RT info:eu-repo/semantics/masterThesis
T1 Estructura electrónica y vibracional de moléculas con interés astrofísico.
A1 Hermosa Muñoz, Javier
A2 Máster Universitario en Astrofísica
K1 Astrofísica
AB In this work we study the properties of certain molecules related to astrophysics. Since the firstdiatomic molecules were discovered, a lot of molecules have been discovered throughout the spaceby using infrared wavelength observations. These observations showed the great variety of complexmolecules in the universe besides the little diatomic molecules. The complex molecules areimportant components inside the galaxy interstellar medium. One example of these complexmolecules are the PAHs (Polycyclic Aromatic Hydrocarbons), such as naphthalene, anthracene orpirene, which are organic compounds containing only hydrogen and carbon. These molecules arecomposed of multiple aromatic rings. Signatures of these PAHs are found in the spectral bands ofthe cosmic infrared emission. A variation in the central wavelength of these bands in different spaceregions have been related with diverse components of the carriers. For instance, if we areconsidering a HII region, the central wavelength drops to a lower wavelength; and if we consider aplanetary nebula, the central wavelength rises to a higher value. Then, the observations of suchdifferences allow us to know the components of the spectral regions.This work studies the electronic structure of three different polycyclic aromatic hydrocarbons:the coronene (C24H12), the corannulene (C20H10) and the C20. The latter has not been found yet inthe space but we include it because we also want to study how the curvature influence in themolecular properties of the polycyclic aromatic hydrocarbons.The three molecules have different geometric structures: the coronene is a planar molecule withsix rings of benzene, the corannulene has a slight curvature with a central cyclopentane surroundedby five rings of benzene. Each molecule has its own particular symmetry, so the simmetry groups ofthese three molecules are not the same. The symmetry group of the coronene is the symmetry groupD6h, the simmetry group C5v is the one corresponding to the corannulene, and finally, the C20 has asimmetry group Ih.In order to calculate some electronic and vibrational properties of these molecules, we use apackage of programs called MOLPRO, which allows us to know the total electronic energy of themolecules, as well as the energy of a lot of monoelectronic levels (occupied or unoccupied).MOLPRO is a set of ab initio program for standard computational chemistry applications. We haveused it using different methods, like Hartree-Fock (HF), Møller-Plesset methods (MP2) or coupledcluster theory (CCS) and density functional theory programs (DFT).We have started with the coronene, and we have used these methods to get the energy andcomparing the energies calculated between them and with some papers we have read. The energiesof the corannulene and of the C20 have been calculated in the same way. After that, we have assigned a symmetry element to several energy levels of the three moleculesaccording to the simmetry near the gap dividing the occupied (HOMO) and the unoccupied(LUMO) levels. Nevertheless, the MOLPRO program is not able to identify properly the simmetrygroups of the three molecules because it is only able to work with abelian point groups, so we hadto assign the energy levels by using some equivalence tables. This assignment has been partiallyconfirmed in the bibliography, including the degeneration of the energy levels. However, the C20levels have been more difficult to assign due to the differences of energy, which were higher thanthe differences we saw in the coronene and the corannulene. That is why the assignment has notbeen made.Afterwards, we have paid special attention to the highest ocuppied molecular orbital (HOMO)and to the lowest unocuppied molecular orbital (LUMO). That is because, as we have stated before,we want to know how the curvature influence in the energy levels and the gap between ocuppiedand unocuppied levels in these three molecules. Again, we can compare the calculations made withthe MOLPRO program with others made before in order to know if our calculations were right.In addition, the normal modes of vibration of the molecules in the ground electronic state havebeen calculated. The goal of these calculations is, using the harmonic approximation, to know howthe atoms in the molecule are moving. We have assigned these normal modes with the aid of thegroup theory in order to discover what normal modes are active to infrared (IR) or Raman. Thefrequencies of these IR active modes are closely related with the peaks of the infrared interstellarbands.Our results show that the HOMO – LUMO gap seems to increase as the curvature increases. It isremarkable in the case of C20, whose curvature is bigger because it is a spheroidal-like molecule,and the gap is quite greater. In fact, we wanted to study the C60, which has been found in the spaceand therefore their characteristics and properties may be more interesting, but the memory of thecomputer we have used was not big enough in order to get these calculations.
YR 2018
FD 2018
LK http://riull.ull.es/xmlui/handle/915/11586
UL http://riull.ull.es/xmlui/handle/915/11586
LA es
DS Repositorio institucional de la Universidad de La Laguna
RD 09-nov-2024