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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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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