Infra-red spectroscopy

Question 1

Wavenumbers

Answer 1

Absorption of electromagnetic radiation in the IR region causes bonds to vibrate, bend or stretch
Specific bonds do this at characteristic wavelengths
We use the wavenumber (reciprocal of wavelength) scale: inverse of the wavelength in cm

Question 2

Applications for infrared analysis

Answer 2

Rapid bond spotting check in synthetic chemistry
Identification of an unknown substance, raw material, pre-cursor comparison to data base
Polymorph detection
Characterise pharmaceutical preparations

Question 3

Instrumentation

Answer 3

Continuous wave: monochromator selects wavenumber, transmission measured, slow (scan in 2-3 mins), old technology
Fourier transform (FTIR): pulse, interferometer, data processed by FT equation, full spectra in one seconds, computerised, multiple scans, attenuated total reflectance (ATR)

Question 4

Nujol mull

Answer 4

Sample mixed with nujol (mineral oil) to form a mull
Spread onto a KBr disc, mounted in instrument
Water free

Question 5

The IR absorption: nuclear motion molecular vibrations

Answer 5

Spring force pointing in stretches the bond
Spring force pointing out compresses the bond
Oscillation is one movement in and out

Question 6

Vibrational modes

Answer 6

Nonlinear molecule with n atoms usually has 3n-6 fundamental vibrational modes
The greater the number of atoms, the more complex the stretching and bending, hence spectrum

Question 7

An infra-red spectrum

Answer 7

Transmission mode (sometimes absorbance)
<1600 cm: fingerprint region- complicated, unique to compound, useful for identification, compare unknown to reference database
>1600 cm: bond spotting region

Question 8

IR: active and inactive

Answer 8

To be IR active a change is dipole moment is essential
A polar bond is usually IR active
A non-polar bond in a symmetrical molecule will absorb weakly or not at all

Question 9

Relationship between mass and frequency

Answer 9

IR absorption depends:
on force constant (bond strength)
on m (not M)

Question 10

Heteroatom-hydrogen bond stretching

Answer 10

Large M, very small m = high dipole
Bonds with more s character (stronger bond) absorb at a higher frequency
If M and m not very different, lower frequency

Question 11

O-H and N-H stretching

Answer 11

Both of these occur around 3300cm, but they look different
Alcohol O-H, broad with rounded tip
Secondary amine (R2NH), broad with one sharp spike
Primary amine (RNH2), broad with two sharp spikes
No signal for a tertiary amine (R3N)

Question 12

Carbonyl stretching

Answer 12

The CO bond of simple ketones, aldehydes and carboxylic acids absorb around 1710 cm
Usually the strongest IR signal
Carboxylic acids will have OH also

Question 13

Carbon-nitrogen stretching

Answer 13

C-N absorbs around 1200 cm
C=N absorbs around 1660 cm and is much stronger than the C=C absorption in the same region
C triple N absorbs strongly just above 2200 cm, the alkyne signal is much weaker and is just below 2200 cm

Question 14

Strengths and limitations

Answer 14

IR alone cannot determine a structure (but it can with NMR
Some signals may be ambiguous
The functional group is usually indicated
The absence of a signal is definite proof that the functional group is absent
Correspondence with a known sample’s IR spectrum confirms the identity of the compound

Question 15

Attenuated total reflectance - FTIR

Answer 15

Pass IR through zinc selenide crystal
Incident IR wave undergoes total reflectance at 0
Entire wave transmitted
Apply a second medium capable of absorbing IR e.g. skin, change in detection proportional to level of evanescence wave