organic chemistry lab
NMR/IR
OBJECTIVES: To conduct NMR and IR on a purified compound. To assign signals and understand the general principles.
Background
NMR (Nuclear Magnetic Resonance).
1H NMR
It is a phenomenon that occurs when nuclei of atoms possessing a spin are placed in a static magnetic field, and subsequently exposed to an oscillating field. For some nuclei (1H, 13C, 15N) the spin is 1/2, for others is 0 (12C, 16O), 1 (2H, 14N). A non-zero spin is a requirement for the NMR phenomenon. The purpose of the radio waves is to supply the energy to change the orientation of the nuclear spins (from ground state to excited state). The equilibrium is then repristinated when the absorbed energy is dissipated as heat. The static field B0 determine the separation, in terms of energy, between the two states (nuclear spin aligned with the field or opposed to the field). The resonance frequency are referenced to a standard signal the tetramethylsilane signal (TMS). There ate three important information that NMR provides: 1) chemical shift, 2) integrals and 3) coupling constant (J)
1) The chemical shift (δ) is the resonance frequency of a nucleus relative to the standard (TMS). Different hydrogens (also called protons) give different NMR signals
2) Integrals refer to the relative number of hydrogens of each signal. It is calculate measuring the area under peak.
3) The coupling constant (J) is the distance in Hz between the line in a multiplet. The magnetic field experienced by a hydrogen is affected by the presence of hydrogens nearby. So for a specif Ha we will observe n+1 lines where n is the number of equivalent adjacent hydrogens (thus is the n+1 rule). The lines of the multiplet are separated by a distance that can be calculate in Hz and is called coupling constant (J). In the chart below you can observe the chemical shift for the different functional groups.
13C NMR
Like 1H, 13C has a spin of ½ therefore is active in NMR. It is very valuable for organic chemist; however, 13C is present only in 1.1% natural abundance, therefore 13C are usually conducted for longer period of time (more scans) and they require more concentrated samples. 13C can couple with 1H giving multiplets and complicating the spectra. Most of the time the 13C are collected using a decoupling, therefore the 13C appear as sharp singlets. In order to determine the number of hydrogens on a specific carbon, there are more complicated NMR experiments such as DEPT. DEPT experiments allow the selective appearance of methine, methylene or methyl group. DEPT 90 will show only CH group, DEPT 135 will show CH and CH3 as positive signals while CH2 will appear as a negative signal. Below a chart with different chemical shifts for different groups.
IR (infrared spectroscopy)
is a measure of the electromagnetic radiation absorbed by a molecule. An IR spectrum is obtained by passing IR radiation through the sample. If light is absorbed the transmission is less than 100%, so we will obtain a plot of absorption (transmittance) vs wavelength. Certain bonds have specific regions of absorption. IR is very useful to obtain information about functional groups. IR by itself is usually non enough to determine the complete structure of a molecule.
2
EXPERIMENTAL
Instructions on how to operate the instruments.
IR Instructions
1. Open "Varian Resolutions" 2. Run a Background (one background is good for about five samples) by clicking on the first icon on the left of the toolbar. 3. Place the sample on the ATR crystal: for liquids (left-hand IR): put on one drop for solids (right-hand IR): put a few crystals in the center of the well, use the wheel to lower the tool to crush the solid (it will release when you are done) 4. Collect a spectrum by clicking on the second icon from the left (collect sample) on the toolbar.
5. On the spreadsheet display, select the spectrum at the top of the list (this is always the one run most recently). 6. Select the peaks by clicking on the icon on the second row of the toolbar. 7. Print the spectrum.
NMR Instructions
Place about 1 drop (liquid) for 1H-NMR (8 drops for 13C- NMR), or 10 mg (solid) for 1H-NMR (50mg for 13C-NMR), of your product into an NMR tube (these are usually in the front of the lab, please return used tubes to the "used" beaker - DO NOT clean them). Add 0.50 ml of an appropriate NMR solvent (usually CDCl3)1 using the pipet provided for that solvent. NMR ‘sees’ all contamination. LABEL NMR tube with the special NMR tube labels (not the stickers used for vial labels, and write your initials on the label. Caution: Do not enter the NMR room if you have a pacemaker! Also, take precautions to keep magnetic cards such as credit cards and I.D.s away from the magnet. 2. At the NMR room (back of DSB 216) in teams of two: Carefully place the sample straight into a spinner (glass does not like to bend!) and make sure that it is at the right height by using the depth gauge. 3. Place the sample into the autosampler. Use the step ladder if necessary, and NEVER lean on the magnet!! Note the slot number (1-8). Warning: Do not insert samples in the slot over the bore (in the back). There may be a sample in that slot, whether visible or not.
4. Approach the computer and log into CHEM 3020. The password is ochemII. 5. Double click on the Delta icon. 6. Choose the silver cylinder icon that looks like the magnet to access the spectrometer. 7. Enter your filename (initialssampleX), any comments, slot #, the solvent (CDCl3). Note that the mouse pointer arrow must be in the box you wish to type into. 8. Choose the experiment (buttons on bottom of page). 9. The computer will now take care of everything. Once the sample is finished, it will go on to the next sample in the queue. The next group can enter new information about their sample without affecting a sample that is running. Your sample may be finished in as early as 10 minutes. Spectra may be picked up during the lab period or later in the week.
From msu.edu