LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

DB 27

M.Sc. DEGREE EXAMINATION – CHEMISTRY

THIRD SEMESTER – November 2008

    CH 3810 – MOLECULAR SPECTROSCOPY

 

 

 

Date : 07-11-08                 Dept. No.                                        Max. : 100 Marks

Time : 9:00 – 12:00

PART A

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

1. Convert 500 nm to cm^-1 and state what sort of transition this corresponds to.
2. What is the resolving power of a mass spectrometer?
3. Define Stokes and anti-Stokes’ radiation.
4. The rotational constant of CO molecule is 1.9313 cm^-1. Determine the transition which gives the most intense spectral line at 300 K.
5. How will you distinguish between (CH₃)₃N and CH₃CH₂CH₂NH₂ by infrared spectroscopy?
6. The magnetic moment vector executes Larmor precession instead of aligning with the magnetic field.
7. Mention any two differences between continuous wave and FT NMR methods.
8. Identify the isomer of C₃H₇ radical which showes 14 lines in its EPR spectrum.
9. Sketch the EPR spectrum of cyclobutyl radical anion C₄H₇
10. Phosphorescence unlike fluorescence cannot be studied in liquid phase. Explain.

PART – B

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

11. What are Einstein’s coefficients? Derive the relation between them.
12. Irradiation of CCl₄ by 435.8 nm radiation gives the Raman lines at 440, 441.9 and 444.7 nm. Calculate the Raman shift for each of these lines.
13. Explain Fourier transform infrared spectrometer.
14. The mass spectrum of an organic compound having the empirical formula C₄H₉NO exhibits peaks at m/z 87 (Molecular ion peak), 86, 72, 71, 59, 44 (base peak) and 43. Predict the structure of the compound and account for the fragments mentioned above.
15. The observed infrared absorption and Raman spectral lines of Nitrate ion (NO₃^–) are given below. Deduce the structure of Nitrate ion and assign the frequencies to particular fundamental modes.

Raman Frequencies (cm-1 )                        IR Frequencies (cm-1) 690 (depolarized)                                          680 (^) –                                                             830 (||)                           1049  (polarized)                                               – 1355 (depolarized)                                        1350 (^)

 

16. The rotational and centrifugal constants of HF¹⁹ molecule are 20.56 cm^-1 and 00213 cm^-1. Estimate the vibrational frequency and the force constant of the molecule.
17. How is FID signal obtained? Mention its significance.
18. What is allylic coupling? Account for the maximum and minimum value of allylic coupling constant.

 

 

19. Explain the use of EPR technique to understand the delocalization of electrons in complexes.
20. What are coherence time and coherence length?
21. Explain how Frank-Condon principle accounts for the intensity variations in electronic spectra.
22. Discuss the effect of magnetic field on the NQR spectra of systems with nuclear spin 1 and 3/2.

 

PART – C

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

23. a) Calculate the energy in Joules of the photon absorbed when ¹⁴N¹⁶O molecule goes from the state v=0, J^’’=0, v= 1, J^’ = 1. Assume that v = 0 and v = 1 states have the same B values.  Given   (cm^-1) = 1904 cm^-1; x_e = 0.00733, r_NO = 0.1151 nm                                 (5)
24. b) Explain the pure rotational spectrum of symmetric top molecule             (5)
25. a) The first three rotational Raman lines of a linear triatomic molecule are at 4.86, 8.14 and 11.36 cm^-1 from the exciting Raman line.  Estimate the rotational constant B and the moment of inertia of the molecule in kg m².                                                                                (4)
26. b) Briefly discuss the MALDI-TOF mass spectrometry                                          (6)
27. a) Explain isomer shift in Mossbauer spectroscopy with an example.                      (4)
28. b) The fundamental band of ²D³⁵Cl is centred at 2011 cm^-1.  Assuming that internuclear distance is constant at 12.88 nm, calculate the wave numbers of the first two lines of each of the P and R branches of DCl                                                                                                                  (6)
29. (a) Explain the principle and interpretation of COSY of a compound                (6)

(b) Calculate the chemical shift, when the frequency of separation between protons of a compound and TMS is 540 Hz at a magnetic flux density of 1.85 T (g_N = 5.585)                    (4)

27. (a) How will you account for the difference in the g-value of free radicals with transition metal complexes                                     (6)

(b)  Calculate the g-value for p⁵ configuration                                                            (4)

28. (a) Discuss the theory of NQR spectroscopy (5)

(b) Explain the broad band absorption and fluorescence exhibited by  the dye-Rhodamine 6 G                                                                                                                                    (5)

 

 

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