LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

GH 40

THIRD SEMESTER – APRIL 2008

CH 3810 – MOLECULAR SPECTROSCOPY

 

 

 

Date : 02-05-08                  Dept. No.                                        Max. : 100 Marks

Time : 9:00 – 12:00

Part A

Answer ALL the questions.             (10 x 2 = 20 Marks)

1. What is ‘resolving power’ of a spectrometer? Mention any one method of improving it.
2. The first line in the rotation spectrum of CO appears at 3.84235 cm^-1. Calculate its

moment of inertia.

3. CO₂ gives two IR absorptions although the molecule has no dipole moment. Why?
4. Explain the principle of mutual exclusion with an example.
5. Mention three important characteristics of lasers which differ from ordinary light.
6. State nitrogen rule.
7. How many signals are expected in the ESR spectrum of isopropyl radical?
8. Write any four advantages of using tetramethyl silane as internal reference compound in recording NMR spectra.
9. An organic compound (molecular formula C₄H₁₀O) contains a triplet centered at ∂ = 1.26 ppm and a quartet at ∂ = 3.38 ppm in its ¹H NMR spectrum. Write the structure of the NMR spectrum. Write the structure of the compound.
10. Acetylenic proton absorbs at upfield compared to ethylenic proton, although the former is attached to a more electronegative carbon. Explain.

PART – B

                                        Answer ANY EIGHT questions              (8 x 5 = 40 Marks)

11. What are the factors governing the intensity of the spectral lines? Explain any two of them.
12. Consider the molecules Br₂, HCl, CCl₄. Which of these will give a pure

(a) rotational microwave spectrum? (b) infrared spectrum?  (c) Raman spectrum

(d) electronic spectrum?

13. The Bond length of ¹H⁸¹Br is 141.44 pm. Bond force constant is 411.5 Nm^-1. Calculate the zero-point energy, the energy of the fundamental vibration ν₀, the rotational constant B and the wave numbers of the lines P₁, P₂, R₀ and R₁
14. State and explain the Franck-Condon Principle. How are the intensity variations of electronic spectra explained by this principle?
15. With an example explain the three most important components of a laser?
16. (a) State the principle of PES. What is the essential difference between XPES and UV-PES?

(b) With an example explain how PES can be applied to molecular structure.    (2+3)

17. What are the sample requirements in NQR spectra? Illustrate the application of NQR

spectra with one example. What is the main drawback of this technique?

18. Discuss hyperfine splitting by taking the ESR spectrum of methyl radical. Sketch the

energy level diagram and indicate the transitions responsible for ESR signals.

19. Discuss the principle of ¹³C NMR spectra and explain two of its applications in the

determination of structure of organic compounds.

20. Write briefly on ‘Lanthanide shift reagents’.
21. Describe the various parts of mass spectrometer and explain the function of each part.
22. Discuss the theory of Mössbauer spectra.

PART – C

                                                     Answer ANY FOUR questions               (4 x 10 = 40 Marks)

23. a) Discuss the factors contributing to the broadening of the spectral lines.                (6)
24. b) The stretching frequency of ¹²C¹⁶O occurs at 2143 cm^-1. Calculate the force constant for this molecule. Assuming the force constant to be the same for ¹²C¹⁷O, calculate the stretching frequency for ¹²C¹⁷                                                                      (4)
25. a) Explain the origin of P, Q, R branches of the rotation-vibration spectra with a diagram. (5)
26. b) Differentiate the origin and nature of Stokes’ lines, Anti-Stokes’ lines and Rayleigh scattering with respect to their position in spectrum and the intensity of the spectral lines. (5)
27. a) What is population inversion? Show that a population inversion can be

achieved in a three level system.          (6)

5416. b) The ionization energy for 3s electron is 30 eV. The kinetic energy of electrons ejected is 5416.07 eV. Find the wavelength of X-rays used for this PES experiment. (1eV = 1.602 x 10^-19J) (4)
5417. a) What is meant by geminal coupling? Explain with suitable examples the effect of hybridization of carbon and the elctronegativity of substituent on the germinal coupling constant. (5)
5418. b) Illustrate long range coupling through п-bond and W–coupling with suitable examples. (5)
5419. a) ESR is observed for atomic hydrogen with an instrument operating at 9.5 GHz. If

the ‘g’ value for the electron in hydrogen atom is 2.0026, what is the strength of the

applied magnetic field? (1 GHz = 10⁹ Hz, μ_e = 9.274 x 10^-24 JT^-1)                        (4)

(b) Describe the FAB technique for the production of molecular ions in mass

spectrometer. What are its advantages and limitations?         (6)

28. (a) Write briefly on ‘McLafferty rearrangement’. (4)

(b) An organic compound with molecular formula C₄H₉Br contains the following

signals in 1H and ¹³C NMR spectra:

PMR data: (a) doublet    ∂ = 1.04 ppm (6H)

(b) multiplet ∂ = 1.95 ppm (1H)

(c) doublet    ∂ = 3.33 ppm (2H)

CMR data: (a) quartet    ∂ = 20.9 ppm

(b) doublet  ∂ = 30.7 ppm

(c) triplet     ∂ = 42.2 ppm

 

Assign a suitable structure to the compound and explain the NMR spectral data.   (6)

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