Structure Determination

• 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 there is only around 1% C13 in organic molecules

  
• How does the C13 NMR spectrum compare to the H NMR spectrum?

  The C13 spectrum is simpler than the H NMR spectrum
• In a C13 NMR spectrum, how many signals are there for equivalent carbon atoms?

  One signal for each set of equivalent C atoms

  
• How many peaks are observed for 1,4 dinitrobenzene in C13 NMR?

  3 peaks

  
• How many peaks are observed for 1,3 dinitrobenzene in C13 NMR?

  4 peaks

  
• How many peaks are observed for 1,2 dinitrobenzene in C13 NMR?

  2 peaks

  
• In a 1H NMR spectrum, how many signals are there for equivalent hydrogen atoms?

  One signal for each set of equivalent H atoms

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

  The intensity is proportional to the number of equivalent H atoms

  
• How many groups of different hydrogen atoms does ethanol have?

  3 groups
• What is the ratio of equivalent H's in ethanol?

  1. 1:2:3
• 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 and will not give peaks in the 1H NMR spectrum

  
• What is the purpose of adding TMS to the sample in NMR spectroscopy?

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

  It only gives one signal and is away from other H signals
• What does the chemical shift (δ) measure in NMR spectroscopy?

  How much the field has shifted away from TMS
• What is the scale used for chemical shift in NMR?

  Parts per million (ppm)
• How does the presence of electronegative groups 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?

  It indicates the number of inequivalent H's on neighboring C atoms plus one

  
• What is the empirical formula calculation based on in elemental analysis?

  Percent composition of each element
• How do you determine the molecular formula from the empirical formula?

  Using the molecular ion peak m/z value from mass spectrum
• What can IR spectra identify in a compound?

  Main bonds or functional groups
• What does the presence of a C=O bond and absence of O-H absorptions indicate?

  The compound must be an ester
• How many different environments are indicated by 4 peaks in C13 NMR?

  4 different environments

  
• What does a singlet in NMR indicate about the adjacent carbon?

  It is adjacent to a carbon with no hydrogens

  
• What does a triplet in NMR indicate about the adjacent carbon?

  It is adjacent to a carbon with two hydrogens
• What does a quartet in NMR indicate about the adjacent carbon?

  It is adjacent to a carbon with three hydrogens
• How do you determine the final structure of a compound using NMR?

  By analyzing the peaks and their integration values

  
• What are the steps to determine the empirical formula from elemental analysis?

  1. Divide the percentage of each element by its atomic mass.
  2. Find the simplest whole number ratio.
  3. Write the empirical formula based on the ratio.
• How do you use NMR spectra to analyze a compound's structure?

  • Identify the number of peaks for different environments.
  • Analyze the integration values for hydrogen counts.
  • Determine the splitting patterns for neighboring hydrogen atoms.
• What is the significance of the molecular ion peak in mass spectrometry?

  • Indicates the molecular weight of the compound.
  • Helps in determining the molecular formula.
• What are the characteristics of TMS in NMR spectroscopy?

  • Provides a single signal for calibration.
  • Non-toxic and inert.
  • Easily removable due to low boiling point.
• What is the role of chemical shift (δ) in NMR spectroscopy?

  • Measures the shift of the signal from TMS.
  • Indicates the electronic environment of the hydrogen atoms.