Measurement of infrared absorption spectra and experiments on structural analysis
Measurement of infrared absorption spectra and experiments on structural analysis
This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.
Operation method
Measurement of infrared absorption spectra and experiments on structural analysis
Principle
Infrared absorption spectroscopy is used to analyze substances by studying the relationship between the structure of the substance and the infrared absorption spectrum, which can be characterized by the position of the absorption peak bands and the intensity of the peaks. Determination of the structure of an unknown substance is an important use of qualitative analysis by infrared spectroscopy. According to the position, intensity and shape of the absorption peaks of the infrared spectra mapped by the experiment, the relationship between the vibration frequency of the group and the molecular structure is used to determine the attribution of the absorption bands, to confirm the group or bond contained in the molecule and to deduce the structure of the molecule, and the steps of the identification are as follows: 1. make a preliminary knowledge of the sample, such as the purity of the sample, the appearance of the sample, the origin of the sample, and the results of the analysis of the elements, as well as physical properties (molecular weight, boiling point, melting point). 2. determine that the unknown substance is unsaturated and has no saturation. 2. Determine the degree of unsaturation of the unknown substance, in order to speculate on the possible structure of the compound; 3. Spectral analysis (1) firstly, search for the characteristic telescopic vibration of the functional group in the functional group region (4000-1300cm-1); (2) further confirm the group according to the absorption of the "fingerprint region" (1300-400cm-1), and then confirm the absorption of the group by the "fingerprint region" (1300-400cm-1). (1300-400cm-1), the presence of the group and its combination with other groups can be further confirmed.
Materials and Instruments
p-Nitrobenzoic acid Acetophenone Move 1. Preparation of infrared spectrometers Caveat 1. Whether the specimen preparation is standardized is directly related to the accuracy of the infrared spectrum, so for liquid samples, attention should be paid to keep the salt tablets dry and transparent, anhydrous ethanol and talcum powder should be used before and after each determination to polish and dry under the infrared lamp. For solid samples after grinding should always pay attention to prevent the absorption of water, otherwise the press out of the tablets easily stained on the mold. 2. The instrument should be protected against shock, moisture and corrosion. Common Problems 1. Graphical analysis based on the spectrum of acetophenone For more product details, please visit Aladdin Scientific website.
KBr Powder (Spectrally Pure) Ethanol Anhydrous (AR) Talc Acetone Degreased Cotton
Nicolet 510P FT-IR Spectrometer FW-4 Model Tablet Press Vacuum Pump Onyx mortar Infrared lamp Tweezers Detachable liquid cell Salt tablets
(1) Turn on the power switch of the infrared spectrometer and wait for the instrument to stabilize for more than 30 minutes before measurement;
(2) Turn on the computer, select win98 system, open the OMNIC E.S.P software; set the experimental parameters in ExperimentSet-up under the Collect menu;
(3) Set experimental parameters: resolution 4 cm-1, scanning times 32, scanning range 4000-400 cm-1; vertical coordinate is Transmittance
2. Infrared spectrogram testing
(1) Preparation and testing of liquid samples
The salt flake of the detachable liquid cuvette was taken out from the dryer and its surface was polished with a little talc mixed with a few drops of anhydrous ethanol under an infrared lamp. The salt flakes were then cleaned with a few drops of anhydrous ethanol and dried under an infrared lamp. Place the salt flake in the center of the hole of the detachable liquid cell, press the other salt flake flat on it, tighten the screws, assemble the liquid cell, place it on the sample holder of the photometer, and carry out the background sweep. Then, disassemble the liquid cell, put a drop of liquid (acetophenone) sample on the salt sheet, press the other salt sheet flat on it (no air bubbles) to assemble the liquid cell. A sample scan was performed as before to obtain an infrared spectrogram of the sample.
At the end of the scan, the liquid absorption cell is disassembled, the sample is promptly washed away with acetone, and the salt tablets are kept in a desiccator.
(2) Preparation and testing of solid samples
Under the infrared light, 1-2mg of the powdered sample, which was ground to about 2μm, was mixed with 100-200mg of spectroscopically pure KBr powder and then ground, and then put into a press mold, and then pressurized with vacuum and pressure of about 10MPa on the tablet press to make a transparent sheet of about 1mm in thickness and about 10mm in diameter. After collecting the background, this piece is mounted on the sample rack, scanning to see if the light transmission rate exceeds 40%, if it reaches, the test results are normal, if it does not reach 40%, according to the situation of increasing or decreasing the amount of samples, and then re-pressing the film.
After scanning, remove the sample holder, take out the sheet, clean the mold, sample holder, etc. as required, and keep it properly.
在 3000 cm-1附近有四个弱吸收峰，这是苯环及 CH3的 C—H 伸缩振动；在 1600～1500 cm-1处有 2～3 个峰，是苯环的骨架振动，所以可判定该化合物有苯环存在；在指纹区 760、692cm-1处有 2 个峰，说明是单取代苯环；在 1687 There are two peaks at 760 and 692 cm-1 in the fingerprint area, indicating that it is a monosubstituted benzene ring; the strong absorption peak at 1687 cm-1 is the C=0 stretching vibration; the strong absorption peak at 1265 cm-1 is the absorption of aromatic ketone; the absorption peaks at 1363 cm-1 and 1430 cm-1 are the C-H symmetric and antisymmetric deformation vibration of CH3, respectively. Therefore, based on the analysis of the above spectra, the structure of this substance is completely consistent with that of acetophenone compared with the standard infrared spectra of acetophenone. Therefore, the structure of this substance was analyzed according to the above spectrum and compared with the standard infrared spectrum of acetophenone.
2. Analyze according to the spectrum of p-nitrobenzoic acid.
The absorption peak at 3020cm-1 is caused by the =C-H expansion vibration on the benzene ring. The absorption peaks at 1605 cm-1 and 1511 cm-1 are caused by the C=C expansion vibration of the benzene ring skeleton. The absorption peak at 817 cm-1 indicates the occurrence of para-substitution on the benzene ring.
The absorption peaks at around 3000 cm-1 and 1400 cm-1 are the absorption of acid, and the absorption peaks at 1530 cm-1 and 1300 cm-1 are the absorption peaks of the group -NO2. So it is presumed to be p-nitrobenzoic acid, and then compared with the standard infrared spectrum of p-nitrobenzoic acid.