IR Lab Report (6).docx

IR Lab Report (6).docx - Jennifer WurmInfrared Spectroscopy...

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Jennifer Wurm—Infrared Spectroscopy: Experiment 6 Introduction Infrared spectroscopy is typical used in organic experiments to identify certain functional groups of a substances with the intent of identifying the molecule. However, in physical chemistry, infrared spectroscopy can be used to analyze certain structural elements such as the allowed vibrational and rotational transitions and their dipole moments. These can be used to determine rotational constants and ultimately bond length. For a molecule to be infrared-active, a non-zero dipole moment must be present. Acetylene is a linear and symmetric molecule without a permanent dipole, however, some of its vibrational modes induce a temporary dipole, causing the molecule to be infrared-active for some vibrations. The symmetric vibrations will not be seen on the infrared-spectrum. To determine which vibrational modes either contain a dipole moment or are symmetric and inactive, symmetry calculations can be used. The vibrations which are active are classified as parallel and perpendicular, and then assigned P, Q, and R branches on the spectrum according to selection rules. In this experiment, acetylene was analyzed by Fourier Transform Infrared Spectroscopy to produce a spectrum from which rotational constants of the ground state and the first vibrational excited state could be determined. Using these, the bond length of C-H was found. Materials and Methods An IR gas cell filled was evacuated using a vacuum system for about 5 minutes. A blank reading of the empty cell was taken by a Fourier Transform Infrared Spectrometer to obtain a background spectrum ranging from 4000-400 cm -1 . The cell was then filled with acetylene gas in the vacuum system from a commercial gas cylinder in increments. The pressure was monitored, and the cell continued to be filled until the pressure reached about 30 Torr. The spectrum of acetylene sample was then obtained by running the FTIR spectrometer which the same parameters.
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Jennifer Wurm—Infrared Spectroscopy: Experiment 6 Results Figure 1 . The IR spectrum of the background for an evacuated IR gas cell was measured. Peaks due to H2O and CO2 are labeled accordingly.1 Figure 2. A sample of C2H2 in an IR glass cell was measured with a FTIR spectrometer to obtain the IR spectrum. The IR- active vibrations are labeled with their corresponding P, Q, and R branches. Impurities due to the background are identified.
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Jennifer Wurm—Infrared Spectroscopy: Experiment 6 Figure 3 . A zoomed in view of the v3 vibration with P and R branches identified. Since this is a
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