IR_spectroscopy-upload by Munimadugu Naresh

Molecular VibrationsandIR Spectroscopy : 

Molecular VibrationsandIR Spectroscopy

Slide 2: 

Vibrations What is a vibration in a molecule? Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds Can a vibration change the dipole moment of a molecule? (Shockwave animation) Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active. Symmetrical stretching/bending does not. Not IR active

Slide 3: 

Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of 2500-15,000 nm or 2.5 – 15 mm) What wavelength of electromagnetic radiation is involved in causing vibrations in molecules? For a vibration at 4111 cm-1 (the stretch in H2), how many vibrations occur in a second? 120 x 1012 vibrations/sec or a vibration every 8 x 10-15 seconds! WOW !!!!! 120 trillion vibration per second!!!!

Slide 4: 

How does the mass influence the vibration? H2 I2 MM =2 g/mole MM =254 g/mole The greater the mass - the lower the wavenumber

Slide 5: 

How much movement occurs in the vibration of a C-C bond? For a C-C bond with a bond length of 154 pm, the variation is about 10 pm. For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm. stretching vibration bending vibration

A little physics of electromagnetic radiation : 

A little physics of electromagnetic radiation Energy (E) E = hn = hc/l where h is Planck’s constant, c is the speed of light, n is frequency or the number of vibrations per second and l is the wavelength Wavenumber (n’) n’ = 1/ l given in cm-1 Period (P) P = 1/n the time between a vibration = hcn’ Energy, frequency, and wavenumber are directly proportional to each other.

What type of vibrations would occur in pentane? : 

What type of vibrations would occur in pentane? Let’s examine the IR spectrum of pentane.

Slide 8: 

IR spectrum Increasing wavenumber (energy, frequency) Increasing absorption of IR radiation Increasing wavelength C-H stretching C-C bending C-H bending

Slide 9: 

IR Spectra of chloroform and deuterochloroform * Spartan ’02 AM1 minimization Shift of peak due to replacement of H with D (2x mass) Increasing absorbance

Some results : 

Some results Calculated values using computational software give lower wave numbers Increasing mass of substituted atoms shifts wave numbers to lower values (Excel spreadsheet) Stretching energies > bending energies > internal rotation energies (occur at higher wavelengths)

Slide 11: 

Does the stretching energy have any relationship to the strength of the bond? W = 6.3286BE + 401.38 r2 = 0.7979

Let’s examine the carbonyl group on three compounds : 

Let’s examine the carbonyl group on three compounds formaldehyde phosgene acetone How does the C=O stretching energy compare for these three molecules? 2053 cm-1 1951 cm-1 2063 cm-1 The carbonyl group has a range of 1700-3000 cm-1.

Functional group analysis in organic compounds : 

Functional group analysis in organic compounds Unlike atomic spectroscopy where sharp energy transitions occur due to well quantized electron transitions, molecular spectroscopy tends to show bands. Molecular vibrations are influenced by the surrounding groups!

Slide 14: 

4000 3000 2000 1000 WAVENUMBER (cm-1) Basic Functional Groups C-H O-H ChC C=C alkenes aromatic C=O C-O C-H O-H bending stretching C-C 400

Use of IR spectra : 

Use of IR spectra Identification of functional groups on a molecule – this is a very important tool in organic chemistry Spectral matching can be done by computer software and library spectra Since absorbance follows Beer’s Law, can do quantitative analysis