UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper II
Q1(m). Microwave activity of C₂H₂, CH₃Cl, C₆H₆ and CO₂
Question
Which of the following molecules are microwave active? Why? C₂H₂, CH₃Cl, C₆H₆, CO₂
Answer in brief
Only CH₃Cl is microwave active: it alone has a permanent dipole moment. C₂H₂, C₆H₆ and CO₂ are symmetric, μ = 0.
Key points
- Gross selection rule: a pure rotational spectrum needs a permanent dipole moment.
- C₂H₂: linear, centrosymmetric (D∞h); the two C–H dipoles are opposite ⇒ μ = 0, inactive.
- CH₃Cl: symmetric top (C₃v), μ ≈ 1.9 D along C–Cl ⇒ active; lines at ν̃ = 2B(J + 1).
- C₆H₆: planar hexagon (D₆h), non-polar ⇒ inactive; CO₂: linear O=C=O, bond dipoles cancel ⇒ inactive.
- The three non-polar molecules show rotational Raman spectra instead (anisotropic polarisability).
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversmicrowave activerotational spectroscopypermanent dipole momentselection rulesCH3Clpoint group
All parts of Question 1
- 1(a) Conditions for ideal behaviour of real gases5 marks
- 1(b) Compressibility factor and Z against P plots5 marks
- 1(c) Formula from corner and face-centre positions5 marks
- 1(d) Crystal systems from the unit-cell constants5 marks
- 1(e) Electrophoretic effect in the Debye–Hückel–Onsager theory5 marks
- 1(f) Activity and activity coefficient: units and magnitudes5 marks
- 1(g) Why a finely powdered adsorbent works better5 marks
- 1(h) Half-life of a first order reaction5 marks
- 1(i) Role of H₃PO₄ in the dichromate titration of Fe²⁺5 marks
- 1(j) Why a voltmeter does not give the EMF5 marks
- 1(k) Well-behaved functions: two tests5 marks
- 1(l) Uncertainty in the velocity of an electron5 marks
- 1(m) Microwave activity of C₂H₂, CH₃Cl, C₆H₆ and CO₂5 marks
- 1(n) Highest vibrational frequency among H₂, D₂ and HD5 marks
- 1(o) Photosensitised reactions5 marks
- 1(p) Energy of one mole of 2500 Å photons5 marks