CH 3810 – MOLECULAR SPECTROSCOPY

Date & Time: 04/05/2009 / 9:00 – 12:00 Dept. No. Max. : 100 Marks

PART A

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

What is S/N ratio? How can it be enhanced?
The rotational constant of a rigid diatomic molecule is 1.566 cm^-1. Determine the transition which gives the most intense line at 300 K.
Differentiate between hot band and overtone in the nature of their transitions.
From the relative abundance of the (M+2)⁺ peaks in the mass spectra, can you distinguish Chloro compounds from Bromo compounds.
What is the basic principle of Mossbauer Spectroscopy?
Calculate the Larmor precessional frequency of a proton when placed in a magnetic field strength of 1.5 T ( g_N = 5.585)
What are diagonal and off-diagonal peaks in 2DNMR.
Sketch the EPR spectrum of HCl radical.
What are the conditions to observe NQR transitions.
Mention the importance of a saturable absorber dye.

PART – B

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

Explain the factors that affect the intensity of spectral lines.
The rotational spectrum of¹²C¹⁶O shows a series of equidistant lines spaced 3.8424 cm^-1 Calculate the rotational constant and the C-O bond length in this molecule.
Explain the pure rotational Raman spectrum of a symmetric top molecule.
A molecule AB₂ has the following IR and Raman spectra. Discuss the molecular structure and assign the observed lines to molecular vibrations.

Frequency (cm^-1 ) IR Raman

3750 very strong –

3650 strong strong, polarized

1595 very strong –

Calculate the recoil velocity of the free Mossbauer nucleus ¹¹⁹Sn when emitting a γ- ray of frequency 5.76 x 10¹⁸ What is the Doppler shift of the γ- ray frequency to an outside observer?
The mass spectrum of a compound having the empirical formula C₅H₈O₂, shows the following peaks at m/e : 28, 43 (Base peak), 85, 100 (Molecular Peak). Identify the compound and account for the fragments.
What is the effect of shielding and deshielding on precessional frequency?
Odd number of bonds give positive J vale while even number of bonds give negative J values – explain.
Discuss the EPR spectrum of a triplet state radical.

Calculate the NQR frequency of a compound that has a C₃ axis of symmetry. Given that e = 4.8 x 10^-10 esu, Q = 0.08 x 10^-24 cm² and q = 2 x 10²⁵
How is pulse operation achieved in Q-switching?
Explain the mechanism of mode locking method of producing laser pulses.

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

a) The fundamental and first overtone transitions of ¹H³⁵Cl are centered at 2886 cm^-1 and 5668 cm^-1 Evaluate the equilibrium vibration frequency, the anhormonicity constant, and the force constant of the molecule. (6)
b) Give any four basic differences between IR and Raman spectra (4)
a) The fundamental band of ¹⁴N¹⁶O is centred at 1904 cm^-1 and its bond length is 0.1151 nm. Calculate the rotational constant B and the first two lines of the P and R branches of the vibration-rotational spectrum of NO. (6)
b) Explain briefly Quadrupole Mass analyzer. (4)
a) Discuss any two applications of Mossbauer spectroscopy. (6)
b) The Raman line associated with a vibrational mode which is both Raman and IR active is found at 460 nm when excited by light of wavelength 435.8 nm. Calculate the wavelength of the corresponding infrared band. (4)
(a) How will you interpret the Hetero nuclear correlation spectrum of a compound?(6)

(b) NMR of benzene is observed at a lower field while acetylene is at a higher field – explain (4)

(a) Explain the importance of the following (i) Electric field gradient (ii) asymmetry parameter (6)

(b) The benzene radical anion has a g value of 2.0025. At what field would you search for resonance in 9.525 GHz instrument. (4)

(a) How will you characterize the delocalization of an unpaired electron in a transition

metal complex? (5)

(b) How will you account for the variation in intensity of electronic transitions? (5)

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