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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                          LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

AD 24

THIRD SEMESTER – NOV 2006

CH 3810 – MOLECULAR SPECTROSCOPY

 

 

Date & Time : 30-10-2006/9.00-12.00      Dept. No.                                                   Max. : 100 Marks

 

 

PART A

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

1. Convert 0.15 nm to Hz and state what sort of transition this corresponds to.
2. What are stokes and anti-stokes lines?
3. Name two techniques to enhance the sensitivity of the spectral lines.
4. What is the basic principle of Mossbauer spectroscopy?
5. From the relative abundance of the (M+2)⁺ peaks in the mass spectra, can you distinguish chloro compounds from Bromo compounds?
6. State the principle of PES. What is the essential difference between XPES and UV-PES?
7. A free electron is placed in a magnetic field of strength 1.3 T. If g_e is 2.0023 and β_e is 9.274 x 10^-24 JT^-1. Find the ESR frequency in GHz.
8. What are the advantages of using TMS as the standard in NMR experiments?
9. Compare the ¹H and ¹³C NMR spectra in terms of the range of chemical shifts and intensity of the peaks (peak areas)
10. What is spin-spin relaxation?

PART – B

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

11. What are the factors that affect the width of the spectral lines? Explain any two of them.
12. The observed infrared absorption and Raman spectral lines of Chlorate ion (ClO₃^–) are given below. Predict the structure of Chlorate ion.

Raman Frequencies (cm-1 )                        IR Frequencies (cm-1 450 (depolarized)                                           434 (^) 610 (polarized)                                               624 (||) 940  (depolarized)                                          950 (^) 982 (polarized)                                               994 (||)

 

13. The frequencies of vibration of the following molecules in their v = 0 states are HCl: 2885 cm^-1; D₂: 2990 cm^-1; DCl: 1990 cm^-1 and HD: 3627 cm^-1, calculate the energy change in the reaction HCl + D₂ à DCl + HD.
14. The mass spectrum of a compound having the empirical formula C₁₁H₁₂O₂, shows the following peaks at m/e : 78,105 (Base peak), 122, 135,176 (Molecular Peak). Identify the compound and account for the fragments.
15. At what temperature will the number of I₂ molecules in the v=1 level be one-tenth of that in the v=0 level? Given = 214.6 cm^-1;x_e = 0.6 cm^-1 and k = 1.38 x 10^-23J K^-1.
16. Explain Quadrupole mass spectrometer.
17. Explain what is Zero Field or Crystal Field Effect in the fine structure of ESR

and the factors that contribute to that.

18. State and explain Franck-Condon Principle. With diagrams explain the

variations in intensity of electronic spectra explained by this principle?

19. What is population inversion? Prove that a population inversion cannot

be achieved in a two level system while it is possible in a three level system.

20. (a) With an example explain Diamagnetic Anisotropy.

(b) The chemical shift of the CH₃ protons in diethyl ether is ∂ = 1.16 ppm and that of the CH₂ protons is 3.36 ppm. What is the difference in local magnetic fields between the two regions of the molecule when the applied field is 16.5 T?

21. Predict

(a) the spin-spin splitting pattern for the protons of neopentane molecule

and the peaks’ intensity distribution.

(b) the hyperfine splitting of the ESR spectrum of ۰CD₃ and ۰CHD₂ radicals.

[note: the spin of D is 1]

22. Explain the principle of 2D NMR highlighting its advantages.

PART – C

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

23. a) Three consecutive lines in the rotational spectrum of a diatomic molecule are observed at 84.544, 101.355 and 118.112 cm^-1.  Assign these lines to their appropriate J^’’ àJ^’ transitions and deduce the values of B and D.
24. b) How are meta stable ions produced and how are they useful in identifying the molecular formula of a compound?
25. a) The fundamental band of HCl is centered at 2886 cm^-1.  Assuming that the internuclear distance is 12.76 nm, calculate the wave number of the first two lines of each of the P and R branches of HCl.
26. b) Explain the pure rotational spectra of symmetric top molecules.
27. a) Explain the fact that the Mossbauer spectrum of [Fe(CN)₅NO]^2- gives two lines whereas [Fe(CN)₆]^4- spectrum gives one line.
28. b) In the rotational Raman spectrum of HCl, the displacements from the exciting lines are represented by Dn = ± (62.4 + 41.6 J) cm^-1.  Calculate the moment of inertia of the HCl molecule.
29. Explain the following with a suitable example:

(a) spin-spin coupling in ¹H-NMR

(b) mechanism of spin-spin coupling for geminal protons

(c) the multiplicity and intensity distribution represented by the ‘coupling tree’

 

27. (a) Explain briefly: (a) spin-spin relaxation (b) Fourier Transformation Technique.

(b) Sketch the esr spectrum of ^°CH₃ and explain its nuclear hyperfine splitting.

 

28. (a) What are asymmetry electric field gradient and asymmetry parameter, η? How

are they related? What is the significance of η?

(b) Taking any laser as example explain the three most important components

of a laser?

 

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