Chapter 16

Question 1

16-1 The IR spectrum of CO shows a vibrational absorption band centered at 2170cm^-1

(a) What is the force constant for the CO bond’?
(b) At what wavenumber would the corresponding peak for CO occur?

Answer 1

Question 2

16.6 Sulfur dioxide is a nonlinear molecule. How many vibrational modes will this compound have? How many IR absorption bands would sulfur dioxide be expected to have?

Answer 2

SO² should have 3n-6 vibrations, or 3 vibrations.

All three are IR active.

Question 3

Answer 3

Bonds will be inactive if no change in dipole moment accompanies the vibration.

(a) Inactive
(b) Active
(c) Active
(d) Active
(e) Inactive
(f) Active
(g) Inactive

Question 4

16-8 What are the advantages of an FTIR spectrometer compared with a dispersive instrument?

Answer 4

The advantages of an FT instrument over a dispersive instrument include speed, increases S/N ratios, higher resolution, highly reproducible and accurate frequency axis and diminished stray
light effects.

Question 5

In a particular trace analysi5 via FTIR, a set of sixteen interferograms were collected. The signal-to-noise ratio (SIN) associated with a particular spectral peak was approximately 5: 1. How many interferograms would have to be collected and averaged if the goal is to obtain a SIN = 20: 1?

Answer 5

Since the S/N ratio goes as the square root of the number of scans, to increase the S/N of a 16 scan interferogram by 4-fold, the number of scan would have to increase by 4-fold or 16-fold.

Thus, 256 (16 scans x 16-fold) scans would be required.