and isolation of new metallofullerenes arises us to perform a computational study for the rationalization and characterization of the new isomers. The Computational Methodology is essential in this kind of thesis. Due to this work is based on theoretical and computational chemistry, we dedicated one chapter explaining the methods that we used to develop the work of this Thesis. The fast evolution of the improvements in computer hardware, and the algorithms and mathematical techniques gave rise to a new way of doing science besides experimental and theoretical studies. Lots of methodologies, software, codes… are emerged in the last decades, and to know which one use or which is the best choice for our systems is sometimes difficult. Thus, a chapter with a brief description of the methodologies used in our calculations is dedicated in this thesis. We performed electronic structure calculations and molecular dynamics simulations. Regarding to the electronic structure calculations, we used the program Amsterdam Density Functional (ADF code). The electronic density was provided by the local density approximation using Becke’s gradient corrected exchange-correlation functional, and Vosko, Wilk, Nusair (VWN) parametrization for correlation, corrected with Perdew’s functional (BP86). Some calculations have also been performed including dispersion corrections by the method of Grimme. The MD simulations have been performed by means of the CPMD program. The description of the electronic structure is based on the expansion of the valence electronic wave functions into a plane wave basis set, which is limited by an energy cutoff of 40 Ry. The interaction between the valence electrons and the ionic cores is treated through the pseudopotential (PP) approximation (Martins-Troullier type). PBE was selected as density functional. In simulations with Ti, Sc, O or Cl atoms, we had to include the nonlinear core corrections (NLCC) in the Martins-Troullier PP. The Nosé-Hoover thermostat for the nuclear degrees of freedom was used to maintain the temperature as constant as possible. Three chapters of the Thesis include the most important results that we have obtained during this years of PhD. One of them is focused on how endohedral metallofullerenes are formed, concretely we focused on Ti@C2n and Sc3N@C2n families. Tremendous advances have been made in the field of nanoscience since the discovery of fullerenes, however, the formation of these molecules still remains such a huge enigma. Different models have been proposed to predict their formation from graphite or amorphous carbon, but none of them are strictly convincing. During the last few years, many experiments of fullerene growth have been greatly developed. Thus, it allowed us to study their formation accurately. This chapter rules out the bottom-up mechanism as a model of fullerene formation through different endohedral metallofullerene families. We have explored this mechanism by means of static DFT and Car-Parrinello molecular dynamics calculations for series of different endohedral fullerenes. A comprehensive exploration of the most favourable isomers, the potential energy surfaces associated with the successive C2 insertions and the topologies of the involved structures, helped us to develop this project. The insertion of a C2 unit to already formed EMF is always an exothermic/exergonic process, independent of the size, insertion site, or symmetry of the cage. In addition, the free energy barriers for each step are attainable at temperature of fullerene formation (2000 K). In this chapter, the most abundant isomers of Ti@C2n (2n=26-48) and Sc3N@C2n (2n=68-80) are formally linked by direct C2 insertions and in a few cases by additional Stone-Wales transformations. Moreover, the second endohedral fullerene to contain a heptagon, Sc2C2@Cs(hept)-C88, is also presented herein, and it is the first example of an endohedral carbide fullerene with a heptagon ring on the carbon cage. Calculations suggest that this endohedral fullerene could be a kinetically-trapped species derived from the recently reported Sc2C2@C2v-C86(9) via a direct C2 insertion. Therefore, the present theoretical studies provide strong support for the CNG mechanism proposed to explain the empirically observed growth of EMFs. However, these results do not exclude shrinkage of fullerenes as an important process when the fullerenes abundance are high and the carbon apour density is low, as Irle and Morokuma have demonstrated. Thus, the endpoint in growth should be controlled by carbon density of the atmosphere surrounding a cage. This is part of the work in collaboration with the experimental groups of Prof. Dunk at the UTEP University at Florida and Prof. Echegoyen at Texas University at el Paso. A chapter based on the oxide clusterfullerenes family, Sc2O@C2n is shown in the Thesis. Sc2O has demonstrated to be a good template for middle size fullerenes, between C70 and C82, permitting to characterize many structures and determining different physical properties. This chapter allows gaining insight into the field of fullerenes containing scandium oxide clusters as well as into experimental and theoretical techniques used to characterize them. The new metallic oxide clusterfullerenes, Sc2O@Td-C76(19151), Sc2O@C2v-C80(31922) and Sc2O@C3v-C82(39717) have been isolated and characterized by mass spectrometry, UV-vis-NIR absorption spectroscopy, cyclic voltammetry, 45Sc NMR spectroscopy, DFT calculations, and single-crystal X-ray diffraction. Our computational studies show that the Sc2O cluster transfers four electrons to the corresponding C2n cage, (Sc2O)4+@(C2n)4_. There are a lot of isomers for each size of cage, and to know which one is the one detected in experiments is a hard task. In addition, a new OCF, Sc3O@C80, has also detected, studied, characterized and described in this chapter. This is part of the work in collaboration with the group of Prof. Chen at the Soochow University at Suzhou. The last chapter of results contains the interesting world of one kind of exohedral fullerenes; the chlorofullerenes. Chlorination has emerged as a powerful tool in fullerene derivatives. Several C2n families (2n=50,60,66,68,etc.) have been found to show cages exohedrally chlorinated. The reaction sites where chlorines are attached, the chlorination pathways and how chlorofullerenes are formed are issues that have been considered in recent years. Herein, we can find the following chlorofullerenes: C66Cl10, C74Cl10 and C78Cl6(C56Cl6). According to our results, low-energy neutral cages are the ones that are functionalized when a chlorine source is introduced in the arc. Chlorination would take place at a temperature significantly lower than 2000 K, once the neutral isomers are formed. Chlorination patterns follow the trend of maximizing strain release on the cage surface, with chlorines at the highly pyramidalized C atoms of the adjacent pentagons. In addition, the relative position of the adjacent pentagons may also play an important role. Therefore, we have demonstrated that neutral C2n cages reacts first with chlorine atoms by free radical addition considering the HOMO and the spin density distributions of the pristine cage and intermediates. In case of C78Cl6(C56Cl6), the perchlorinated cyclopentadiene reacts when the chlorofullerene C78Cl6 is formed by 1,4-cycloaddition. Part of this work was done during my stay in Xiamen University, thus it is in collaboration with Prof. Su-Yuan Xie. The last chapter of the Thesis summarizes the most important achievements and conclusions remarks of all this work. Although computational studies are not as easy as it seems, we have succeeded in most of the goals we have faced. In addition, most of the projects resulted in suitable and in agreement with experiments. A brief chapter-by-chapter summary of the concluding remarks for each project is presented.