Molecular Spectroscopy Report No. 3 Due: April 27, 2015 Monday

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Molecular Spectroscopy Report No. 3

Due: April 27, 2015 Monday

You can get the atomic mass of elements from the reference books available in the library. Do not take average atomic mass of elements because isotopes should be treated separately.

Problem 1) How many normal modes of vibration does the CO2 molecule have? Describe the motions involved and comment on their infrared and Raman activity.

Problem 2) Consider the following molecules H2, HCl, CO2, H2O, CH3Cl, CH2Cl2, H2O2, NH3, SF6. Which of them will give (a) a pure rotational spectrum, (b) a vibrational rotational spectrum, (c) a pure rotational Raman spectrum and (d) a vibrational Raman spectrum?

Problem 3) In the Raman spectrum of CCl4 obtained by using the mercury arc line at 438.53 nm for excitation, lines are observed at 440.00, 441.88, and 444.70 nm. Calculate the Raman shift for each of these lines in terms of wave number.

Problem 4) The infrared and Raman activities of the N2O molecule are listed in Table 4.3 of the text. Answer the following questions from the data listed in the Table.

a) What is the molecular structure? Describe it based on the spectroscopic data.

b) Identify the vibrational modes of the three bands observed.

c) How to distinguish the symmetric and asymmetric stretching vibrations?

Problem 5) The infrared and Raman spectroscopic data of A2B2 molecule are list in the following table. Deduce what you can about the structure of the molecule and assign the observed vibrations to particular modes as far as possible.

Position (cm^-1) Infrared spectrum Raman spectrum

2701 inactive strong intensity

2439 strong intensity inactive

1762 inactive strong intensity

537 very strong intensity inactive

505 inactive ?

Problem 6) For a molecule belonging to the D2h point group, deduce whether the following vibrational transitions, all from the zero-point level, are allowed in the infrared and/or Raman spectrum, stating the direction of the transition moment and/or the component of the polarizability involved:

a) to the v=2 level of the b1g vibration;

b) to the v=1 level of an au or b2u vibration;

c) to the combination level involving v=1 of a b1uvibration and v=1 of a b3g vibration;

d) to the combination level involving v=2 of an au vibration and v=1 of a b2g vibration.

Problem 7) Consider the molecule CH3Cl.

a) To what point group does the molecule belong?

b) How many normal modes of vibration doer the molecule have?

c) What are the symmetries of the normal modes of vibration for this molecule?

d) Which of the vibrational modes of this molecule are infrared active?

e) Which of the vibrational modes of this molecule are Raman active?

Problem 8) How would you expect the intensities of the C≡C stretching vibration to compare in the infrared and in the Raman spectrum of these compounds: (a) HC≡CH; (b) HC≡CCl.

Problem 9) For the ICl molecule, the following spectroscopic constants are listed as ωe= 384.293 cm^-1, ωeχe= 1.501 cm^-1, Be= 0.1141587 cm^-1, and αe = 0.0005354 cm^-1.

a) Predict the pure rotational Raman spectrum. What will be the Raman shift of the two lines closest to the exciting laser line?

b) Predict the pattern of the Stokes vibration-rotation Raman spectrum. What will be the Raman shifts of the S(0) and O(0) lines from the exciting line at 514.5 nm?

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Molecular Spectroscopy Report No. 3 Due: April 27, 2015 Monday