Kearns_Experiment 5

Kearns_Experiment 5 - Experiment 5 Vibronic Spectrum of...

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Experiment 5, Vibronic Spectrum of Iodine By UV-Vis Absorption Author: Jeremy Kearns Group 6 Section 1, M 1:25 PM – 4:25 PM Partner: Tyler Smith 2/21/2011 Abstract: This experiment uses a UV-Vis in order to observe the vibronic spectrum of iodine. By comparison with a known absorption, the peaks of an absorption spectrum of iodine was labeled. The change in vibrational energy was then graphed against the quantum level. This allowed one to solve for the harmonic frequency, ω e , and first anharmonicity parameter, ω e x e . These values were found to be 129.96 cm -1 and 0.9362 cm -1 respectively. The dissociation energy, D e , and Morse parameter, β, were found to be 4510 cm -1 and 1.878 Å -1 . These values were then used to graph the Morse potential against interatomic radius. It was shown that for a real diatomic molecule, energy levels are not equally spaced and the Morse potential does not follow a perfect parabolic trend and is therefore not a harmonic oscillator. I. Introduction The absorption of different molecules can be shown using UV-Vis absorption. Although it is known that the diatomic structure of Iodine is two Iodine atomics with a single bond between them, the simple dumbbell model for the occupation of molecular energy levels presented in general chemistry is not completely accurate. Diatomic iodine does not strictly have
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Experiment 5, Vibronic Spectrum of Iodine By UV-Vis Absorption the shape of the rigid dumbbell. Molecules vibrate and, through absorbing a photon, can change the arrangement of electrons, thus changing the shape of the molecule. UV-Vis can be used to show the vibrational levels in the ground and excited states of iodine. Analysis of the absorption spectra from the UV-Vis can lead to a detailed description of the inter-atomic potential between two iodine atoms in diatomic iodine. According to the Born-Oppenheimer approximation, each electronic state in a diatomic molecule has a characteristic nuclear potential function, V(R), which relates the interatomic potential energy as a function of interatomic distance, R. For low vibrational energies, the interatomic potential can be simplified to that of a model of a harmonic oscillator. In this model, the potential energy curve would be a parabola and the energy levels would be equally spaced. Real atoms do not behave like the simple model of a harmonic oscillator. As the interatomic distance approaches infinity, the chemical bond stretches and gets weaker, resulting in dissociation. When the interatomic distance shortens, interatomic repulsions raise the potential more quickly than that in a harmonic oscillator model. This shows that real molecules are anharmonic. The spacing between energy levels decreases as the potential increase. As the vibrational energy of a molecule approaches the dissociation energy, the spacing between energy levels approaches zero. The vibrational energy levels of an anharmonic oscillator can be described by a power
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Kearns_Experiment 5 - Experiment 5 Vibronic Spectrum of...

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