XII

Lecture Notes Chem 524 -- (Part 16)

XII. Infrared Spectroscopy — (Read Chap 14)

A. Regions: -- near IR (800-2500 nm — quartz optics/W-I lamp, diode detect)

anharmonic vib, overtone and combination bands.

-- mid IR (2500-20000 nm, 2.5-20m, 400-500 cm^-1)

(glowers, diode on TGS det, FTIR best, salt optics)

fundamental vibrations, fingerprint pattern in vibrational modes.

-- far IR (20m à ? , 400 cm^-1 à ? )

(difficult sources, detector, S/N) torsions, lattice vib, large amplitude mode

B. Dispersive IR (still find around, rarely made new)

-- some principle as uv/vis double beam -- chop between ref and samp, meas. difference

-- multiple gratings/filters to cover range - scale sometimes change, near to mid-IR

-- homemade for special purposes (TAK group) - modulation or kinetics meanurement,

-- also laser based spectrometers use dispersive element for mode separation

C. Fourier Transform — (dominate all usage now and commercial market, single beam)

-- High end — air bearing for moving mirror, cooled SiC source, multiple detector (TGS, MCT), high resolution <1 cm^-1 à 0.001 cm^-1 (rare), multiple beam splitter, purged

--(old white light fiducial mark, current method count laser fringes)

--external bench--emission/ Raman/ reflection/ modulation / etc.

-- computer capable multiple tasks (vary scan speed, number, averaging, data processing, deconvolution, subtraction, searches)

--Low end — mechanical bearing, corner cube mirrors, restricted speed and lower resolution, >1 cm^-1, uncooled source (lower T), unpurged,

-- computer/software--more limited processing/automated

-- unusual designs — swing, Genzel, PE 1700 (pivot), 1800 (double B/S), Bomem DA (vertical drop), Bomem Michelson (pivot), sliding wedge

D. Beam Splitter — Heart of FTIR — (typical: KBr/Ge for mid IR)

1. Modulation efficiency: varies as (2RT)I — max for R = 0.5 where (R+T)=1

ideal: I(d) = 0.5 I(n) cos (2pnd) --note: can be polarized reflection.

Polarizing B/S — Martin Puplett -- I_p(d) = 0.5 (n) [1 + cos 2pnd]

2. Other regions: coated quartz -- near IR -- change source

mylar (must not accoustical couple to BS)-- far IR -- change detector

E. Sampling is big issue in IR -- solvent interference -- need for short path

1. Gas -- multipass cell (better with tune laser)

2. Liquid — short path/salt — KBr/CaF₂/ZuSe … window & spacer

-- solvent must not dissolve cell / restrict region

-- path from interference fringes b = n/2(Dn) (Fig 14-15)

3. Small sample — beam condenser

-- microscope big appl now/autovials/bio

-- solids reflection — diffuse — powder — specular — IRAS surface and interface study

F. Applications

1. Qualitative Analyses — major use

-- group frequencies characterize band pattern

-- library searches identify compounds

2. Quantitative — problem low e, short path (due to solvent)

3. Noise limit — typically Johnson: s_A/A ~ s_0t/E_r (-1/TlnT)

G. FTIR can get great S/N, >10³ for A > 0.1

1. Baseline correction (single beam) precise subtraction (incl. H₂O, CO₂ vapor)

2. Resolution enhance — 2nd derivative

-- Fourier self-deconvolution (emphasize high res part)

-- Component fitting

H. Accessories

1. ATR — sample absorbance close to surface of all through reflectance/evanescent wave penetration, can study films, liquids (solutions) or flow

2. GC/LC detection — 2D idea -- spectrum for each chromatographic peak — qualitative analysis of components--identification

3. Microscope — multichannel detector (MCT array detector) -- 3D ideal spectrum for each image pixel -- qualitative analysis

4. 2-D correlation spectra — perturb sample observe changes in phase with perturbation