LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

LM 33

SECOND SEMESTER – APRIL 2006

                                               CH 2950 – MOLECULAR SPECTROSCOPY

(Also equivalent to CH 2802)

 

 

Date & Time : 21-04-2006/FORENOON     Dept. No.                                                       Max. : 100 Marks

 

 

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

1. Indicate which one will have higher λ_max value and explain

• CH₂ = CH₂ and CH₂ = CH – CH = CH₂
• C₆H₆ and C₆H₅OCH₃

2. The ¹H NMR of C₄H₉Cl gave a single peak at 1.80 δ as a singlet. Give the structure of the compound.
3. Sketch the EPR spectrum of C₆H₅^• radical and explain.
4. Which one has a higher ν(C=O) value? Explain

O                                                O

||                                                  ||

CH₃ – C – CH₃       and    CH₂ = CH – C – CH₃

5. How is the mass of metastable peak calculated in the mass spectrum? Explain with one example.
6. Identify the point groups for the following molecules:

(a)POCl₃    (b) SF₆     (c) Br₂          (d) Ni(CN)₄^2- (square planar)

7. Explain the rule of mutual exclusion with a suitable example.
8. Explain the spectroscopic transitions (a) fundamental (b) hot bands using proper quantum
9. Explain the meaning and significance of x, y, and z in the T₂ representation of the T_d point group as shown below

T_d      E          8C₃      3C₂      6S₄       6σ_d

T₂      3          0          -1         -1         1          (x,y,z)

 

10. Identify the symmetry operations present in the point groups C₁, C_i, and C_s. Give one example for each.

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

11. How does the solvent polarity affect the λ_max of the compound?
12. How are the following differentiated by the infrared spectral studies?

• aliphatic and aromatic compounds
• ethanol and diethyl ether (3+2)

13. Write a note on Nuclear Overhauser effect.
14. Sketch the Mossbauer spectrum of [Fe(CN)₆]^3- and explain.
15. Discuss the detectors used in UV-Visible double beam spectrophotometer.
16. Discuss the McLafferty rearrangement in the mass spectral fragmentation pattern.
17. Explain the factors for the broadening of spectral lines.
18. How are P, Q, and R branches of absorption bands obtained in vibration-rotation spectra of molecules? In which type of molecules Q branch is not observed and why?
19. (a) When do we say two symmetry operations are in the same class? Explain with an example.
    • The equilibrium vibration frequency of the HBr molecule is 2649.7 cm^-1 and the anharmonicity constant is 0.0171; what, at 300K, is the intensity of the ‘hot band’ (v=1 to v=2 transition) relative to that of the fundamental (v=0 to v=1)?    (2+3)

20. A microwave spectrometer capable of operating only between 60 and 90 cm^-1 was used to observe the rotational spectra of HI. Absorptions were measured as follows:

HI (cm-1)

64.275 77.130 89.985

Find B, I and r for each of the molecule, and determine the J values between which transitions occur for the first line listed above.

21. (a) Explain the following with a suitable molecule for each:

(i) Principal axis of rotation    (b) Inversion Center

(b) Explain the meaning of a E_g representation in the character table.  (2+2+1)

 

 

 

22. Give the reduction formula and reduce the following reducible representation.

C_2h      E          C₂        i          σ_h          

8          0          6          2

 

C_2h       E          C₂        i           σ_h

A_g        1          1          1          1          R_z              x², y², z², xy

B_g           1          -1         1          -1         R_x, R_y        xz, yz

A_u        1          1          -1         -1         z

B_u        1          -1         -1         1         x, y

 

 

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

23. How are the UV-Visible spectral studies useful in (a) charge-transfer transition
    (b) electronic effect in the organic molecules? (5+5)
24. Describe the following:

• Proton noise decoupled ¹³C-NMR
• Spin-spin coupling in ¹H NMR                                                               (5+5)

25. Discuss the principle and instrumentation involved in the EPR spectroscopy. Explain the nuclear hyperfine splitting. (4+3+3)
26. (a) State and explain Franck-Condon Principle. How are intensity variations of electronic spectra explained by this principle?

(b) The Bond length of NO is 115.1 pm. Bond force constant is 1595 Nm^-1. Calculate (a) Zero-point energy and the energy of the fundamental vibration ν₀.
(b) Calculate the rotational constant B. (c) Calculate the wave numbers of the lines P₁, P₂, R₀ and R₁                                                                                      (5+5)

27. (a) What are Stokes and anti-Stokes lines and Rayleigh lines? Compare their intensities in the Raman vibrational spectrum of a compound.

(b) Consider the molecules N₂, CCl₄, CH₃Cl and H₂O. Predict which of them will show (a) Pure rotational spectrum (b) infrared absorption (c) electronic transition. Give reasons for your choice.                                                                      (5+5)

28. Find the number, symmetry species of the infrared and Raman active vibrations of CH₄, which belongs to T_d 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)

 

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)

 

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