|CM2101 (Principles of Spectroscopy) |
|Experiment 1: Rovibrational Spectrum of Hydrogen Chloride |
To measure the infra-red (IR) spectrum of gaseous HCl using Fourier Transform Infra-Red (FTIR) spectrometer, and analyse the rotational fine structures.
The experiment aims to identify various parameters of gaseous HCl related to quantum mechanics by using high-resolution IR spectrum and graphical method of analysis. The values that are being investigated specifically are νe (equilibrium ...view middle of the document...
In addition, with regards to the rotational energy levels of a diatomic molecule, the molecule is first approximated to a rigid rotor where its bond length does not change. Another important thing that is assumed is that there is no interaction between the vibrational and rotational motions of a molecule. Only after the properties of the molecule have been studied and understood, modifications to the harmonic oscillator and rigid rotor approximations are done, taking care the anharmonic properties, as well as the actual vibration-rotation interactions and centrifugal distortions of the bond to complete the picture.
The vibrational and rotational fine structures can be studied by obtaining an infra-red spectrum by using Fourier Transform Infra-Red (FTIR) spectrometer. IR radiation is passed though a sample, resulting in the absorption and transmittance of the radiation at a certain range of wavelengths. The signals are encoded in an interferogram which is then decoded via a mathematical tool of analysis known as the Fourier transformation. The spectrum obtained thus acts as a molecular fingerprint of the sample under consideration. The advantages of using FTIR over dispersive techniques include higher speed and sensitivity, as well as mechanical simplicity in which the moving mirror is the only continuously moving part in the instrument.
|Chemicals |Concentrated H2SO4 solution |Solid CaCl2 |
| |Solid NaCl2 |Nitrogen gas |
| | | |
Preparation of Background Spectra
To the empty 10-cm IR cell, nitrogen gas was introduced in order to drive off any other gases that might interfere with the experiment. Two background spectra were subsequently obtained: one over the range of 4000 – 600cm-1 (resolution of 4 cm-1) and another over the range of 3200 – 2500 cm-1 (resolution of 0.5 cm-1). The spectra were saved in the system to be used later.
Determination of HCl Spectra
Figure 1: Experimental setup to obtain gaseous HCl
As shown in Figure 1 above, solid NaCl was placed in a conical flask that was connected to a burette containing concentrated H2SO4 solution and a U-tube containing solid CaCl2. The U-tube was subsequently connected into the IR cell. Following that, a few drops of H2SO4 were allowed to come to contact with NaCl, resulting in the production of HCl gas that would be passed into the IR cell. The sample was then scanned over the range of 4000 – 600cm-1 (resolution of 4 cm-1) and another over the range of 3200 – 2500 cm-1 (resolution of 0.5 cm-1) after proper subtraction of the background readings had been done.
When the partial pressure of the HCl was too high as seen by the...