Application of IR Raman Spectroscopy

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Application of IRRaman Spectroscopy

• 3 IR regions
• Structure and Functional Group
• Absorption IR
• Reflection IR
• Photoacoustic IR
• IR Emission
• Micro

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Mid-IR

• Mid-IR absorption
• Samples
• Placed in cell (salt)
• Combined with oil
• Need cell that does not absorb IR
• KBr, NaCl
• Tends to absorb water
• Gases
• Solutions
• Solvent issues
• Dissolution of cell

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Analysis

• Can calculate group frequencies
• C-H, CO, CC, O-H
• Variations of frequencies for group
• Fingerprint region
• Compare to standards
• Absorption of inorganics
• Sulphate, phosphate, nitrate, carbonate
• Search spectra against library

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Mid-IR
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Interpretation

• Alcohols and amines display strong broad O-H and
  N-H stretching bands in the region 3400-3100 cm-1
  
• bands are broadened due to hydrogen bonding and a
  sharp 'non-bonded' peak can around 3400 cm-1 .
• Alkene and alkyne C-H bonds display sharp
  stretching absorptions in the region 3100-3000
  cm-1
• bands are of medium intensity often obscured
  (i.e., OH).
• Triple bond stretching absorptions occur in the
  region 2400-2200 cm-1
• Nitriles are generally of medium intensity and
  are clearly defined
• Alkynes absorb weakly unless they are highly
  asymmetric
• symmetrical alkynes do not show absorption bands
• Carbonyl stretching bands occur in the region
  1800-1700 cm-1
• bands are generally very strong and broad
• Carbonyl compounds (acyl halides, esters) are
  generally at higher wave number than simple
  ketones and aldehydes
• amides are the lowest, absorbing in the region
  1700-1650 cm-1
• Carbon-carbon double bond stretching occurs in
  the region around 1650-1600 cm-1
• bands are generally sharp and of medium intensity
• Aromatic compounds will display a series of sharp
  bands
• Carbon-oxygen single bonds display stretching
  bands in the region 1200-1100 cm-1
• bands are generally strong and broad

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Quantitative IR

• Difficult to obtain reliable quantitative data
  based on IR
• Deviations from Beers law
• Narrow Bands and wide slit widths required
• Require calibration sources
• Complex spectra
• Weak beam
• Lack of reference cell
• Need to normalize refraction
• Take reference and sample with same cell

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Other methods

• Reflectance IR
• Measurement of absorbance from reflected IR
• Surface measurement
• Photoacoustic IR
• can use tunable laser
• Near IR
• 700 nm to 2500 nm
• Quantitative analysis of samples
• CH, NH, and OH
• Low absorption
• Emission IR

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Raman Spectroscopy

• Scattering of light
• Fraction of scattered light in the visible
  differs from incident beam
• Difference based on molecular structure
• Based on quantized vibrational changes
• Difference between incident and scattered light
  is in mid-IR region
• No water interference
• Can examine aqueous samples
• Quartz or glass cells can be used
• Competition with fluorescence

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Raman Spectroscopy

• Theory
• Instrumentation
• Application
• Method
• Excitation with UV or NIR
• Measurement of scatter at 90
• Measurement 1E-5 of incident beam

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Theory

• 3 types of scattered radiation
• Stokes
• Lower energy than Anti-Stokes
• Named from fluorescence behavior
• More intense
• Used for Raman measurements
• Anti-Stokes
• No fluorescence interference
• Rayleigh
• Most intense
• Same as incident radiation
• Shift patterns independent of incident radiation
  wavelength

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Theory

• Excitation
• From ground or 1st vibrationally excited state
• Population of excited state from Boltzmanns
  equation
• Molecule populates virtual states with energy
  from photon
• Can be effected by temperature
• Elastic scattering is Rayleigh
• Energy scatteredenergy incident
• Energy difference due to ? ground and 1st excited
  state
• hn-DE is Stokes scattering
• HnDE is anti-Stokes scattering

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Theory

• Variation in polarizability of bond with length
• Electric field (E) due to excitation frequency
  with E0
• Dipole moment (m) based on polarizability of bond
  (a)
• For Raman activity a must vary with distance
  along bond
• a0 is polarizability at req

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Theory

• Equation has Rayleigh, Stokes, and Anti-Stokes
  component
• Complementary to IR absorbance
• Overlap not complete

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Instrumentation

• Laser source
• Ar (488 nm, 514.5 nm)
• Kr (530.9 nm, 647.1 nm)
• He/Ne (623 nm)
• Diode (782 nm or 830 nm)
• Nd/YAG (1064 nm)
• Tunable lasers
• Intensity proportional to n4
• Consider energy and chemical effect of absorbing
  energy

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Instrumentation

• Sample holder
• Glass
• Laser focusing allows small sample size
• Liquid and solid samples can be examined
• Use of fiber optics

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Applications

• Laser microprobes
• Use of laser permits small sampling area
• Resonance Raman
• Use electronic absorption peak
• Low concentrations can be examined
• Lifetimes on 10 fs
• Surface enhanced Raman
• Increase of sensitivity by 1000 to 1E6

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