The Spectral Game

Developed by Jean-Claude Bradley, Robert Lancashire, Andrew Lang and Antony Williams


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UsefulChem 3/1/09 post

Spectroscopy Resources

Organic Structure Determination (Open Courseware)
Spectroscopy Page (Organic Chemistry Wikibook)
Dhavale NMR presentation

The following taken from Organic Chemistry II at Drexel University taught by Jean-Claude Bradley:

Nuclear Magnetic Resonance Spectroscopy (NMR)

  • Reusch
  • can view every H and C in molecules as peaks
  • scale in ppm (delta scale), relative to tetramethylsilane (TMS), defined as zero
  • in a typical NMR plot, higher ppm are on the left (low field, more deshielded)
  • integration corresponds to number of Hs
  • electronegative elements deshield
  • adjacent and non-equivalent (2-3 bonds) H's couple with each other and split peaks (N+1 rule), giving coupling constant J
  • J for alkenes: cis 10 Hz, trans 15 Hz, geminal 2 Hz
  • J for aromatics: ortho 8 Hz, meta 2 Hz
  • CH2 next to chiral center are diastereotopic and the H's are not equivalent
  • alkanes 0.9-1.5 ppm
  • aromatics 7-8 ppm
  • alkenes 5-6 ppm
  • aldehydes 9-10 ppm
  • terminal alkyne H 2.5 ppm
  • benzylic or next to ketone or aldehyde 2.1 - 2.3 ppm
  • CH2-X (X = O,N,halogen) 3 - 4 ppm
  • carboxylic acid 10-12 ppm, although may be very broad
  • amine and alcohols may be broad, very variable and not undergo 3 bond coupling
  • in ultrapure samples, H of OH may exhibit 3 bond coupling and splitting
  • hydrogen bonding H's can be replaced with H NMR invisible D's by adding D2O or CD3OD
  • C NMR do not show C-C coupling because only about 1% of C's are C-13
  • C NMR are usually proton spin decoupled so that only singlets are obtained
  • C NMR shifts: carbonyl 160-210 ppm, aromatic 100-150 ppm, alkynes 75-95 ppm, alkanes 0-35 ppm


Click on images for explanation of NMR assignment:


Infrared Spectroscopy (IR)

  • Reusch
  • measures the bending and stretching of bonds
  • higher wavenumber (in cm^-1) corresponds to higher energy
  • heavier atom gives lower energy and lower wavenumbers
  • higher bond energies (e.g. single -> double -> triple bonds) give higher wavenumbers
  • useful range 1600-3500 cm^-1 (any absorptions below that are in the fingerprint region)
  • C=C unconjugated alkene 1640-1680
  • C=C conjugated alkene 1620-1640
  • C=C aromatic 1600
  • C-H (sp3) 2900-3000
  • C-H (sp2) 3000-3100
  • C-H (sp) 3300
  • hydrogen bonded N-H, O-H broad near 3300, CO2H very broad 2500-3500
  • C=O ketone, aldehydes, carboxylic acids 1710
  • C=O conjugated ketone or aldehyde 1680
  • C=O esters 1730-1740
  • C=O amides 1640-1680
  • C-H aldehyde 2700 and 2800
  • C=N 1660
  • CN (nitrile) 2250
  • CC alkyne 2150