Infrared Spectroscopy and Mass Spectrometry
2Introduction •Spectroscopy is a technique used to determine the structure of a compound •Most techniques arenondestructive •Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength
3Types of Spectroscopy •Infrared (IR) spectroscopymeasures the bond vibration frequencies in a molecule and is used to determine functional groups •Mass spectrometry (MS)fragments the molecule and measures their mass.MS can give the molecular weight of the compound and functional groups •Nuclear magnetic resonance (NMR)spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present •Ultraviolet (UV) spectroscopyuses electronic transitions to determine bonding patterns
4Electromagnetic Spectrum •Frequency and wavelength are inversely proportional c=lnl= c/n wherecis the speed of light (3 x 1010cm/sec) •Energy of thephoton is given by E =hn wherehis Planck’s constant (6.62 x 10-37 kJ sec)
5The Electromagnetic Spectrum
6The Infrared Region (IR) •Below red •Wavelengths are usually 2.5 x 10-4 to 25 x 10-4 cm •Units are wavenumbers, or cm -1 400 to 4000 cm-1 •Wavenumbers are proportional to frequency and therefore to energy
Interaction between Light and Matter •Energy levels are separated by an energy gap (∆E) •Absorption ofexactlythis energy causes changes •Energy is temporarily stored and released back–usually in form of heat
Energy Gap •The energy needed to cause vibrations depends on nature of bond
Vibrational Excitation •Stretching = continuous change in interatomic distance along axis of bond between two atoms •Bending = change in bond angle
10Molecular Vibrations •Bond is stretched, a restoring force pulls the two atoms together toward their equilibrium bond length •Bond is compressed, the restoring force pushes the two atoms apart •If the bond is stretched or compressed and then released, the atoms vibrate
Molecular Vibrations Frequency of stretching vibration depends onmassof atoms andstiffnessof the bond •Group of bonds with similar bond energies: Frequency decreases with increasing atomic weight
Molecular Vibrations •Stronger bonds are generally stiffer •Stronger bonds usually vibrate faster than weaker bonds •Group of bonds having atoms of similar masses: Frequency increases with bond energy
13Vibrational Modes •A nonlinear molecule withnatoms has 3n–6 fundamental vibrational modes
14IR Spectrum •Two main regions •Diagnostic regionis between 1600–3500 cm-1: has the most characteristic vibrations and the most predictable vibrations–fewer peaks •Fingerprint regionis between 600–1400 cm-1 : has most of complex vibrations
IR Absorbance Spectrum •Different types of bonds will appear in different regions of the IR spectrum
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