Infrared Spectroscopy Flashcards
Infrared Spectroscopy (IR):
Measures vibrations of bonds to identify functional groups
Ultraviolet Spectroscopy (UV):
Measures electronic transitions to provide information on the electronic bonding in the sample
Mass Spectrometry (MS):
Breaks the molecule into fragments (destructive) and measures the MW of fragments to give information to the structure and functional groups present
Nuclear Magnetic Resonance (NMR):
Uses electromagnetic fields to measure spinning of nuclei to identify chemical environments leading to identification of functional groups and structure of the sample
What is he spectrum called?
Electromagnetic Spectrum
Electromagnetic Radiations
Travel at the speed of light (c) but at different frequencies and wavelengths
Frequency (v)
= number of complete wave cycles that pass a fixed point per second, Hz (cycles per second)
Wavelength (λ) =
distance between any two peaks
Relationship:
λ = c/v, or c = λv, or v = c/ λ
Relationship:
λ = c/v, or c = λv, or v = c/ λ
Photons =
massless packs of energy E = hv (h = Planck’s Constant)
Molecule struck by a photon:
may absorb the photon’s energy and will result in the molecule’s energy increasing by an amount equal to the photon’s energy E = hv
Infrared Region
Molecular vibrations can be detected in the infrared region.
The covalent bonds between atoms act like springs. If the bond is stretched / compressed and then released, the atoms vibrate
Heavier atoms vibrate more ______.
Frequency decreases with __________ atomic mass/
slowly
increasing
Stronger bonds are more ______ so they vibrate faster.
More ______ needed to stretch / compress (higher frequency)
rigid
force
What are the three vibrational modes?
1 symmetric stretching
2) antisymmetric stretching
3) Bonding (scissoring)
Non-linear:
Non-linear molecules with n atoms will have 3n – 6 vibrational modes (i.e. 6 atoms = 12 modes)
What are the combinations and overtunes of fundamental vibrations called?
complex vibrations
IR-Active and IR-Inactive vibrations
The bond must have a dipole moment.
The vibration must change the dipole moment of the molecule
IR-Inactive:
charicteristic
Symmetrical / has no dipole moment
Infrared Spectroscopy of Hydrocarbons
stronger bonds absorb at higher frequencies because of their greater stiffness
c-c bond stretching
- isolated
- conjugated
- aromatic
Different O-H stretching frequencies: important
Alcohol (e.g. CH3OH); 3300 cm-1, Broad
Acid (e.g. CH3COOH)l 3300 cm-1, Broad
Different O-Hstretching frequesncies:
Alcohol (e.g. CH3OH); 3300 cm-1, Broad
Acid (e.g. CH3COOH)l 3300 cm-1, Broad
Different C-O frequencies
Alcohols and ethers: 1000 – 1200 cm-1
N-H Stretching Frequencies
Amine: 3300 cm-1 , Broad
1 Spike = N-H of secondary amine (R2NH)
2 Spikes = N-H of primary amine (RNH2)
C=O stretching frequencies:
Intense absorption at 1700 cm-1
Small overtone peaks at 3400 cm-1
Aldehyde: 1725 cm-1
Ketone: 1710 cm-1
C-H stretching frequencies:
Aldehyde has two: 2700 and 2800 cm-1
Infrared Spectroscopy of Ketones & Aldehydes
ketone - 1710cm-1
aldehyde - 1724 / 2700, 2800
Strengths and Limitations of IR Spectroscopy
- Ability to indicate functional groups in the compound
- Highlights the absence of other functional groups that would give strong absorptions if they were present
- Can confirm identity of a compound by comparison with a known sample
- It is non-destructive (sample can be retrieved)
what is an overtone in a graph?
any resonant frequency above the fundamental frequency
(does not mean anything)
revice sprectrum
lecture slides - identify absorption location/ intensity of each functional group
What is IR:
a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves
