IR, NMR, Mass Spec

• What type of waves does NMR spectroscopy use?

  Radio Waves
• What type of waves does IR use?

  Infrared
• Molecular bonds can vibrate by bending or stretching
• The energy necessary to cause vibration depends on the type of bond
• The frequencies that are absorbed by the sample in IR tell us the type of bonds that are present
• What compounds are transparent to IR light?

  NaCl and KBr
• wavenumber equals the frequency divided by the speed of light
• The wavenumber for a stretching vibration depends on the bond strength and the mass of the atoms bonded together
• In IR, as mass increases, wavenumber decreases
• In IR, as bond strength increases, wavenumber increases
• The region above 1500cm-1 is called the diagnostic region and the region below is called the fingerprint region
• Resonance results in a lower wavenumber
• When a bond undergoes a stretching vibration, its dipole moment also oscillates, which creates an electrical field surrounding the bond
• The more polar the bond, the greater the opportunity for interaction between the waves, which leads to stronger IR signals
• Primary and secondary amines exhibit medium broad N-H stretching signals due to hydrogen bonding
• Due to polarity, O-H bonds are broader than N-H bonds
• A single molecule can only vibrate symmetrically or asymmetrically at any given moment
• Mass spectrometry is used to determine the molar mass and formula for a compound
• In mass spec, a compound is vaporized and then ionized, and the masses of the ions are detected and graphed
• The most common method of ionizing molecules is by electron impact (EI)
• If the radical cation remains intact, it is known as the molecular ion or parent ion
• The base peak is the tallest peak in the spectrum, and it tells us which fragment is the most stable in the positively charged state
• The molecular ion peak tells you the mass of the compound
• An odd massed molecular ion peak may indicate an odd number of N atoms in the compound, and an even massed molecular ion peak may indicate an even number of N atoms
• Molecules with chlorine have M+2 peaks in a 4:1 ratio
• Molecules with bromine have M+2 peaks in a 1:1 ratio
• Mass spec only detects charged fragments, and the name of the peak is the weight of the radical that is lost, and the height is the weight of the cation that remains
• Fragmentation will be more prevalent when more stable fragments are produced
• Alcohols generally undergo two main types of fragmentation: alpha cleavage and dehydration
• Amines generally undergo alpha cleavage
• Carbonyls generally undergo McLafferty arrangement
• One C12 atom is exactly 12 amu, because the amu scale is based on the mass of C12
• In Electrospray Ionization (ESI), a high voltage needle sprays a liquid solution of an analyte into a vacuum causing ionization; it is a softer technique
• Alkanes follow the formula CnHn+2 because they are saturated
• Adding a degree of unsaturation (1 pi bond or a ring) decreases the number of H atoms by two
• For the HDI scale (how many degrees of unsaturation), a halogen is treated as if it were a hydrogen atom, an oxygen atom does not affect the HDI, and a nitrogen increases the number of expected hydrogen atoms by one
• The HDI formula is 1/2(2C+2+N-H-X)
• If the HDI is 0, the molecule cannot have any rings, double bonds, or triple bonds
• NMR involves an interaction between electromagnetic radiation (light) and the nucleus of an atom
• If the total number of neutrons and protons is an odd number, the atoms will have a net nuclear spin