10-1 Application of IR Raman Spectroscopy 3 IR regions Structure and Functional Group Absorption IR Reflection IR Photoacoustic IR IR Emission Micro

10-2 Mid-IR Mid-IR absorption §Samples àPlaced in cell (salt) àCombined with oil §Need cell that does not absorb IR àKBr, NaCl *Tends to absorb water §Gases §Solutions àSolvent issues *Dissolution of cell

10-3 Analysis Can calculate group frequencies §C-H, C=O, C=C, O-H àVariations of frequencies for group Fingerprint region §Compare to standards §Absorption of inorganics àSulphate, phosphate, nitrate, carbonate Search spectra against library

10-4 Mid-IR

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10-8 Interpretation Alcohols and amines display strong broad O-H and N-H stretching bands in the region 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 cm -1 §bands are of medium intensity often obscured (i.e., OH). Triple bond stretching absorptions occur in the region cm -1 §Nitriles are generally of medium intensity and are clearly defined §Alkynes absorb weakly unless they are highly asymmetric àsymmetrical alkynes do not show absorption bands Carbonyl stretching bands occur in the region 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 cm -1 Carbon-carbon double bond stretching occurs in the region around 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 cm -1 §bands are generally strong and broad

10-9 Quantitative IR Difficult to obtain reliable quantitative data based on IR §Deviations from Beer’s law àNarrow Bands and wide slit widths required *Require calibration sources §Complex spectra §Weak beam §Lack of reference cell àNeed to normalize refraction *Take reference and sample with same cell

10-10 Other methods Reflectance IR §Measurement of absorbance from reflected IR àSurface measurement Photoacoustic IR §can use tunable laser Near IR §700 nm to 2500 nm àQuantitative analysis of samples *CH, NH, and OH ØLow absorption Emission IR

10-11 Raman Spectroscopy Scattering of light §Fraction of scattered light in the visible differs from incident beam àDifference based on molecular structure *Based on quantized vibrational changes *Difference between incident and scattered light is in mid-IR region §No water interference àCan examine aqueous samples §Quartz or glass cells can be used §Competition with fluorescence

10-12 Raman Spectroscopy Theory Instrumentation Application Method §Excitation with UV or NIR §Measurement of scatter at 90 ° àMeasurement 1E-5 of incident beam

10-13 Theory 3 types of scattered radiation §Stokes àLower energy than Anti- Stokes *Named from fluorescence behavior àMore intense àUsed for Raman measurements §Anti-Stokes àNo fluorescence interference §Rayleigh àMost intense àSame as incident radiation Shift patterns independent of incident radiation wavelength

10-14 Theory Excitation §From ground or 1 st vibrationally excited state àPopulation of excited state from Boltzmann’s equation *Molecule populates virtual states with energy from photon *Can be effected by temperature §Elastic scattering is Rayleigh àEnergy scattered=energy incident §Energy difference due to ∆ ground and 1 st excited state  h  E is Stokes scattering  H  E is anti-Stokes scattering

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10-16 Theory Variation in polarizability of bond with length Electric field (E) due to excitation frequency with E 0 Dipole moment (m) based on polarizability of bond (  ) For Raman activity  must vary with distance along bond    is polarizability at r eq

10-17 Theory Equation has Rayleigh, Stokes, and Anti-Stokes component Complementary to IR absorbance §Overlap not complete

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10-19 Instrumentation Laser source §Ar (488 nm, nm) §Kr (530.9 nm, nm) §He/Ne (623 nm) §Diode (782 nm or 830 nm) §Nd/YAG (1064 nm) §Tunable lasers  Intensity proportional to 4 *Consider energy and chemical effect of absorbing energy

10-20 Instrumentation Sample holder §Glass §Laser focusing allows small sample size §Liquid and solid samples can be examined §Use of fiber optics

10-21 Applications Laser microprobes §Use of laser permits small sampling area Resonance Raman §Use electronic absorption peak §Low concentrations can be examined àLifetimes on 10 fs Surface enhanced Raman §Increase of sensitivity by 1000 to 1E6

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