Vibrational Rotational Lab of HCL
VIBRATIONAL-ROTATIONAL SPECTRA OF HCl
Physical Chemistry Laboratory II, CHEM 3155.001
April 20, 2012
Introduction and Objective
The experimental objective of this lab was to collect an IR spectrum of gaseous HCl and from it the experimental rotational constant, B, and fundamental vibration frequency, v0, can be calculated(1). The concept of infrared spectroscopy deals with the infrared region of the electromagnetic spectrum. Molecules absorb at specific resonant frequencies that are characteristic of their structure. It will match the frequency of the bond or group that vibrates because molecules are constantly in motion, both intermolecular vibrational and molecular rotational motion. The different modes are only IR active if it is associated with a dipole. Also, infrared absorption or emission can only occur at allowed transition levels. The frequencies of electromagnetic radiation absorbed or emitted by the transition between two of these levels for a diatomic molecule fall within the range of the infrared wavelengths. This allows the transitions to be measured using the method of IR for diatomic molecules, such as HCl. Because only specific transition levels are allowed, it is concluded that the values are quantized and quantum mechanical results are related to molecular motion. To understand the information contained in the HCl spectrum, we must take into account the vibrational and rotational energy levels of the molecule. One way the energy contributions from the various sources within the molecule can be separated and treated as independent contributions, equation 1, is through the Born-Oppenheimer approximation.
E=EVIB+EROT= Ev+EJ (1)
Now each of the separate energies can be modeled independently. The rotational energy of the molecule can be modeled as a rigid rotor, which treats molecules as fixed masses on a spinning bar. The exact expression of rotational energy levels for rigid rotors can be obtained by solving the Schrödinger
Physical Chemistry Laboratory II, CHEM 3155.001
April 20, 2012
Introduction and Objective
The experimental objective of this lab was to collect an IR spectrum of gaseous HCl and from it the experimental rotational constant, B, and fundamental vibration frequency, v0, can be calculated(1). The concept of infrared spectroscopy deals with the infrared region of the electromagnetic spectrum. Molecules absorb at specific resonant frequencies that are characteristic of their structure. It will match the frequency of the bond or group that vibrates because molecules are constantly in motion, both intermolecular vibrational and molecular rotational motion. The different modes are only IR active if it is associated with a dipole. Also, infrared absorption or emission can only occur at allowed transition levels. The frequencies of electromagnetic radiation absorbed or emitted by the transition between two of these levels for a diatomic molecule fall within the range of the infrared wavelengths. This allows the transitions to be measured using the method of IR for diatomic molecules, such as HCl. Because only specific transition levels are allowed, it is concluded that the values are quantized and quantum mechanical results are related to molecular motion. To understand the information contained in the HCl spectrum, we must take into account the vibrational and rotational energy levels of the molecule. One way the energy contributions from the various sources within the molecule can be separated and treated as independent contributions, equation 1, is through the Born-Oppenheimer approximation.
E=EVIB+EROT= Ev+EJ (1)
Now each of the separate energies can be modeled independently. The rotational energy of the molecule can be modeled as a rigid rotor, which treats molecules as fixed masses on a spinning bar. The exact expression of rotational energy levels for rigid rotors can be obtained by solving the Schrödinger
References: (1) Today’s Laboratory Handout, University of Texas at Tyler, Fall 2011, pp. 1-4.