Vibrational rotational spectra of hcl and dcl lab report

Vibrational Rotational Spectra of Gases

Introduction

This laboratory experiment will give the students a practical experience of the concepts of the infrared spectroscopy of molecules and the different types of energies studied in CHM 365 and CHM 304. In this lab, the students will be able to characterize heterodiatomic gaseous molecules using FTIR spectroscopy. This is very crucial technique for the determination of molecular structure. This technique is very important in several applications, such as the analysis of greenhouse gases, e.g., CO2, H2O, O3, and Chlorofluorocarbons, chemical warfare agents, and changes to the concentration of atmospheric gases. This experiment will enable the students to calculate the bond lengths and determine the isotopic effects for HCl and DCl.

The Harmonic Oscillator model can be applied to study the vibrational and rotational transitions of heteronuclear diatomic molecules. In this case, the potential energy can be calculated as:

where k is the force constant of the bond. The Schrödinger equation for a particle undergoing harmonic motion can be modified to give an equation to calculate the allowed vibrational energy levels:

where h is Planck's constant, n is the vibrational frequency, and the vibrational quantum number v = 0, 1, 2, ....

In the same time, diatomic molecules undergo rotation through space. The rigid rotor model may be used to approximate the rotational contribution to the IR spectrum of a diatomic molecule. For a rigid rotor, the allowed energy levels may be calculated as:

where J is the rotational quantum number (with integer values

0, 1, 2, ...) and h is Planck's constant.

Adding these vibrational and rotational energy terms gives a first approximation of the value of its energy levels.

Procedure

Part A. IR Spectra of Gases

In this experiment, several vibrational-rotational infrared bands of CO2 will be recorded at medium-to-high resolution (ca. l cm-1). These spectra will be analyzed to extract information about hot bands.

1) Pump out the system for 10-15 minutes (have IR cell attached with its valve open).

2) Close off the line to the cell.

3) Take your background spectrum. Use the highest resolution (0.4 cm-1 if possible), set the signal gain to 1 and the number of scans to 16 or greater.

4) Return cell to apparatus. Add approximately 1atm of CO2. Record IR absorption spectrum. Two spectral ranges are of interest: 550 − 750 cm-1 and 2250 − 2450 cm-1. At this high pressure you should observe that absorption extends quite far from the maxima due to rotational unresolved P and R branches.

5) Repeat the same with 150 Torr of CO2. In the region of bending mode absorption, besides seeing unresolved rotational P and R branches, you should see sharp lines at 618, 648, 720 cm-1 and maybe more. These lines are due to hot bands.

6) Repeat the same with a piece (less than 1 gram) of dry ice. Did the hot bands disappear?

7) Repeat the relevant parts of the procedure to obtain a spectrum for Argon, and Nitrogen Gas at atmospheric pressure.

Data Analysis

Make sure to cover these questions in your discussion section.

How many vibrations does this molecule have and which ones you see in the IR spectra?

Explain (without calculations) why some vibrations appear to have all three branches, P,Q, and R, while others have only P and R.

What are the hot bands and overtones? Did you observe either of them and how did you identified them as such?

Instrumentation

1- Perkin-Elmer FTIR spectrometer

2- Gas flow cell with KBr windows from Perkin Elmer for IR spectroscopy

3- Universal 10cm Gas Cell Mount

Part B. Isotope Effect (Optional)

HCl and DCl

Students will learn about the isotope effect with reference to HCl and DCl. The most abundant form of HCl is 1H35Cl. Another isotope of chlorine, 37Cl, has a high natural abundance, however, and the lines for 1H37Cl are obvious in a high-resolution spectrum of HCl, right next to the 1H35Cl lines. In fact, the isotopic abundance of 35Cl and 37Cl may be calculated from the relative absorbance values in the IR spectrum (since absorbance is proportional to concentration). Though the change of an isotope (e.g., 35Cl to 37Cl) does not affect the equilibrium bond length or the force constant. For the molecule, varying an isotope does change the overall mass of the molecule. Since the reduced mass affects the vibrational and rotational behavior of a molecule, the energy of its transitions is changed.

In this experiment, rotational fine structure of the infrared vibrational spectrum of HCl and DCl will be recorded at medium-to-high resolution (ca. l cm-1). These spectra will be analyzed to extract rotational constants and obtain the moment of inertia of the molecule and thus the internuclear separation. The pure vibrational frequencies will also be measured.

1 HCl and DCl can be synthesized by the addition of D2O to sulfuric acid (1:1) and