This is effectively the particle-in-the-box that you know from your physical chemistry class. In other words, it can be assumed that each electron moves in a constant potential, which rises sharply to infinity at the ends. These is effectively delocalized over the entire molecule suggests a simple model to approximate the behavior of the conjugated systems: a free electron that can move within the confines of the molecule (or its conjugated part). The visible absorption of the conjugated molecules comes from these delocalized -states, which are very different from all the other electronic states and orbitals in the molecule. This is illustrated in Figure 1 for a butadiene.įigure 1: Formation of delocalized π-electronic orbitals in a conjugated linear hydrocarbon. The electronic strucutre of conjugated molecules is unique in that it has delocalized π-orbitals, which are formed from sp2 hybridized p atomic orbitals of carbon above and below the plane of the molecules (conjugated molecules are planar – just in case you didn’t know that). This includes linear as well as cyclic (aromatic) compounds. Conjugated systems Conjugated systems are those with alternating single and double bonds. those that do absorb visible light, are often called dyes. As consequence, they do not absorb visible light but a higher energy (shorter wavelength) ultraviolet (UV) light and are not colored. In most other molecules the energy difference between the ground and excited states is much higher. Those compounds which are colored (i.e., absorb in the visible) generally have some weakly bound or delocalized electrons such as the odd electron in a free radical or the electrons in a conjugated organic molecule. In many substances, the lowest excited electronic state is more than 280 kJ above the ground state and no visible spectrum is observed. Spectroscopy in the visible region Absorption bands in the visible region of the spectrum (350 - 700 nm) correspond to transitions from the ground state of a molecule to an excited electronic state which is 160 to 280 kJ above the ground state. 3) Test the applicability and limitations of the simple free electron particle-in-a-box model for the description of the electronic states of conjugated systems. 2) Practice the application of the quantum mechanical formalism in the context of the particle-in-a-box model to predict experimental observables and compare with experiment. Objectives 1) Establish the connection between the fundamental principles of quantum mechanics - energy quantization – via the simple model of the particle-in-a-box and real world absorption spectroscopy. CHEM 4525: Physical Chemistry Laboratory IĮxperiment 2 – Spectrophotometry of Conjugated Dyes 1.
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