228-ftir reviewws12

8/2/2019 228-FTIR ReviewWS12

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FTIR Review

Chem 228

WS/2012


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The Electromagnetic Spectrum

Radiation in the infrared region (IR) corresponds to

stretching, bending, etc. of chemical bonds.


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Modeling of IR Absorption

Bonded atoms can be modeled as masses on springs.


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k is proportional to the bond strength.

m is the atomic mass of the bonded atoms.


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IR Spectra IR frequencies which cause resonance of the

masses/springs are absorbed selectively.

IR absorption measurements are therefore

useful for determination of organic functional

groups. (~4000-400 cm-1)

Symmetrical groups may NOT show up in IR

due to requirement for a bond dipole moment.


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Fingerprint IR Absorptions In addition to the fundamental resonance

frequencies, absorptions corresponding to

harmonics (overtones, combinations) occur inthe fingerprint region of the IR spectrum

(~1500-400 cm-1).

Pattern matching in the IR fingerprint region

is useful to identify the specific compound

involved.


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Grating IR Spectrometer


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FTIR Spectrometer


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Nicolet 750M

agna-IR This is the FTIR in the organic lab.


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FTIR of Liquid Samples NaCl plates are used for liquids since glass

would absorb in the IR.

A thin layer of liquid is sandwiched betweentwo NaCl plates and placed in the beam path.


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Liquid Samples-Sealed Cells Sealed cells allow IR of volatile liquids

and quantitative measurements, since

the sample thickness can be controlled.


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Liquid or Solid Samples-ATR ATR=Attenuated Total Reflectance

IR beam is reflected off surface of

sample via a high RI prism (ZnSe).


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FTIR of Solid Samples Solids are usually run as KBr disks or Nujol (mineral oil) mulls.

KBr disk is prepared by grinding the sample with KBr and

pressing in a die to form a transparent disk.

Nujol mull is prepared by grinding the sample in mineral oil.


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FTIR of Solid Samples KBr disks can be hard to make well.

Spectral artifacts (extra peaks) may occur

if the disk is poorly made.

Nujol mulls show hydrocarbon IR

spectrum superimposed on sample

peaks.


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FTIR of Solid Samples KBr disk holder.


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FTIRM

icroscope Microscope uses mirrors to avoid the need for

NaCl lenses.

(We do not currently have one of these.)


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FTIRM

icroscopy


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Fiber Optic FTIR Dip Probe Useful for continuous process monitoring.


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FTIR Spectral Interpretation Most organic functional groups absorb

somewhere between 4000-400 cm-1.

Most organic compounds contain C-H

single bond absorptions, which make a

convenient visual reference.


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FTIR Spectral Interpretation Normal organic IR range is 4000-400 cm-1

Energy is directly related to cm-1

E=(k/m)0.5, so going from C-C to C=C to triple

bonds should shift peak location to left

(higher energy) on plot. Substituting D for H would shift peak location

to right (lower energy) on plot.


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FTIR Spectral Interpretation Table (visual)


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2,3-dimethylbutane


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n-hexane vs 2,3-dimethylbutane IRs are very similar-not the best method to

identify, since C skeleton is the main difference.


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13C NMR would be best for

n-hexane vs 2,3-dimethylbutane n-hexane 2,3-dimethylbutane

1H

13C


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toluene


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1-hexene


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1-heptyne


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Heptyl cyanide


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3-heptanone


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heptaldehyde


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1-hexanol


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1-propanol vs 2-propanol IR shows OH group.

Pattern matching in the fingerprint region or

NMR could determine which alcohol.


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1-hexylamine


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Heptanoic acid


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Ethyl acetate


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Butyric anhydride


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IR-Problems Impurities also absorb, making spectral

interpretation difficult or misleading.

(eg. moisture in sample)

Film thickness for liquids may alter

relative intensity of peaks.

KBr disk technique for solids maygenerate artifacts.


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Sample Thickness Effect on FTIR

Too thick

Thickness OK

228-ftir reviewws12

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