NMR

• What are the two main types of NMR spectroscopy?

  13C NMR and 1H NMR

  
• Why is modern NMR sensitive enough to give a full spectrum for C13?

  Because only around 1% C13 exists in organic molecules
• How does the C13 NMR spectrum compare to the H NMR spectrum?

  The C13 spectrum is simpler than the H NMR
• What does each signal in a C13 NMR spectrum represent?

  • One signal for each set of equivalent C atoms

  
• In a 1H NMR spectrum, what does each signal represent?

  One signal for each set of equivalent H atoms

  
• What does the intensity of each signal in 1H NMR indicate?

  It is proportional to the number of equivalent H atoms

  
• What are the groups of hydrogen atoms in ethanol?

  • 3 groups of different hydrogen atoms
  • Ratios: 3:1:2
• What type of solvents are used in NMR spectroscopy?

  Inert solvents with no 1H atoms

  
• Why are solvents like CCl4 and CDCl3 used in NMR?

  They have no H atoms to give peaks
• What is the purpose of adding TMS to the NMR sample?

  To calibrate the spectrum
• Why is TMS used as a calibration compound?

  • Only gives one signal
  • Signal is away from other H signals
  • Strong signal, small amount needed
  • Non-toxic and inert
  • Low boiling point for easy removal
• What does the chemical shift (δ) measure in NMR spectroscopy?

  How much the field has shifted from TMS

  
• What is the scale used for chemical shift in NMR?

  Parts per million (ppm)

  
• How does electronegativity affect the δ value in NMR?

  More electronegative groups give a greater shift
• How do hydrogen atoms bonded to nitrogen or oxygen appear in 1H NMR spectra?

  They usually appear as singlets

  
• What does the splitting of peaks in 1H NMR indicate?

  Number of inequivalent H’s on neighboring C atoms + 1

  
• What are the types of splitting in 1H NMR signals?

  • Singlet: 1 peak (0 neighboring H)
  • Doublet: 2 peaks (1 neighboring H)
  • Triplet: 3 peaks (2 neighboring H)
  • Quartet: 4 peaks (3 neighboring H)

  
• What does the integration trace in NMR indicate?

  It shows the relative number of H atoms
• How do you determine the empirical formula from elemental analysis?

  1. Divide percentage by atomic mass
  2. Find the simplest ratio
• How do you determine the molecular formula using the molecular ion peak?

  Use m/z value from mass spectrum
• What can IR spectra identify in a compound?

  Main bonds and functional groups
• What does the presence of a C=O bond indicate in a compound?

  It could be an ester or carboxylic acid
• How many different environments are indicated by 4 peaks in C13 NMR?

  Four different environments

  
• What does a singlet in NMR indicate about neighboring hydrogens?

  Adjacent to a carbon with no hydrogens

  
• What does a triplet in NMR indicate about neighboring hydrogens?

  Adjacent to a carbon with two hydrogens
• What does a quartet in NMR indicate about neighboring hydrogens?

  Adjacent to a carbon with three hydrogens
• How do you deduce the structure of a compound from NMR data?

  1. Analyze peak patterns and areas
  2. Identify functional groups from IR
  3. Use empirical and molecular formulas
  4. Combine data to propose structure
• What is the empirical formula derived from the elemental analysis percentages?

  C4H8O
• What does the absence of O-H absorptions in IR spectra suggest?

  It must be an ester
• What is the molecular formula determined from the molecular ion peak m/z value of 144?

  C8H16O2
• What does the peak at δ 4 in NMR indicate?

  H–C–O with area 2 suggests CH2

  
• What does the peak at δ 2.2 in NMR indicate?

  H–C=O with area 2 suggests CH2

  
• What does the peak at δ 1.2 in NMR indicate?

  1. CH3 with area 3 suggests CH3

  
• What is the final structure deduced from the NMR data?

  CCH3CCH3CH2COOCH2CH3