Infrared spectroscopy depends upon a change in the vibrational state - - PowerPoint PPT Presentation

Infrared spectroscopy

• depends upon a change in the vibrational state of a molecule
• here, ν = about 1014 Hz which corresponds to λ = 2.5 - 16 x 10-6 m
• normally for convenience use wavenumbers, 1/λ cm-1, when referring to the IR frequency

causing bond vibration 4000 - 200 cm-1 (higher E) (lower E)

C C CH3 H C C CH3 H

Eo E* ∆E = E* - Eo = hν

The vibrational behavior of the chemical bonds in molecules is well-described by Hooke’s Law

ν π µ

= ( /

) / 1 2 c k

so the frequency of the vibration depends upon

• the masses of the bonded atoms: the greater the difference, the lower the frequency
• the strength of the bond: the weaker the bond, the lower the frequency
• a change in the dipole moment of the bond as it vibrates

hν

How an IR spectrometer works detector source sample (a heater) (film sandwiched (thermometer) between salt plates)

• as the wavelength of radiation is varied, is the detector hot or not?

hν

absorbance 100 = cold 0 = hot ν X X X X X X