LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

LM 37

SECOND SEMESTER – APRIL 2007

CH 2950/CH 2802 – MOLECULAR SPECTROSCOPY

 

 

 

Date & Time: 19/04/2007 / 1:00 – 4:00     Dept. No.                                                             Max. : 100 Marks

 

 

Part-A   ANSWER ALL QUESTIONS (10 x 2 = 20)

 

1. Give the range of frequency values of (a) C=O (b) C-H (c) N-H   (d) O-H
2. Mention the standard used in EPR. Draw its structure.
3. A compound C₆H₁₂ has a single singlet signal in its ¹H NMR spectrum. Identify the structure.
4. Mention the high intense mass spectral peaks in (a) C₆H₅CH₃ (b) CH₃CHO
5. Mention the possible electronic transitions in (a) C₆H₅COCH₃   (b) C₆H₆
6. Identify the point groups for the following molecules:

• NH₃ (b) CCl₄               (c) Cl₂          (d) C₆H₆

7. Identify which of the following molecules will give both rotational and IR

• Cl₂ (b) CH₄  (c) CH₃Br   (d) H₂O

8. What is the difference between a fundamental and a hot band?
9. What is unique about C₁ point group? Give an example.
10. Explain the meaning of the letters A and u in the irreducible representation A_u of the character table.

Part-B   ANSWER ANY EIGHT QUESTIONS (8 x 5 = 40)

11. How are the charge-transfer compounds studied by UV-Visible spectral studies?
12. What is nuclear overhauser effect (NOE)? Explain with one example.
13. How are infrared spectral studies useful in the study of hydrogen bonding?
14. What are isotopic peaks and meta-stable peaks? Explain with one example.
15. Sketch the EPR spectrum of C₆H₅⁰ (phenyl radical) and explain.
16. Sketch and explain the Mossbauer spectra of [Fe(CN)₆]^4- and [Fe(CN)₆]^3-.
17. Explain how P, Q, and R branches of absorption bands are obtained in vibration-rotation spectra of molecules?
18. What are stokes and anti-stokes lines? Compare their intensities in the Raman vibrational spectrum of a compound.
19. The fundamental and first overtone transitions of CO are centred at 2143.26 cm^-1 and 4260.04 cm^-1 respectively. Evaluate the equilibrium vibration frequency and the zero-point energy.

 

 

 

 

20. (a) What is similarity transformation?

(b)When do we say two symmetry operations are in the same class? Illustrate with a suitable

example.                                                                                                            (2+3)

21. (a) Explain ‘Fermi Resonance’ with a suitable example.

(b) If a spectral line has a natural line width of 0.17 cm^-1, what is the life-time of the excited

species?                                                                                                              (3+2)

22. Reduce the following reducible representation

T_d    E       8C₃        3C₂        6S₄         6σ_d

4     1         0         0         2

 

into irreducible representation using the reduction formula and the T_d character  table given

below:

 

 

T_d          E          8C₃      3C₂      6S₄       6σ_d

 

 

A₁        1          1          1         1         1                                 x²+y²+z²

A₂        1          1          1         -1         -1

E          2          -1         2         0         0                                 (2z²-x²-y², x²-y²)

T₁         3          0          -1         1         -1         (R_x,R_y,R_z)

T₂         3          0          -1         -1         1         (x,y,z)              (xy,xz,yz)

 

 

Part-C   ANSWER ANY FOUR QUESTIONS (4 x 10 = 40)

 

23. Discuss the principle and instrumentation involved in ¹H NMR.
24. Discuss the Woodward-Hoffmann’s rules for conjugated dienes and α, β–unsaturated ketones.
25. Write a note on

(a) McLafferty Rearrangement

(b) Influence of the solvent λ_max in the UV-Visible spectral studies.

26. (a) State and explain Franck-Condon Principle. How are intensity variations of electronic spectra explained by this principle?

(b) Assume the following data for the molecule ¹H¹⁹F. Bond length = 91.68 pm, Bond force constant = 965.7 Nm^-1, Atomic masses: ¹H = 1.66 x 10^-27 kg, ¹⁹F = 31.55 x 10^-27 kg. Calculate the zero-point energy, the energy of the fundamental vibration ν_0, the rotational constant B, and the wave numbers of the lines P₁ and R₀.                                                                 (5+5)

 

27. (a) Outline the causes for the broadening of spectral lines.

(b)  What is transition dipole moment? How does it determine whether a transition between two energy levels is allowed or forbidden?                                      (6+4)

 

 

 

28. Find the number, symmetry species of the infrared and Raman active vibrations of NH₃, which belongs to C_3V point group. State how many of them are coincident.

(You may, if you wish, use the table of f(R) given below for solving this).

 

Operation:      E     σ       i       C2      C3     C4     C5         C6     S3     S4       S5       S6         S8               f(R):       3       1    -3      -1       0       1     1.618      2      -2      -1     0.382     0       0.414   For any Cn, f(R) = 1 + 2cos(2π/n),            For any Sn, f(R) = -1 + 2cos(2π/n)

 

 

D_3V     E       2C₃     3σ_v

 

A₁    1       1         1              z                      x² +y², z²

A₂     1       1         -1             R_z

E       2       -1         0         (x,y) (R_x,R_y)        (x²-y²,xy) (xz,yz)

 

 

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