1、Application of IR Raman Spectroscopy,3 IR regions Structure and Functional GroupAbsorption IR Reflection IR Photoacoustic IR IR Emission Micro,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 So
2、lvent issues Dissolution of cell,Analysis,Can calculate group frequencies C-H, C=O, C=C, O-H Variations of frequencies for group Fingerprint region Compare to standards Absorption of inorganics Sulphate, phosphate, nitrate, carbonate Search spectra against library,Mid-IR,Interpretation,Alcohols and
3、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 inten
4、sity 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
5、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 th
6、e 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,Quantitative IR,Difficult to obtain reliable
7、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,Other methods,Reflectance IR Measurement of absorbance
8、 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,Raman Spectroscopy,Scattering of light Fraction of scattered light in the visible differs from incident beam Difference b
9、ased 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,Raman Spectroscopy,Theory Instrumentation Applicati
10、onMethod Excitation with UV or NIR Measurement of scatter at 90 Measurement 1E-5 of incident beam,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 Mos
11、t intense Same as incident radiation Shift patterns independent of incident radiation wavelength,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 temperat
12、ure Elastic scattering is Rayleigh Energy scattered=energy incident Energy difference due to ground and 1st excited state hn-DE is Stokes scattering Hn+DE is anti-Stokes scattering,Theory,Variation in polarizability of bond with length Electric field (E) due to excitation frequency with E0 Dipole mo
13、ment (m) based on polarizability of bond (a) For Raman activity a must vary with distance along bond a0 is polarizability at req,Theory,Equation has Rayleigh, Stokes, and Anti-Stokes component Complementary to IR absorbance Overlap not complete,Instrumentation,Laser source Ar (488 nm, 514.5 nm) Kr (
14、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,Instrumentation,Sample holder Glass Laser focusing allows small sample size Liquid and solid samples can be examined Use of fiber optics,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,
