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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper II

Q15(b). Normal modes of CS₂ and OCS; IR activity

Question

Sketch the fundamental vibration modes of CS₂ and OCS. Which of these are IR active? [sketches of the vibrational modes are expected]

Answer in brief

Each has 4 modes (3 fundamentals; the bend is doubly degenerate). CS₂: bend and antisymmetric stretch IR active, symmetric stretch inactive. OCS: all three IR active.

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Topic notes: vibrational and electronic spectra (force constant, isotope effect, overtones, Franck–Condon), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversnormal modesIR activeCS2OCSlinear triatomic moleculemutual exclusion

All parts of Question 15