Transcript 9&10_chp16&17_slides

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
 (cm 1 ) 
1


v
c
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
1
 m
2
k
m
k is force constant
A system of two masses connected by a spring?
m 
1
2
k


m1m2
reduced mass
m1  m2
1.1.6 Quantum treatment
1 h
E  (  )
2 2
k
1
 (  )h m

2
E0 = ½ hm ground vibrational state (=0)
E1 = 3/2hm first excited vibrational state (=1)
E = hm
Vibrational selection rule:  = 1
since levels are equally spaced- should see one absorption frequency
2.1 FT-IR
2.1.1 Time-domain spectroscopy - changes in radiant power with time
Unfortunately, no detector can respond on
10-14
Fig. 7-41 (p.207)
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  roundtrip) waves are out of phase at beamsplitting mirror –signal
Difference in path lengths called
retardation 
Plot  vs. signal – cosine wave with
frequency proportional to light frequency Fig. 7-43 (p.208)
but signal varies at much lower frequency
One full cycle when movable mirror moves distance /2 (round trip = )
The velocity of moving mirror vMM
time for mirror to move λ/2  

2v M
frequency of signal at the detector f 
optical frequency of radiation 
1


2v M

c

v M  1.5 cm/s
frequency of signal at the detector  10-10  optical frequency of radiation
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
 (cm ) 
1
1

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)
2.2 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, ..
3.1 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)
Fig. 17-3 (p.457)
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
2b  N
interferen ce fringes between
two wavelengt h 1 and 2
N 
b
2b
1

2b
2
 2b 1  2b 2
N
2( 1  2 )
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)
Fig. 17-5 (p.460)
Fig. 17-5 (continued, p.461)
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
Diffuse-reflection spectrometry
• Advantages (powdered samples)
• Instrumentation
(an adapter fitting into the cell
component of FT-IR)
f ( R ' ) 
(1 R ' ) 2
2 R '

k
s
k  2.303c
reflected intensity of the sample
R ' 
nonabsorpi ng standard KCl
Fig. 17-11 (p.471)
Comparison of the absorbance
spectrum (a) for carbazole with its
diffuse-reflectance spectrum (b)
Fig. 17-12 (p.471)
 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)