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Chapters 15&16 An Introduction to Infrared Spectrometry 1 Theory of IR Absorption Spectrometry. 1.1 Introduction 1.1.1 Energy of IR photon Near IR 12,800-4000 cm -1 Mid IR 4000-200 cm -1 Far IR 200-10 cm -1
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Chapters 15&16 An Introduction to Infrared Spectrometry1 Theory of IR Absorption Spectrometry 1.1 Introduction 1.1.1 Energy of IR photon Near IR 12,800-4000 cm-1 Mid IR 4000-200 cm-1 Far IR 200-10 cm-1 Energy of IR photons insufficient to cause electronic excitation but can cause vibrational or rotational excitation 1.1.2 Dipole moment changes during vibrations Magnitude of dipole moment determined by a. charge b. separation (vibrational or rotation causes varying separation)
* Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation (only in this case the alternating E field can interact with the molecule and cause change in amplitude of one of its motions) Molecule with permanent dipole moments are IR active
1.1.3 Types of molecular vibrations basic categories: - stretching: change in bond length, symmetric or asymmetric - bending: change in the angle, scissoring, wagging, rocking, twisting/torsion
H2CO vibrations 1 = 2790 cm-1 CH2 sys stretch 2 = 1756 cm-1 CO stretch 3 = 1482 cm-1 CH2 scissors 4 = 1165 cm-1 out-of-plane wagging 5 = 2846 cm-1 aym CH stretch 6 = 1221 cm-1 in-plane CH2 rock
1.1.4 Vibrational modes Translation Rotation Vibration Non-Linear 3 3 3N-6 Linear 3 2 3N-5 Only some vibrational modes are IR active
1.1.5 Classical vibrational motion A system of two masses connected by a spring? 1.1.6 Quantum treatment E0 = ½ hm ground vibrational state (=0) E1 = 3/2hm first excited vibrational state (=1) E = hm Vibrational selection rule: = 1 since levels are equally spaced- should see one absorption frequency
2 Instrumentation 2.1 FT-IR 2.1.1 Time-domain spectroscopy - changes in radiant power with time Fig. 7-41 (p.207) Unfortunately, no detector can respond on 10-14 s time scale
2.1.2 Michelson interferometer Use Michelson interferometer to measure signal proportional to time varying signal - If moving mirror moves 1/4 (1/2 round-trip) waves are out of phase at beam-splitting mirror – no signal -If moving mirror moves 1/2 (1 round-trip) waves are out of phase at beam-splitting mirror –signal Difference in path lengths called retardation Plot vs. signal – cosine wave with frequency proportional to light frequency but signal varies at much lower frequency Fig. 7-43 (p.208)
One full cycle when movable mirror moves distance /2 (round trip = ) The velocity of moving mirror vMM Bolometer, photoconducting IR detector, etc. can “see” changes on 10-4 s
Time-domain signal frequency-domain signal Fig. 7-44 (p.210)
2.1.3 Resolution Two closely spaced lines only separated if one complete “beat” is recorded. As lines get closer together, must increase Mirror motion is 1/2, resolution is governed by distance moving mirror travels. 2.1.4 Advantages of FT-IR (reading assignment, will be in exam)
2 Instrumentation (continued) 2.2 IR sources Fig. 6-22 (p.153)
2.3 IR transducer Pyroelectric - TGS (Triglycine sulfate) based on pyroelectric effect (temperature dependent capacitance) fast enough for FT-IR (but less sensitive than thermocouple) most common detector for FT-IR Photoconducting semiconductors (e.g., PbS, MCT, etc) resistance decreases with increase photon flux (promotion of electrons to conduction band) MCT (mercury telluride-cadmium telluride) about x100 sensitive than TGS, cooled to N2(liquid) temp to reduce thermal noise. Thermal couple– based on temperature detection (heating effect of radiation) poor sensitivity slow (ms response time) – not suitable for FT-IR
*Anharmonic oscillator: a. Electron repulsion (steeper at small distance) b. Dissociation (bond breaks at large distance) Consequences: -Harmonic at low -E becomes smaller at high -Selection rules fails = 1, 2, 3, ..
3 Absorption Spectrometry 3.1 Sample handling IR (especially FT-IR) is very widely used for • qualitative • quantitative Analysis of • gases • liquids • solids Most time-consuming part is sample handling
3.1.1 Gases fill gas cell (1) transparent windows (NaCl / KBr) (2) long path length (10cm-10m) – few molecules
3.1.2 Liquids fill liquid cell (1) dissolved in transparent solvent – not water (attacks the window) (2) short path length (0.015-1mm) – solvent absorbs In the following figure, horizontal lines indicate useful regions Fig. 17-1 (p.456)
Determining cell pathlength • Get interference pattern due to interference of waves that are reflected between the cell windows (empty cell) Determination of the distance between the salt plates Fig. 17-4 (p.458)
3.1.3 Solids (1) make semi-transparent pellet with KBr (2) grind and mix with mineral oil to form mull. One drop (film) between NaCl plates.
3.2 Qualitative Analysis (1) Identify functional groups (group frequency region, 3600-1250 cm-1) (2) Compare with standard spectra containing these functional groups (fingerprint region, 1200 – 600 cm-1) - use computerized spectral search engines - use IR assignments in conjunction with other info (e.g., chemical, physical, spectroscopic)
Group frequencies • Approximately calculated from masses and spring constants • Variation due to coupling • Compared to correlation charts/database (Table 17-6, p462-463)
3.3 Quantitative Analysis IR more difficult than UV-Vis because (1) narrow bands (variation in ) (2) complex spectra (3) weak incident beam (4) low transducer sensitivity (5) solvent absorption IR mostly used for rapid qualitative but not quantitative analysis
4 IR Reflection Spectrometry • Diffuse-reflection spectrometry • Advantages (powdered samples) • Instrumentation • (an adapter fitting into the cell • component of FT-IR) Fig. 17-11 (p.471)
Comparison of the absorbance spectrum (a) for carbazole with its diffuse-reflectance spectrum (b) Fig. 17-12 (p.471)
5 IR Emission Spectrometry Orbiting Mars Global Surveyor-Thermal Emission Spectra, providing measurement of the Martian Surface and atmosphere mini thermal-emission spectrometer measured by the Mars rover Spirit, indicating composition of nearly soils and rocks Fig. 17-18 (p.477)