PG – 721

*PG721*

II Semester M.Sc. Degree Examination, July/August 2013 (RS) (2006-07 Onwards) CHEMISTRY C-204 : SPECTROSCOPY – I Time : 3 Hours

Max. Marks : 80

Instruction : Answer question No. 1 and any five of the remaining. 1. Answer ten questions of the following :

(2×10=20)

a) Prove that in BF3 molecule, C3σv is not equal to σ vC 3 . b) Find the result of the operation, Snn when (i) n is odd and (ii) n is even. c) What do the Mulliken symbols, Big and A2u signify ? d) List out the symmetry elements in CHCl3 molecule. e) Distinguish between symmetric top and asymmetric top molecules. f) What is meant by Stark effect ? g) The Cu K α X-ray radiation has a wavelength of 1.545 A . Express this energy in wave number and frequency.

h) What is the principle involved in Raman spectroscopy ? i) Explain the rule of mutual exclusion. j) Distinguish between fluorescence and phosphorescence. k) Plot schematically the time domain signals from two spectral lines having the same frequency but different widths. l) Explain Franck Condon principle. 2. a) Identify the symmetry elements present in the molecules and assign the appropriate point groups. i) PtCl42–

ii)

HCN

iii) CH2Cl2 and iv) B2H6 b) What are character tables ? Obtain the character table for H2O molecule. c) Give the matrix representation for E, i, Cn, σ and Sn.

(4+4+4) P.T.O.

*PG721*

PG – 721 3. a) Construct the group multiplication table for C3V point group.

b) Deduce the matrix representation for rotation of a vector with respect to Z-axis and evaluate the matrix for C2 and C3. c) Show that three reflections of ammonia constitute a class.

(4+4+4)

4. a) Give a brief account of symmetry and spin forbidden transitions. b) With suitable examples, discuss the rotational spectra of polyatomic molecules. c) The spacing between the first and second line of the rotational spectrum of a diatomic molecule is 40.91 cm–1 while that between the ninth and tenth line is 37.68 cm–1. What is the % elongation of the bond upon centrifugal distortion ? (4+5+3) 5. a) What are the advantages of FTIR spectroscopy over IR spectroscopy ? b) Using classical mechanics, deduce the centrifugal distoration constant for a diatomic vibrating rotor. c) A series of molecular transitions required the following energies (in kcal mol–1) in order to occur. Indicate the spectral region in which you would expect the absorption of radiation and calculate the wavelength of radiation. i) 50 and ii) 0.01.

(3+5+4)

6. a) Molecules of the type XY2 can have one of he following structures i) Y-Y-X and ii) Y-X-Y Predict the IR and Raman spectra for each of these structure models. b) Describe the electronic spectrum of HCHO. c) Draw the Jablonski diagram and indicate various photophysical processes that occur. (4+4+4) 7. a) Discuss how Frank-Condon principle is helpful to explain intensities of vibrational structures. b) State the selection rules for electronic transitions. When are these rules relaxed ? c) What are pi- and δ molecular orbitals ? Calculate the bond order for O −2 and O +2 .

(4+4+4) —————————