structural spectroscopy

• most molecules vibrate at a frequency corresponding to the IR part of the spectrum
• to be IR active molecular vibrations that occur must result in a net change of the dipole of the molecule (change in positions of + and -)
• stretching and bending are types of molecular vibrations that are active
• stretching is a change in bond length

  this requires energy and low wavelengths
• bending is a change in the angle between bonds
• asymmetric stretching causes a net change in dipole because the centre of charge distribution changes
• asymmetric stretching occurs at increased frequency, decreased wavelength and increased wave number
• IR samples can be prepared in vapour phase, in solution, in solid state, or liquid state
• single bonds <E double bonds <E triple bonds <E bonds to H
• 3300

  alkyne C-H
• > 3000

  C-H in unsaturated group
• <3000

  C-H in a saturated group
• 2850 - 2700

  aldehyde C-H - both symmetric and asymmetric stretching
• 3500 - 2500

  O-H absorption bands

  broad peaked due to hydrogen bonding
• 3500 - 3300

  N-H absorption bands

  sharper peaks than OH (less h bonding)
• amines are classified (primary to tertiary) based on how many R groups they are bonded to
• primary amines have 2 stretches
• secondary amines have one stretch
• bonds to H occur at 4000 - 2500
• triple bond absorption bands occur in the region of 2500 - 1900
• 2260

  nitrile R-C (triple bond) N
• 2200

  alkyne R-C (triple bond) C-R

  will have C-H stretch to if C-H bonds present
• double bond absorption bands are in the region of 1900-1500
• 1680 - 1500

  alkene double bonds
• 1600 - 1500

  double bonds in aromatic rings
• 1870 - 1660

  carbonyl bond
• induction, resonance and ring size affect the frequency of carbonyl groups
• occurrence of resonance pairs lowers the wavenumber of carbonyl groups by 40-45 cm-1 because electrons have more mobility which weakens the C=O bond and therefore lowers the frequency
• decreasing the ring size of compounds increases the ring strain which increases the wave number of carbonyl groups
• esters can be identified by looking for the C-O stretch (1300-1050)
• carboxylic acids can be identified by looking for the C-O stretch (1300-1050) and looking for the O-H stretch (3500 - 3300)

  if there is no/little H bonding the OH stretch = (3500 - 3300)

  if there is H bonding the OH stretch = (3500 - 2500)
• to identify aldehydes look for a C-H stretch around 2850-2700 (pair of peaks)
• carbonyl groups have a frequency of about 1715
• An inductive effect in chemistry refers to the tendency of electronegative atoms or groups to attract electrons through sigma bonds from a less electronegative atom and results in an unequal sharing of electrons in the sigma bonds.
• as the electronegativity of groups surrounding the C=O bond increases this induction strengthens the C=O bond, and increases the stretching frequency
• frequency is affected by type of bond excitation (bending or stretching), strength of bond, types of atoms bonded to gether
• as strength of bond increases the v increases
• NMR detects nuclei and says what kind of environment they are in
• atomic nuclei spin in parallel to each other

  to make them spin anti-parrelel (with more E) we use radiowaves

  when we take the radiowaves away the nuclei relax

  when they relax they emit a tiny radio frequency of electromagnetic radiation

  this is NMR
• neutrons + protons = odd number then nucleus has a half integer spin (fraction)