Infrared Spectroscopy Gives information about the functional groups in a molecule.

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


CHCH33


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


(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


conjugated conjugated trienetriene plus plus two methyl groupstwo methyl groups

LycopeneLycopene


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


++••


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.


++••

The molecular ion dissociates to a cationThe molecular ion dissociates to a cationand a radical.and a radical.


++ ••

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


++

++++

++

+

+

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

++ ++ ++ ++

+ +


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.

Infrared Spectroscopy Gives information about the functional groups in a molecule.

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