17 - spectroscopy

• What is mass spectra used for?

  Mass spectra can be used to identify the molecular mass of an organic product and gain further information about its structure.
• What is the M⁺ peak?

  The last, even numbered, peak on the mass spectrum.
• What does the M⁺ peak tell you?

  It indicates the molecular mass of the molecular ion and hence the original molecule.
• Where is the M+1 peak?

  Its a small peak one unit after the M⁺ peak.
• Why does the M+1 peak exist?

  It exists because 1.1% of carbon is present as the carbon-13 isotope.
• Fragmentation


  The process by which a mass spectrometer break down molecular ions into smaller pieces known as fragments.
• What are the other peaks in a mass spectrum caused by?

  They are caused by fragment ions formed from the brakdown of the molecular ion.
• What are atoms in molecules joined by?

  Covalent bonds.
• What do covalent bonds possess and do around a central point?

  Covalent bonds possess energy and vibrate naturally around a central point.
• What does the amount a covalent bond vibrates increase with?

  The amount of vibration is increasing with temperature.
• What happens when bonds absorb infrared radiation?

  They bend or stretch more.
• What is a stretch vibration?

  It's a rythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases.
• What is a bend vibration?

  It's a change in bond angle.
• What does the amount a bond stretched or bends depend on?

  1. The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms.

  2. The strength of the bond - stronger bonds vibrate faster than weaker bonds.
• Fingerprint region


  Below 1500 cm⁻¹ is known as the fingerprint region.

  It is difficult to predict the identity of a functional group from a peak in this region.
• Infrared spectroscopy


  It is used as a means of identifying the functional groups present in organic molecules.
• What is the peak that all organic compounds produce between 2850 and 3100 cm⁻¹?

  It's a characteristic peak from the presence of C-H bonds. It's often confused with the O-H peak in alcohols.
• What is the range of the absorbance peak of an O-H goup in an alcohol?

  3200 - 3600 cm⁻¹.
• What does the absorbance peak of an O-H group in an alcohol look like?

  Large smooth peak.

  
• What is the range of the absorbance peak of a C=O bond in a ketone or aldehyde?

  1630 - 1820 cm⁻¹.
• What does the aborbance peak of a C=O bond in a ketone or aldehyde look like?

  Smooth sharp peak.

  
• What are the two absorbance peak found in an IR spectrum of a carboxylic acid?

  - C=O peak.

  - O-H (carboxylic acid) peak.
• What is the range of the absorbance peak of a C=O bond in a carboxylic acid?

  1630 -1820 cm⁻¹.
• What is the range of the absorbance peak of a O-H bond in a carboxylic acid?

  2500 - 3330 cm⁻¹.
• What does the aborbance peak of a O-H bond in a carboxylic acid look like?

  Rigid broad peak.