Infrared spectroscopy

(IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum.

advantages : - easy - inexpensive. - non destructive method.

What is IR absorptionIt deals with the absorption of light which

result In a vibrational transition in the IR region.

The IR region of most interest to an organic chemist lies between 4000 cm-1 to 600 cm-1.

IR can be used both as a qualitative and a quantitative tool.

Qualitative:

Elucidation of the structure:Each functional group has its own characteristic IR freq. that can be used to identify this group.

Optical isomers can be identified.

transition

fundamental

Stretching vibration

Symmetric

Asymmetric

Bending vibration

OOP In the plane

Non fundamental (over tone)

instrument

A beam of infrared light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. The beams are both reflected back towards a detector, however first they pass through a splitter which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained.

A reference is used for two reasons:This prevents fluctuations in the output of the

source affecting the data This allows the effects of the solvent to be

cancelled out (the reference is usually a pure form of the solvent the sample is in)

instrument 1-Source of light. 2-Sample cell: glass or quartz can not be used → they

absorb IR radiation.The sample cell is made of alkali

halide( window) can be circular or rectangular.

3- monochromator 4- detector.

Before running the sample , the instrument must be calibrated by using polystyrene film,

If the instrument is calibrated well it will give 11 peak in different areas (4000-650) cm-1

4000-2000 +- 8 2000- 650 +- 4

Sampling Sample used may be liquid ,gas or solid. 1- liquid sample:A drop of liquid sample is placed b/w two

plates ( sandwich) (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used). The plates are transparent to the infrared light and will not introduce any lines onto the spectra. Some salt plates are highly soluble in water, so the sample and washing reagents must be anhydrous (without water)..

2- gas sample → special kind of cells.

3-solid samples:a)- mulls: dissolve the sample in a mineral oil called

nujol oil → one drop into the window . nujol oil has an absorbent in IR so it reading

should be removed.

b)- deposit film :Dissolve the sample in a volatile solvent →the solvent will

volatile and a deposit film remains.

c)-Pellets: solid sample is diluted with KBr or Nacl (1:200), mix well then put in a special room where it is

compressed to a transparent pellets.

The spectra is divided into 3 regionsWhich are:

4000 cm-1 1300 cm-1 909 cm-1 650cm-1% T 1- high

frequency ( 4000- 1300) cm-1→ main functional group ex.- OH , NH2

3- intermediate frequency ( 1300-909 ) cm-1 → finger print region( if two compound have same absorption in low or high region it will not overlap in the intermediate region).

2-Low frequency ( 909-650) cm-1 → if the group is non aromatic.

λμm

Factor effecting absorption:

1- type of vibration.

2- type of atoms.

3- type of bond

1- type of vibration:Non fundamental needs energy more than fundamental.

Stretching need energy more then bending.

2- types of atoms: different atoms requires more energy than similar atoms. E.x:

c=c 1600 cm-1 c=o 1700 cm-1

3- types of bond : single → 600 cm-1 double → 1600 cm-1

triple → 2000 – 2100 cm-1

Interpretation of the chart:

to describe an absorption band we identify

1- peak shape. 2- position . 3- intensity.

1- shape ; ex. broad , Sharpe or forked not all functional group have a characteristic shape so we will depend on the position more.

2-intensity

3- position ex. 650 or 2001 cm-1

1- Cm-1 = 1/μm x 10 ⁴ Cm-1 = 1/nm x 10 ⁷ Cm-1 = 1/λ

2- v` = frequency. π = 3.14 c = light speed 3x10 10 cm/sec

k = force constant dyne / cm μ= mass g

3- vibration mod ( if linear or non linear ):

a- 3n-5 linear b- 3n-6 non linear.

conjugationWhen I say the compound is aromatic?1- c=c stretching.

2- c-H stretching aromatic above 3000 cm-1

3- c-H bending OOP at 1000 – 650 cm-1

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