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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper II covers organic chemistry and spectroscopy.

UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II

Q8(c)(iii). Fundamental vibrations of CO₂ and their IR activity

Question

How many fundamental vibrational frequencies would be expected to be observed in the IR spectrum of CO₂? Sketch them. Identify the IR-active vibrations with justification.

Answer in brief

Linear CO₂ has 3N − 5 = 4 normal modes and three fundamental frequencies; two appear in the IR: the antisymmetric stretch (2349 cm⁻¹) and the bend (667 cm⁻¹).

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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 modesCO₂ vibrationsIR active3N − 5mutual exclusion rule

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