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Infrared SpectroscopyInfrared Spectroscopy
Gives information about the functional groups in a molecule
region of infrared that is most useful lies betweenregion of infrared that is most useful lies between2.5-16 2.5-16 m (4000-625 cmm (4000-625 cm-1-1))
depends on transitions between vibrational depends on transitions between vibrational energy statesenergy states
stretchingstretching
bendingbending
Infrared SpectroscopyInfrared Spectroscopy
Fig. 13.30
Fig. 13.31
IR Spectrum of HexaneIR Spectrum of Hexane
Fig. 13.32
Fig. 13.33
Fig. 13.34
Fig. 13.35
Fig. 13.36
OH
O
CH3
O
OH
OH
Ultraviolet-Visible (UV-VIS) Ultraviolet-Visible (UV-VIS)
SpectroscopySpectroscopy
Gives information about conjugated electron systems
gaps between electron energy gaps between electron energy levels are greater than thoselevels are greater than thosebetween vibrational levelsbetween vibrational levels
gap corresponds to wavelengthsgap corresponds to wavelengthsbetween 200 and 800 nmbetween 200 and 800 nm
Transitions between electron energy statesTransitions between electron energy states
EE = = hh
X-axis is wavelength in nm X-axis is wavelength in nm
Y axis is a measure of absorption of Y axis is a measure of absorption of electromagnetic radiation expressed as electromagnetic radiation expressed as molar absorptivity (molar absorptivity ())
maxmax is the wavelength of maximum is the wavelength of maximum
absorption and is related to electronic absorption and is related to electronic makeup of molecule,especially makeup of molecule,especially electrons electrons
Conventions in UV-VISConventions in UV-VIS
maxmax 230 nm 230 nm
maxmax 2630 2630
UV Spectrum of cis,trans-1,3-cyclooctadieneUV Spectrum of cis,trans-1,3-cyclooctadiene
* Transition in Alkenes* Transition in Alkenes
HOMO-LUMO energy gap is affected by HOMO-LUMO energy gap is affected by substituents on double bondsubstituents on double bond
as HOMO-LUMO energy difference as HOMO-LUMO energy difference decreases (smaller decreases (smaller EE), ), maxmax shifts to longer shifts to longer
wavelengthswavelengths
Methyl groups on double bond cause Methyl groups on double bond cause maxmax
to shift to longer wavelengthsto shift to longer wavelengths
CC CC
HH
HH
HH
HH
CC CC
HH
HH CHCH33
maxmax 170 nm 170 nm
CHCH33
maxmax 188 nm 188 nm
Extending conjugation has a larger effect Extending conjugation has a larger effect on on maxmax; shift is again to longer ; shift is again to longer
wavelengthswavelengths
CC CC
HH
HH
HH
HH
CC CC
HH
HH
maxmax 170 nm 170 nm maxmax 217 nm 217 nm
HH
CC CC
HH
HHHH
maxmax 217 nm 217 nm
(conjugated (conjugated dienediene))
HH
CC CC
HH
HH CC CC
HH
HHHH
CC CC
HH CHCH33
HH
HH
CC CC
HH33CC
HH CC CC
HH
HH
maxmax 263 nm 263 nm
conjugated conjugated trienetriene plus plus two methyl groupstwo methyl groups
LycopeneLycopene
maxmax 505 nm 505 nm
orange-red pigment in tomatoesorange-red pigment in tomatoes
Mass SpectrometryMass Spectrometry
mass spec is different because it is not mass spec is different because it is not related to electromagnetic radiationrelated to electromagnetic radiation
Atom or molecule is hit by high-energy electron Atom or molecule is hit by high-energy electron from an electron beam at 10evfrom an electron beam at 10ev
ee––
beambeam
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
forming a positively charged, odd-electron forming a positively charged, odd-electron species called the species called the molecular ionmolecular ion
ee––
++••
Molecular ion passes between poles of a Molecular ion passes between poles of a magnet and is deflected by magnetic fieldmagnet and is deflected by magnetic field
deflection depends deflection depends on mass-to-charge ratioon mass-to-charge ratio
highest m/z highest m/z deflected leastdeflected least
lowest m/z lowest m/z deflected mostdeflected most
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++••
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
If the only ion that is present is the molecular If the only ion that is present is the molecular ion, mass spectrometry provides a way to ion, mass spectrometry provides a way to measure the molecular weight of a compound measure the molecular weight of a compound and is often used for this purpose.and is often used for this purpose.
However, the molecular ion often fragments to However, the molecular ion often fragments to a mixture of species of lower m/z.a mixture of species of lower m/z.
The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++••
The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++ ••
Usually several fragmentation pathways are Usually several fragmentation pathways are available and a mixture of ions is produced.available and a mixture of ions is produced.
mixture of ions of mixture of ions of different mass different mass gives separate peak gives separate peak for each m/zfor each m/z
intensity of peak intensity of peak proportional to proportional to percentage of each percentage of each ion of different ion of different mass in mixturemass in mixture
separation of peaks separation of peaks depends on relative depends on relative massmass
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
++
++++
++
+
+
mixture of ions of mixture of ions of different mass different mass gives separate peak gives separate peak for each m/zfor each m/z
intensity of peak intensity of peak proportional to proportional to percentage of each percentage of each ion of different massion of different mass
separation of peaks separation of peaks depends on relative depends on relative massmass
++ ++ ++ ++
+ +
Principles of Electron-Impact Mass SpectrometryPrinciples of Electron-Impact Mass Spectrometry
Fig. 13.39
m/z m/z = 78= 78
Some molecules undergo very little Some molecules undergo very little fragmentationfragmentation
Benzene is an example. The major peak Benzene is an example. The major peak corresponds to the molecular ion.corresponds to the molecular ion.
The largest peak is called the base peak and is The largest peak is called the base peak and is reference to 100 to give relative intensity.reference to 100 to give relative intensity.
IsotopesIsotopesin Chlorobenzenein Chlorobenzene
3535ClCl 3737ClCl
Isotopic ClustersIsotopic Clustersin Chlorobenzenein Chlorobenzene
ion responsible for ion responsible for m/m/zz 77 peak does 77 peak does not contain Clnot contain Cl
HH
HH
HH
HH
HH ++
Alkanes undergo extensive fragmentationAlkanes undergo extensive fragmentation
Decane, CDecane, C1010HH2222
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
The largest peak may not be the parent ion, The largest peak may not be the parent ion, or may not be visible at all.or may not be visible at all.
Propylbenzene fragments mostlyPropylbenzene fragments mostlyat the benzylic positionat the benzylic position
9191
CHCH22—CH—CH22CHCH33
120120
Molecular FormulaMolecular Formula
as aas a
Clue to StructureClue to Structure
Molecular WeightsMolecular Weights
One of the first pieces of information we try to One of the first pieces of information we try to obtain when determining a molecular obtain when determining a molecular structure is the molecular formula.structure is the molecular formula.
We can gain some information about molecular We can gain some information about molecular formula from the molecular weight. formula from the molecular weight.
Mass spectrometry makes it relatively easy to Mass spectrometry makes it relatively easy to determine molecular weights.determine molecular weights.
The Nitrogen RuleThe Nitrogen Rule
A molecule with an A molecule with an odd number of odd number of nitrogens has an odd nitrogens has an odd molecular weight.molecular weight.
A molecule that A molecule that contains only C, H, contains only C, H, and O or which has and O or which has an even number of an even number of nitrogens has an nitrogens has an even molecular even molecular weight.weight.
NNHH22 9393
138138
NNHH22OO22NN
183183
NNHH22OO22NN
NNOO22
Exact Molecular WeightsExact Molecular Weights
CHCH33(CH(CH22))55CHCH33
HeptaneHeptane
CHCH33COCO
OO Cyclopropyl acetateCyclopropyl acetate
Molecular formulaMolecular formula
Molecular weightMolecular weight
CC77HH1616 CC55HH88OO22
100100 100100
Exact massExact mass 100.1253100.1253 100.0524100.0524
Mass spectrometry can measure exact masses.Mass spectrometry can measure exact masses.
Therefore, mass spectrometry can be used to Therefore, mass spectrometry can be used to distinguish between molecular formulas.distinguish between molecular formulas.
Index of Hydrogen DeficiencyIndex of Hydrogen DeficiencyDegree of UnsaturationDegree of Unsaturation
relates molecular formulas to relates molecular formulas to multiple bonds and ringsmultiple bonds and rings
For a molecular formula, CFor a molecular formula, CccHHhhNNnnOOooXXxx, the
degree of unsaturation can be calculated by:
Degree = ½ (2c + 2 - h - x + n)
Rings versus Multiple BondsRings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum ofIndex of hydrogen deficiency tells us the sum ofrings plus multiple bonds.rings plus multiple bonds.
Using catalytic hydrogenation, the number ofUsing catalytic hydrogenation, the number ofmultiple bonds can be determined.multiple bonds can be determined.