CHEM3541 Experiment A - A-1 Study of UV-Visible Spectra of...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
A-1 Study of UV-Visible Spectra of Diphenyl Polyenes Using “Particle -in-a- Box” Model Introduction You may already be aware that molecules and photons can interact in various ways. For example, infrared photons may excite vibrational modes in molecules, while photons in the visible to near ultraviolet region have energies that can excite electrons out of their molecular orbitals into other bound states (formerly unoccupied molecular orbitals). So, each peak in a UV-Visible spectrum corresponds to the excitation of an electron from some occupied molecular orbital to some unoccupied molecular orbital. In this experiment, we shall treat the molecular orbitals in a conjugated carbon-carbon system as one dimensional particle-in-a-box wavefunctions. Transitions from one molecular orbital to another will be treated as transitions from one 1-D particle in a box state to another. The objective of this laboratory experiment is to interpret UV-Visible spectrum of three diphenyl polyenes. The UV- Visible transitions are compared to a particle-in-a-box model and also to transitions calculated using the software package HyperChem. trans,trans-1,4-diphenyl-1,3-butadiene 1,6-diphenyl-1,3,5-hexatriene 1,8-diphenyl-1,3,5,7-octatetraene
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
A-2 Theory The visible bands of the diphenyl compounds arise from electronic transitions involving the π electrons along the polyene chain. The wavelength of these bands depends on the spacing of the electronic energy levels. The π electrons of conjugated compounds represent a good example for performing particle-in-a- box calculations. In the series of diphenyl compounds, the length of the box is taken to be the distance between the phenyl rings and the phenyl rings represent the walls of the box. For each molecule, count the number of π electrons in the conjugated system, determine the 1 -D particle in a box quantum number of the highest occupied molecular orbital (HOMO) for each molecule, assuming that a pair of electron goes into each orbital. For instance, if the s ystem has two π electrons, then the-particle-in-a box quantum number for the HOMO is n=1. The corresponding 1- D particle-in-a-box quantum number of the lo