HW 11-IR spectroscopy key

# HW 11-IR spectroscopy key - (1)

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Unformatted text preview: (1)  The IR spectrum of CO shows a vibra:onal band centered at 2170 cm‐1.  (a)  What is the force constant for the CO bond?  Wavenumbers (in cm‐1) = 5.3 x 10‐12 s/cm (k/µ)1/2  With k in N/m and  µ in kg  µ =  1.1 x 10‐26 kg and k = 1843.6 N/m  (b) At what wavenumber would the corresponding peak for 14CO occur?  µ =  1.25 x 10‐26 kg, and with the force constant from above the vibra:on would occur at 2035 cm‐1  (2) How many vibra:onal modes does sulfur dioxide have? How many are IR ac:ve?   SO2 is bent so 3N‐6 = 3 possible modes.  To be IR ac:ve the mode must involve a change in dipole moment. The dipole changes during  all 3 vibra:ons so all are ac:ve.  (3) Consider the possible C‐H stretching modes of the ethene molecule, C2H4.  The symmetric stretch has all four H’s moving away from their respec:ve C atoms simultaneously while in  the assymetric stretch two H on one C move away while the other two are moving in.  Which of these vibra:ons is IR ac:ve?  The asymmetric stretch is ac:ve.  (4) What length of mirror drive in an FTIR spectrometer would be required to provide a resolu:on of 0.050  cm‐1?  The wavenumber resolu:on = 1/δ, where δ is called the retarda:on.The mirror drive required for a desired  resolu:on is ½ the retarda:on.   Thus, δ = 20 cm and the mirror drive = 10 cm.  ...
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## This note was uploaded on 11/16/2011 for the course CHEM 4011 at Colorado.

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