Even though I accidentally spilled most of my product while pouring it into the separatory funnel, the product left was identical to everyone else’s by comparison of the UV-vis spectrum. Discussion One possible source of error in this experiment occurred when I spilled the product. Although a small amount of it remained in the beaker, I decided to pipet the spilled product off the desk to save as much as I could in order to complete the experiment. At first I thought this might cause my product to be somewhat impure, but after looking at the UV-vis spectra, it was obvious this had no effect on the outcome. My spectrum looked perfectly fine.
UV-vis can be very helpful in finding out how pure your product is and what the components are by looking where the absorption bands are and the extent to which the components were absorbed. UV-vis works by electrons moving from the ground state to a higher excited state. The types of transitions possible are. The sigma to sigma star transitions take place at 200-700 nm and are usually not seen in UV-vis spectrum. N to sigma star transitions require less energy than sigma to sigma star transitions and occur at 150-250 nm. The last type of transitions occur between 200 and 700 nm and are the type which occur in the Cu(TTP) spectrum.
Peaks that are shifted to shorter wavelengths caused by transitions are named blue shifts. The peaks that are shown in our Cu(TTP) spectrum are caused by transitions and are classified as red shifts. “This is caused by attractive polarisation forces between the solvent and the absorber, which lower the energy levels of both the excited and unexcited states. This effect is greater for the excited state, and so the energy difference between the excited and unexcited states is slightly reduced – resulting in a small red shift. This effect also influences transitions but is overshadowed by the blue shift resulting from solvation of lone pairs” (UV-vis Absorption http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/uvvisab1.htm)
Questions and Answers
1. Why does Cu(TTP) elute before H2TTP? The copper would elute first. The reason for this is because of the acidity of the hydrogen atoms. Since the hydrogens are acidic, they want to “hold” onto the silica gel, which will cause them to elute after a longer time period. 2. The rates at which compounds elute from a silica gel column depend on any pretreatment of the silica gel. In which case would a compound elute faster: down a column made of silica gel that had been previously heated at 150 degrees Celsius under vacuum for 8 hours, or down a column made of silica gel that had been sitting open in the laboratory for a few days? Explain.
When you heat silica gel, water evaporates off the gel leaving open sites where polar molecules can bond. When you let silica gel out in the open, it will collect water molecules on it. Compounds would elute faster down a column that has been heated at 150 degrees for 8 hours because the gel would be dry. 3. A mixture of cis and trans isomers of the neutral complex Cr(CO)4[P(C6H5]2 is loaded onto a silica gel and eluted with CHCl3. Which isomer would elute first, and why? The trans isomer would elute first.
Skoog Douglas; Holler F. James; Nieman Timothy A., Principles of Instrumental Analysis, fifth edition, Thompson Learning: 1998. Quinn 6 UV-Vis Absorption Spectroscopy-Theory. http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/uvvisab1.htm.