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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 I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper I

Q8(a). Why all F atoms of PF₅ look alike in the ¹⁹F NMR spectrum

Question

'All the F atoms appear indistinguishable in the ¹⁹F NMR spectrum of the sp³d-hybridised PF₅ molecule.' Explain how.

Answer in brief

PF₅ is fluxional: its axial and equatorial fluorines exchange by Berry pseudorotation much faster than NMR can follow, so only one averaged signal is seen, split into a doublet by ³¹P.

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Topic notes: VSEPR, hybridisation and Bent's rule, with every Civil Services, IFoS and Geo-Scientist PYQ →

Topic notes: main group chemistry: boranes, borazine, phosphazenes, silicates, silicones, interhalogens, xenon and sulfur–nitrogen compounds, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversBerry pseudorotationfluxional molecules¹⁹F NMRPF₅NMR time scaletrigonal bipyramid

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