Lucas Paquin CHEM 166A April 30 th , 2018 Molecular Spectroscopy of Anthracene Introduction All molecules have several degrees of freedom, representing the various kinds of energy they possess . Some examples include translational, rotational, vibrational, and electronic energies. In this experiment , rotational, vibrational, and electronic energies are studied, all of which are quantized. The subject of this experiment is the polyatomic molecule known as anthracene (C 14 H 10 ). Anthracene has three benzene rings fused together in a “linear chain”. In its most stable electronic state, called the ground state, the molecule has three degrees of rotational freedom and more than 50 degrees of vibrational freedom, sometimes called the “normal” modes of vibration. The objective is to learn something about these different quantized degrees of freedom in anthracene, and how they influence the dynamical properties of the molecule in both its ground state and its lowest excited “singlet” state. This was done using UV-visible absorption, fluorescence, and IR spectroscopy to investigate the relationship between vibrations in anthracene's ground electronic state and in its first excited singlet state. First, a UV-visible spectrum of anthracene in solution was obtained to define the positions of its absorption bands. Next, one of the absorption bands was chosen and its fluorescence spectrum was measured. Then, its fluorescence excitation spectrum was measured, and the similarity to the absorption spectrum was analyzed. Lastly, the IR vibrational spectrum of the ground state was measured. Then, the vibrational energy level spacings detected in anthracene's emission spectrum will be compared with those observed in its IR and UV-visible absorption/excitation spectra and used to draw conclusions regarding anthracene's geometry and bonding in the two electronic states. This is done to discover how its dynamical behavior following the absorption of light is influenced by differences in these properties (1) . Figure 1 shows the potential energy curves for anthracene with the S0, S1, and S2 states. Each state is labeled with the first few vibrational states. The arrows labeled A and B represent what transitions were analyzed with the different spectroscopic analyses performed in this lab. The arrow labeled A represents the absorption of energy and an electron moving to an excited state, which was measure using UV-Vis spectroscopy. The arrow labeled B represents the radiative fluorescence, or the relaxation of an electron from an excited state to the ground state – which was measured using the fluorimeter. Figure 1: potential energy curves showing what the generic coordinate dependence of the potential energies of the lowest three electronic states of anthracene look like.
Methodology Previously prepared samples of anthracene in heptane at different concentrations (4, 40, and 400 µM) were put into cuvettes and analyzed using the Shimadzu UV-2450 UV-Vis spectrophotometer & UVProbe software. Once the spectrum was obtained, two of the
You've reached the end of your free preview.
Want to read all 18 pages?
- Summer '19
- Mole, Excitation Peaks