Henry SuMay 8, 2003
Lab Experiment #6CHE 331
Dr. Rahni
Analysis of Acetic Acid Solutions by NMR
Purpose
The purpose of this laboratory experiment is to generate NMR data for different concentrations of an acetic acid solution.
Theory
Nuclear magnetic resonance(NMR) spectroscopy is one of the most widely used spectroscopic tools in organic chemistry. It is based on the fact that certain nuclei have magnetic moments that are normally oriented in a random fashion. When a magnetic field is applied to these nuclei, the magnetic moments of the nuclei align either with, as in the state(lower energy) or against, as in the state(higher energy), the magnetic field.
If the nuclei are irradiated with the electromagnetic radiation, some of the nuclei will be excited into the state. The absorption that corresponds to this excitation will occur at different frequencies, depending on the atom’s environment. Thus, the nuclear magnetic moments are affected by neighboring atoms that possess magnetic moments.
The output that results from such techniques is a chart of frequency vs. absorption of energy. Frequency decreases towards the right and the magnetic field increases toward the right. A chemical shift() of the spectrometer frequency is plotted on the x-axis. This process of NMR is commonly used to study 1H nuclei(protons) or 13C nuclei.
A few terms to be familiar with in discussing NMR are the TMS, which is the reference peak, found at the far right of the chart. Deshielding effects, or electron withdrawing effects, that forces the nuclei to resonate at a lower field than normal, and thus moves the peak to the left. Chlorine atoms, for example, can cause this phenomenon to occur. Shielding effects, or electron-donating effects, cause the nuclei to resonate at a higher field, and thus moves the peak to the right. Silicon atoms, for example, can cause this phenomenon to occur. Coupling occurs if two magnetically different protons are found within three bonds of each other. Taking these factors into consideration, we can examine the charts that are produced and find out more information about compounds that are being examined.
Proton NMR is usually used to determine the relative number of protons and their relative chemical environments. It can also be used to show how many adjacent protons there are by splitting patterns, or the number of adjacent hydrogens. A peak will be split into (n+1) peaks, where n represents the number of adjacent hydrogens. Finally, proton NMR can be used to show certain functional groups.
Carbon NMR is used to determine the number of different carbon atoms with their relative chemical environments. In spin-coupled NMR, the number of hydrogens per carbon can also be determined. NMR can subsequently determine the number of nonequivalent nuclei, which is determined by the number of peaks; the magnetic environment of a nucleus, which is determined by the chemical shift; the relative number of nuclei, which is determined by integrating and comparing peak areas; and the number of neighboring nuclei, which is, again, determined by the splitting pattern.
Apparatus
The following materials will be needed in order to perform this experiment:
Material / QuantityNMR Spectrometer, 60 MHz / 1
NMR tubes 5mm od / 10
Pipets 0.5 ml and 1.0 ml / 10
Procedure
1-Make appropriate dilutions of acetic acid solutions.
10 ppm, 20 ppm, 40 ppm, 50 ppm, 60 ppm, 80 ppm
2-Run NMR spectroscopy on each solution
-See NMR manual
Data and Calculations
See accompanying NMR data.
Conclusion:
Upon completion of this laboratory experiment, NMR data was generated for acetic acid solutions of concentrations 10 ppm, 20 ppm, 40 ppm, 50 ppm, 60 ppm, 80 ppm.