Infrared SpectroscopyChem 4010/5326:

Organic Spectroscopic Analysis

© 2015 Andrew Harned

Infrared Spectroscopy

Hooke’s LawBonds can be thought of like a spring

Wavenumbers can be approximated by Hooke’s law

√ 2πc1

ν =(m1 + m2)

m1m2K • m = mass of atom

K = force constant of bond

weaker bonds (lower K), lower ν

Hooke’s Law

if mass of either atom changes, ν changes

Bonds can be thought of like a spring

Wavenumbers can be approximated by Hooke’s law

√ 2πc1

ν =(m1 + m2)

m1m2K • m = mass of atom

K = force constant of bond

Infrared SpectroscopyOnly vibrations producing a change in dipole moment are observed

i.e. “large” electronegativity differences

What does this mean?

Infrared Spectroscopy

Fundamental VibrationsFundamental stretching and bending vibrations for CH2

Similar vibrations for other groups

Stretching occurs at higher wavenumbers than bending

Sample Handling•  Gas, liquid or solid•  Solution or neat liquids

•  Can subtract out solvent by taking background/reference spectrum•  Solids can be deposited as thin film

•  evaporation from some solvent-  CCl4 is best, but now expensive/hard to find-  CHCl3 & CH2Cl2 next best, do not obscure much of spectrum

•  pressed into KBr pellet-  can be difficult to get right

•  mixed with Nujol (petroleum oil, high boiling)-  obscures aliphatic region

•  Samples typically applied to polished NaCl plate (IR inactive)•  Teflon tape can also be used if stretched thin•  Adventitious water can be a problem

•  False ID of ROH, RCO2H, R2NH

Some Newer IR Techniques•  ATR allows for direct examination of samples in the solid or liquid phase without further preparation.

Attenuated Total Reflectance

•  The sample is applied to a crystal (ZnSe, Ge, Diamond) with high refractive index and pressure is applied.•  The IR beam is passed thorugh the crystal such that it reflects off the internal surface in contact with the sample at least once.

https://en.wikipedia.org/wiki/Attenuated_total_reflectance

•  The reflection forms an evanescent wave that penetrates 0.5–2 µM into the sample.•  The attenuated energy from each evanescent wave is passed back into the beam

Some Newer IR TechniquesReactIR

•  Because the evanescent wave only penetrates a few microns into the sample, highly absorbing samples are not a problem. Only the sample in intimate contact with the crystal is detected.•  This means that strongly absorbing solvents are not a problem.•  ReactIR technology allows IR data to be collected from solutions (i.e., reactions) in real time under relevant conditions.

ReactIR 15 from Mettler ToledoRueping, M.; Bootwicha, T.; Sugiono, E. Beilstein J. Org. Chem. 2012, 8, 300.

What does spectrum look like?

① ② ③ ④

4 Main Regions

Potential ProblemsOvertones

1st overtones at twice normal νtypically weak

Potential ProblemsFermi Resonance

Overtone falls close to a fundamental band. Intensity is greatly enhanced.Usually results in “doubling” of fundamental band

A Word Of Caution•  IR rarely used as a stand alone technique

•  Quick, easy, reliable method for determining presence or absence of functional groups

•  Very limited information about connectivity

•  Often 1H NMR & 13C NMR will give the same information and can be more reliable

•  IR is much cheaper and some functional groups are NMR silent (e.g. diazos, isocyanates, and azides)

InterpretationHydrogen stretching region: O-H

–O-H (3600-3400 cm-1): appearance dependent on concentration

Broadness due to H-bondingIntermolecular H-bonding: dilution diminishes, sharpens peak

Intramolecular H-bonding: dilution does nothing

InterpretationHydrogen stretching region: N-H

–N-H (3400-3200 cm-1)

InterpretationHydrogen stretching region: C-H

–C-H (3300-2700 cm-1): exact position dependent on hydridization

Interpretationsp carbon region

InterpretationDouble bond region: C=O

–C=O (1850-1640 cm-1): exact position dependent on substitutionsand is close to being diagnostic

InterpretationDouble bond region: C=O

Structural effects: true for all members of the family

InterpretationDouble bond region: C=O

Structural effects: true for all members of the family

InterpretationDouble bond region: C=O

Structural effects: true for all members of the family

InterpretationDouble bond region: C=O

Structural effects: true for all members of the family

InterpretationDouble bond region: C=O

Structural effects: true for all members of the family

InterpretationDouble bond region: C=O

Carbonyls besides ketones and aldehydes

InterpretationDouble bond region: C=O

Carbonyls besides ketones and aldehydes

InterpretationDouble bond region: C=O

Carbonyls besides ketones and aldehydes

InterpretationDouble bond region: C=C

–C=C (1680-1580 cm-1): exact position dependent on substitutionsmedium to weak absorbances

conjugated

Phenyl & Aromaticseveral bands

~1620 & 1580 cm-1

diagnostic peakselsewhere

InterpretationFingerprint region

–1600-600 cm-1: unique to each compound

-  a few unique bands-  lots of overlap-  often used along with other regions to confirm (unique to each compound)

InterpretationFingerprint region

–1600-600 cm-1: unique to each compound

InterpretationFingerprint region

–1600-600 cm-1: unique to each compound

InterpretationFingerprint region

–1600-600 cm-1: unique to each compound

InterpretationFingerprint region

– aromatic C–H bending

InterpretationFingerprint region

– aromatic overtones

Can be quite useful in determining substitution around benzene rings, but are often very weak and hard to see.

NMR has become more reliable.

InterpretationFingerprint region

–1600-600 cm-1: unique to each compound

Can complicate identification of C=C and aromatic substitution