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IGNOU MCH-20 (July 2025 – June 2026) Assignment Questions
1. Answer any five of the following questions in brief:
(a) Explain the origin of atomic spectrum.
(b) What is meant by anisotropy of chemical bond? Give an example.
(c) What makes TMS a molecule of choice to act as a reference in the NMR spectrometry?
(d) What is meant by polarizability of a molecule?
(e) In what way is NMR spectrometry different from other molecular spectroscopic methods?
f) What is the importance of 13C-NMR spectroscopy in structure determination?
(g) What is the requirement of the vibrational mode of a molecule to be Raman active?
(h) What is the photoelectric effect? Write the equation of Einstein’s photoelectric law.
2. (a) (i) 5G services in India are provided at a frequency of 3300 MHz. Calculate the wavelength of the radiation.
(ii) Explain why the atomic spectra are line spectra whereas the molecular spectra are band spectra.
(b) A photon of visible light has a wavelength of 550 nm. Calculate its
(i) wavenumber
(ii) frequency
(iii) energy per mol
3. (a) What is meant by the Balmer series of hydrogen? What will be the wave number of the longest wavelength in the Balmer series?
(b) A photoelectron spectrum of a compound containing nitrogen (N), aluminium (Al), and fluorine (F) shows three strong peaks at the following binding energies:
• Peak X: ~686 Ev
• Peak Y: ~399 eV
• Peak Z: ~73 eV
Find the best option from the choices given below that best explains the relative positioning of these peaks in terms of binding energy.
(i) Peak X corresponds to F 1s electrons, which have the highest binding energy because fluorine’s high electronegativity and small size give its 1s electrons a large effective nuclear attraction. Peak Y is N 1s and Peak Z is Al 2p.
(ii) Nitrogen has more valence electrons than fluorine or aluminium, therefore its peak appears at the highest binding energy.
(iii) The spectrum shows more aluminium atoms than nitrogen or fluorine, so its peak appears at the lowest binding energy due to dilution.
(iv) Aluminium has the highest nuclear charge, so its 2p peak should have the largest binding energy, but shielding effects reverse this trend. Justify your answer.
4. (a) (i) What information about the molecule can be obtained from a rotation spectrum?
(ii) What is meant by a selection rule? Give the selection rule for a rotating diatomic molecule modelled as a rigid rotator.
(b) Assuming that the force constants of H-Cl and D-Cl are the same, calculate the fundamental vibration frequency of D-Cl if that of H-Cl is 2890 cm-1.
5. (a) In Raman spectra, the Stokes line are more intense than the anti-Stokes lines. Why?
(b) The rotational Raman spectrum of a diatomic molecule has the first line of a series of Stokes and anti-Stokes lines at 346 cm-1, relative of the V of incident light. What would be its bond length.
(c) CN+ has bond length 129 pm. Calculate the wavenumbers of the first four lines in its microwave spectrum.
[Given: mass of C=12.000 u and mass of N=14.007 u]
6. (a) (i) Calculate the number of vibrational modes for a molecule of methane.
(ii) The frequencies of the first and second overtones respectively are exactly double and triple of the fundamental vibration frequency. Comment.
(b) Raman rotational spectra of H2 was observed with Hg light with Rayleigh line at 2294 cm-1. Stokes lines were observed at 22581, 22337, 22094, 21851 cm-1. Draw the rotational energy level diagram with Stokes and anti Stokes lines.
7. (a) (i) Electronic transitions are called vertical transitions. Comment.
(ii) A light source of wavelength 𝜆 illuminates a metal surface and electron are ejected with the maximum kinetic energy 2 eV. If the same surface is illuminated by a light source of wavelength 𝜆/3, then calculate the maximum kinetic energy of the ejected electron. (Work function of metal is 1 eV)
(b) (i) How does the rule of Mutual Exclusion help in predicting the symmetry in case of H2O and CO2 molecules?
(ii) The molecule X2Y has 3 fundamental vibrational frequencies. One is Raman active and two are IR active. Identify the structure of the molecule.
8. (a) (i) Explain why the 13C13C spin splitting is not observed in 13C-NMR spectrum?
(ii) Predict the number of signal and multiplicity of peaks in the 13C spectrum of the following compounds:
a) 2,5-Dimethylhexane
b) Cyclohexanol
(b) (i) What is meant by AX spin system in NMR? Draw a schematic energy level diagram for this system and show the allowed transitions.
9. (a) (i) Calculate the ESR frequency of an unpaired electron in a magnetic field of 3000 G (0.3 T).
[Given: g = 2.0023; 𝜇𝐵 = 9.2741 × 10−24 𝐽𝑇−1; and h = 6.626×10−34 𝐽𝑠]
(ii) Which of the following systems will show electron spin resonance (ESR) spectrum? (i) H (ii) H2 (iii) Na+ (iv) Cl- (v) CH3 (vi) NO2
(b) What are different mechanisms that contribute to the relaxation of nuclear spins in the excited state? Explain any one of them.
10. (a) (i) What is Mossbauer effect?
(ii) The lifetime of 57Fe (excited state) is 1.52 x 107s. If the excited state is 14.3 keV above the ground state determine the linewidth 𝜏 and ratio 𝜏 /E.
(b) Write short notes on the following.
(a) Isomer shift
(b) Spin-Spin splitting
