Chem143 Lab 4 Martin Case Corinne Sadlowski 1 H NMR Spectroscopy of Caffeine http en wikipedia org wiki NMR spectroscopy http www bmrb wisc edu metabolomics gen metab summary 5 php molName caffeine Write up for this lab to be completed on 10 13 and 10 15 Reagents and Equipment Recrystallized caffeine sample from Lab 3 5 10 mg Deuterochloroform CDCl3 0 1 TMS 0 5 0 6 mL NMR tube No pre lab is required This Lab follows directly from Lab 3 Purpose An organic synthesis almost always results in a product The O critical question for the scientist is is it the product that I want CH3 The organic chemists of the late 19th and early 20th centuries H3C N developed a wide variety of degradation reactions to reduce N complex molecules to more simple pieces Libraries of the TLC Rfs of these pieces were assembled Chemically degraded N O N unknowns were run on TLC together with the libraries of standard fragments and a chemist would infer the structure from the CH3 fragments In addition the combustion chemistry of the elements was well understood and accurate measurements of NOx CO2 caffeine and H2O volumes could be used to determine the N C and H proportions in an unknown molecule In the 1950s mass spectrometry became the key tool for structure determination By bombarding an unknown sample with electrons or ions in a high vacuum a mass spectrometer would report masses of both the unknown and its fragments By knowing what kinds of bonds were likely to break in the mass spectrometer a chemist could reconstruct an unknown molecule from the masses of the fragments In 1946 Felix Bloch and Edward Mills Purcell refined the technique of proton NMR for use on liquids and solids for which they were awarded the 1952 Nobel Prize for physics They noticed that magnetic nuclei like 1H could absorb radio frequency RF energy when placed in a magnetic field When this absorption occurs the nucleus is described as being in resonance Hydrogen atoms in different chemical environments within a molecule resonate at different frequencies for the same magnetic field strength These frequencies constitute the proton NMR spectrum of the molecule and the report structural information about the molecule NMR spectroscopy is currently the technique that gives the maximum structural information in the shortest time and is second only to TLC in routine organic lab analysis In this lab you will record the 1H NMR spectrum of your caffeine from Lab 3 using a 500 MHz 12 Tesla NMR spectrometer 1 Chem143 Lab 4 Martin Case Corinne Sadlowski Experimental Procedure 1 Sample preparation Weigh 5 10 mg of your recrystallized caffeine from Lab 3 into a small snap top vial Add 0 5 0 6 mL CDCl3 and swirl to dissolve Use a pipette to trsansfer all of the solution into a clean dry NMR tube Cap the tube 2 Acquiring the NMR spectrum Pairs of students will acquire NMR spectra under the supervision of the instructor While you re waiting and after you have run your spectrum use the table below to assign your spectrum NMRGuide 3 5 Copyright 1998 2003 BRUKER Biospin All rights reserved Write up Include a copy of your spectrum with your write up Do the chemical shifts of the protons in your sample agree a with your expectations and b with an authentic spectrum You can find the NMR spectrum of caffeine in water at the URL at the top of the page Check the box labeled 1D 1H spectrum and update the data display Note that the peak at 4 85 ppm is H2O which should not be present in your sample To what extent do you think the 1H NMR spectrum is a good way to find the molecular structure of caffeine What similar technique might provide more structural information 2 Chem143 Lab 4 Martin Case Corinne Sadlowski Waste disposal Deuterochloroform is regulated as hazardous waste in the same way as regular chloroform Dispose of your NMR sample as chemical waste in the appropriately labeled waste container Rinse the tube with acetone and discard the washings in the same waste container Safety The NMR magnet can interfere with medical pacemaker implants Be sure to let the instructor know if this is likely to be an issue for you 3