Ch 12 IR and Mass Spectrometry Flashcards
Infrared spectroscopy
Measures bond vibration freq.s in molecule
Used to determine functional group
Mass spectrometry
Fragments molecules and measures masses
Does not involve direct approach of light
Nuclear magnetic resonance NMR
Detects signals from hydrogen (and c and other atoms)
Can be used to distinguish isomers
Most informatics techniques
Ultraviolet spectroscopy
Uses electron transitions to determine bonding patterns
Invisible light region of electromagnetic spectrum
Spectroscopy
Analytical technique to help determine structure
Use of absorption, emission, or scattering of emagnetic radiation by matter to study physical processes
Wavelength
Physical distance in direction of propagation a photon makes one complete oscillation
Relationship of frequency and wavelength
Inversely proportiona
Photon
Discrete packet of energy w magnetic field and electric field components
These fields are perpendicular/orthogonal to each other and to direction of propagation of photon
Flip direction as photon travels
Frequency
Number of flips/oscillations that occur in one second
E=hv
Energy is plancks constant x frequency
v=c/λ
Frequency is spd of light divided by wavelength
E=hc/λ
Energy is plancks x spd of light / wavelength
Infrared region
Vibrational IR : 2.5 micrometers to 25 micrometers
Absorption of IR radiation in this region causes bonds to change from lower vibrational energy level to higher (causes chemical bonds)
Wave numbers
Frequency of IR radiation expressed in wave numbers
Number of waves per centimeter (Can be directly translated to v)
Vibrational IR from 4000 cm^-1 to 400cm^-1
Higher wave number, higher energy
Covalent bonds
Two vibrating masses connected by chemical bonds
As vibrates, energy changes btwn KE and PE and vv
Total energy EsubK + EsubP is proportional to v of vibration
Energies associated with bonding vibrations
Are quantized –> in a molecule, only certain vibrational energy levels are allowed
Usually btwn 2-10kcal/mol
(Within IR rgn of 1-11)
IR absorption positions
Affected by
- strength of bond (stronger bonds at higher freqs)
- masses of atoms in bond (lighter masses at higher freqs)
- type of vibration observed (stretching at higher freqs than bending)
IR absorption intensity
-overall peak intensity directly related to [ ] of sample
-relative peak intensity is additive
large number of similar grps will increase intensity of given peak
-relative peak intensity influ. by dipole moment
Stretching frequencies
Heavier mass, decrease stretching frequency
Frequency increases with increasing bond energies
How many vibrations allowed for nonlinear molecule?
3n-6
Simplest vibrational motions
Stretching and bending
Fingerprint region
600-1400cm^-1
Complex vibrations
How many molecules will give same IR spectrum?
None
Except enantiomers
Stretching freq.s are _________ than corresponding bending freq.s
Higher
It is easier to bend a bond then stretch/compress it
Bonds to hydrogen have _________ stretching freq.s than those to heavier atoms
Higher
Triple bonds have ________ stretching freq.s that corresponding double bonds
Higher
Double bonds have ________ frequencies than single bonds
Higher
Except for bonds to hydrogen
O-H
3200-3650cm^-1
Strong and broad
Strong bc polar–> polar bonds, strong IR absorption
N-H
3100-3500cm^-1
Medium
C-H
2850-3300
Medium to strong
C=O
1630-1810
Strong
Very important
C=C
1600-1680
Weak
Usu hard to see
C-O
1050-1250
Strong
For a molecule to absorb IR radiation, bond undergoing vibration…
- must be polar
- it’s vibrations must cause periodic change in bond movement
(Dipole moment leads to active IR absorption)
IR inactive
Covalent bonds that don’t meet criteria (polar and vib.s that cause periodic change in movement)
C-C double and triple bonds of symmet. substituted alkenes and alkynes don’t absorb rad. bc they aren’t polar bonds
Compression/stretch doesn’t lead to dipole moment
FT-IR spectrometer
Faster Uses interferometer Better sensitivity Less energy req. from source Scan takes ~1-2 sec Laser beam keeps it calibrated
IR spectroscopy of hydrocarbons
Stronger bonds absorb at higher freq.s
C-C, C=C, C|||C
Conjug lowers freq
Isolated C=C,conjugated C=C, aromatic C=C
IR absorption
C-C
C=C
C|||C
1200cm^-1
1660
2200
(Weak or absent if internal, bc C atoms not diff from each other)
IR absorption
Isol. C=C
Conj. C=C
Aromatic C=C
1640-1680 cm^-1
1620-1640
~1600
C-H bond stretching
Bonds w higher s character absorb at higher frequency
Sp^3 CH absorption freq
Just below 3000cm^-1
Sp^2 CH absorption freq
Just above 3000cm-1 ( to left)
Sp CH absorption freq
At 3300cm^-1
OH absorption freq and pattern
Around 3300cm^-1
Broad with rounded tip (don’t confuse with amines)
Looks different bc O is more eneg than N, so looks diff
Secondary amine freq and absorption pattern
Around 3300cm^-1
R2NH- broad with one sharp spike
Only one sharp spike bc only one N-H bond
Primary amine freq and absorption pattern
Around 3300cm^-1
RNH2- broad with two sharp spikes (one per N-H bond)
C=O bond absorption freq
Around 1710cm^-1
1715+/- 5cm^-1
Usually strongest IR signal
Carboxylic acids IR freq
Will have C=O and O-H absorption freqs (broad btwn 2500-3500cm^-1)
O-H absorbs broadly bc of strong H bonding- gigantic broad stretch confirmed by C=O
Aldehydes absorption freq
C=O and two C-H signals around 2700-2800cm^-1
