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Molecular Spectroscopy
B Y: LELAND BREEDLOVE, ANDREW HARTFORD, ROMAN HODSON, AND
KANDYSS NAJJAR
ROMAN HODSON
Theory and Introduction
FTIR Spectroscopy
University of California at Davis Chemistry Department. FTIR Block Diagram [Image] Retrieved April 14, 2015.
• Time domain data to frequency domain data
• Need fast time scales• Light is split and reflected
off a motorized mirror• Fourier transforms
interferogram into a spectrum
Rotational/Vibrational Energy Levels
Vibrational Energy: Rotational Energy: G(v) = (v + ½)νe F(J) = BJ(J+1)
MIT. (n.d.). Principles of Molecular Spectroscopy. Retrieved March 23, 2015, from http://web.mit.edu/ 5.33/www/lec/spec4.pdf.
• Rotational levels nested between vibrational levels
• J is rotational quantum number, v is vibrational quantum number
• Total energy is the sum of the two
• Selection rules
P and R Branches
UC Davis. (n.d.). Rovibrational Spectroscopy. Retrieved April 14, 2015, from http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Rotational_Spectroscopy/ Rovibrational_Spectroscopy
• Higher rotational levels can be populated at room temperature
• ΔJ = +1, rotational transition added to vibrational energy (R)
• ΔJ = -1, low wavenumber side of branch (P)
Harmonic Oscillator
University of Liverpool. (n.d.). Vibrational Spectroscopy. Retrieved March 13, 2015, from http://osxs.ch.liv.ac.uk/java/spectrovibcd1-CE-final.html.
• All energy level spacing is equal (hν)
• Equilibrium bond length is the same at all energy levels
• Forbids vibrational transitions of Δv ≠ ± 1
• Does not account for bond dissociation/repulsion
Anharmonic Oscillator
University of Liverpool. (n.d.). Vibrational Spectroscopy. Retrieved March 13, 2015, from http://osxs.ch.liv.ac.uk/java/spectrovibcd1-CE-final.html.
• Shows equilibrium bond length changes
• The spacing between energy levels decreases at higher quantum numbers
• Models bond dissociation/repulsion
• Allows for overtone transitions: Δv > ±1
Absorption
• ΔE = hν• A = εlc• A = -log(I/Io)• Use range of
wavelengths• Provides information
about the excited state energy levels
UC Davis. (n.d.). Infrared: Theory. Retrieved April 14, 2015, from http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/Infrared%3A_Theory
Emission
Microscopy Resource Center. Jablonski Energy Diagram; Excitation and Emission Spectrum [Image] Retrieved April 14, 2015.
• ΔE = hν• Use single wavelength
to excite to a particular excited state
• Electrons relax back to various vibrational levels in the ground state
• Provides information concerning the ground state
Franck-Condon Principle
UC Davis. (n.d.). Selection Rules and Transition Moment Integral. Retrieved April 14, 2015, from http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Fundamentals/ Selection_rules_and_transition_moment_integral
• Born-Oppenheimer approximation
• Demonstrates vibronic transitions
• The wavefunctions in ground and excited state must overlap
• Peak intensity is proportional to amount of overlap
Analysis of carbon monoxide through its rovibrational
spectrum
ANDREW HARTFORD
Experimental method
Evacuated and collected background spectrum of gas sample cell using FTIR spectrophotometer
Filled sample cell with 100 mmHg CO, collected spectra at resolutions of 4, 2, 1, 0.5, 0.25 cm-1
Stored gas sample in desiccator when not in use
Results
2000 2050 2100 2150 2200 22500
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
2728-28
26
-27
25
-26
2423
-25
-24
22 21
-23
20
-22
1918
-21-20
17 16
-19
15
-18
1113
-17
14 12
-16
-15-14
-13
8
-12
10
-11
9
-10
7
-9
-8
6
-7
5
-6
-5
4 3
-4-3
2 1
-2
-1 1
0
2
1
3 4
2
5
43
6
5
7
6
8
7
9
8
10
9
11
10
12
11
13
12
14
13
15
14
16
17
1516
18
19
17
20
18
21
19
22
22
23
20
24
21
25
23
2627
2524 26
28
27
2928
Wavelength (cm-1)
Abso
rban
ce (
AU)
P-Branch R-BranchP-Branch R-Branch
J values
Fundamental Absorbance Spectrum (CO) between 1950-2275 cm-1
First overtone (CO) between 4100-4400 cm-1
4125 4175 4225 4275 43250.08
0.1
0.12
0.14
0.16
0.18
0.2
24
-24
23
-23
22
-22
21 20
-21-20
19
-19
18
-18
1716
-17
1514
-16
12
-15
13
-14-13
-12
11
-11
10
-10
9 8
-9-8
7
-7
6 4
-6
5
-5
3
-4
2
-3
1
-2
-1
0
1
2
1
3
2
4
3
5 6
4 5
7
6
8
9
7
10
9
11
8 10
12
1314
1211
15
13
16
1415
1718
1920
1618
21
1719
2223
20 2221
2425
2324
Wavelength (cm-1)
Abso
rban
ce (
AU)
P-Branch R-Branch
J-values
Wavelength vs. m values (Fundamental spectrum)
-30 -20 -10 0 10 20 301900
1950
2000
2050
2100
2150
2200
2250
f(x) = − 5.71059552062594E-05 x³ − 0.0142504848197964 x² + 3.76634079344824 x + 2142.8071931871R² = 0.999693385573565f(x) = − 0.0143372950823478 x² + 3.73758863579973 x + 2142.82216461854R² = 0.999682062419989f(x) = 3.72337699623958 x + 2138.73722924753R² = 0.996445029356287
m values
Wav
elen
gth
(cm
-1)
Cubic
Quadratic
Linear
Wavelength vs. m values (First overtone spectrum)
-30 -20 -10 0 10 20 304000
4050
4100
4150
4200
4250
4300
4350f(x) = 3.76728929831439 x + 4252.2003626029R² = 0.98623969853831f(x) = − 0.0349102489872732 x² + 3.80184373919751 x + 4259.61006637168R² = 0.999973319486877
f(x) = − 1.05406482050996E-05 x³ − 0.0348941899747725 x² + 3.80578473873728 x + 4259.60800019687R² = 0.999973525196163
m values
Wav
elen
gth
(cm
-1)
Linear
Quadratic
Cubic
Constatnts
Fundamental (cm-1) First Overtone (cm-1)
2142.9 4259.6
Equilibrium Frequency
(cm-1)
αe
(cm-1)Be (cm-
1)De
(cm-1)χe (cm-
1)
Experimental Value 2168.8 0.014
3 1.90 1.5 x 10-5
0.00599
Literature Value19 2169.8 0.017
5 1.9313 6.2 x 10-6
0.00612
Percent error 0.0461% 18.3% 1.62% 142% 2.12%
Molecular constants
Moment of Inertia (kg
m2)
Equilibrium bond (Å)
Force Constant
(N/m)
Experimental Value
1.47 x 10-46 1.14 1903
Literature Value19
1.4490 x 10-
461.1281 1902
Percent error
1.45% 1.05% 0.0526%
Global Warming Potentials of Greenhouse Gases
Kandyss Najjar
http://commons.wikimedia.org/wiki/File:Earth's_greenhouse_effect_(US_EPA,_2012).png
Brief Theory
GWP - heat trapped by greenhouse gases when exposed to IR radiation emitted from the Earth quantity, strength, and location of IR absorption bands Researchers and political activists – effects on climate
change
Radiation forcing capacity – sum of IR spectrum and emission of Earth blackbody radiation Equivalent to the GWP relative to gas atmospheric lifetime
RFC can be determined relative to a reference gas Normally CO2
**Elrod, M. J. J. Chem. Ed. 1999, 76, 1702-05.
Brief Theory (Cont.)
The radiation forcing capacity is given by Equation 1 RFA – radiation forcing capacity per 1 kg increase of
sample A(t) – time decay of a pulse of sample RFR and R(t) – same, but for reference
Equation 2 – determine GWP in terms of mass instead of molecule τ – atmospheric lifetime (years) MW – molecular mass (g/mol)
Equation 1 Equation 2
**Elrod, M. J. J. Chem. Ed. 1999, 76, 1702-05.
Experimental
NaCl cell evacuated as per Week 1’s procedureUsing OMNIC – background spectrum
Range: 495 – 1600 cm-1
Resolution: 1 cm-1
Filled gas cell with: CH4 – 60.0 Torr N2O – 60.1 Torr
IR spectra taken for both gases Range: 495 – 1600 cm-1
Resolution: 1 cm-1
http://www.specac.com/assets/products/49903cada5f6c.jpg
Expected Results
Nitrous Oxide Fundamental: ~600 cm-1
First Overtone: ~1300 cm-1
Methane First Overtone: ~ 1200 cm-1 to ~ 1400 cm-1
N2O
CH4
Molecular Spectroscopy. University of Texas at Austin - Chemistry Department. Canvas.utexas.edu (accessed April 26, 2015).
Experimental Results
IR spectra obtainedwere very similar tothe expected spectraNitrous Oxide
Fundamental: ~600 cm-1
1st Overtone: ~1300 cm-1
Methane 1st Overtone: ~ 1200 cm-1 to ~ 1400 cm-1
495.00 695.00 895.00 1095.00 1295.00 1495.00-0.10
0.10
0.30
0.50
0.70
0.90
Wavenumber (cm-1)
Abs
orba
nce
(AU
)
N2O
495.00 695.00 895.00 1095.00 1295.00 1495.00-0.05
0.15
0.35
0.55
0.75
0.95
Wavenumber (cm-1)
Abs
orba
nce
(AU
)
CH4
Calculating Global Warming Potential (GWP)
Converted IR spectra to CSV filesUsing Excel, constructed table
505 – 1495 cm-1, in increments of 10 cm-1
=lookup function to add corresponding IR absorbance data
Inserted table into provided “Global Warming Potential Model” spreadsheet Path length: 10 cm Lifetime, formula weight, and gas pressure in cell**
N2O – 120 years, 44.01 g/mol, 60.1 Torr CH4 – 15 years, 16.04 g/mol, 60.0 Torr
Time Horizons: 20, 100, and 500 years**Elrod, M. J. J. Chem. Ed. 1999, 76, 1702-05.
Results
GHG Lifetime (Years)**
Time Horizon (Years)
Calculated GWP
Literature GWP**
Percent Difference (%)
N2O 120
20 73.3 93 21.1
100 69.3 88 21.3
500 60.9 77 20.9
CH4 15
20 33.3 37 10.0
100 11.6 13 10.9
500 5.9 6 2.46
**Elrod, M. J. J. Chem. Ed. 1999, 76, 1702-05.
Short Summary
IR spectra obtained match expected spectra Nitrous Oxide
Fundamental: ~600 cm-1
1st Overtone: ~1300 cm-1
Methane 1st Overtone: ~ 1200 cm-1 to ~ 1400 cm-1
N2O is a more effective greenhouse gas Larger atmospheric lifetime Larger GWP more efficient at trapping heat within
atmosphere
LELAND BREEDLOVE
Absorbance And Emission Of Iodine Gas
Experimental
I2 absorption spectra Halogen lamp Detector inline with beam
I2 emission spectra Argon LASER Detector arranged 90° to laser Filter used to maximize area of interest
Absorption
I2 Absorption Bandhead Energy versus v’ + ½
5 10 15 20 25 30 35 40 4515500
16000
16500
17000
17500
18000
18500
19000
19500f(x) = − 0.00758860569380775 x³ − 0.438066846118949 x² + 119.225915924138 x + 15689.9357002699R² = 0.999860764677182
v' + 1/2
Wav
enum
bers
(cm
-1)
Emission
500 550 600 650 700 750 8000
5000
10000
15000
20000
25000
1
2
3 4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
2021
2223
2425 26
2728
2930
31
3234
36
Wavelength (nm)
Inte
nsity
I2 Emission Bandhead Energy vs v” + ½
0 5 10 15 20 25 30 35 400
1000
2000
3000
4000
5000
6000
7000
8000
f(x) = − 0.00122088262316558 x⁴ + 0.0754204610070116 x³ − 2.14035247166619 x² + 224.925209425968 x − 138.153683425675R² = 0.999964332382799
v" + 1/2
Wav
enum
ber
(cm
-1)
B-State and X-State Constants
Spectroscopic constants
Experimental values (cm-1)
Literature Values12 (cm-
1)
Percent Error (%)
G”(0) 138.15 107.11 29.0
v”e 224.93 214.53 4.85
vex”e 2.14 0.6130 249
vey”e -0.5177 0.0754 73303.88
D”o 11731.229 12440.2 5.70
E(I*) 7927.4 7602.98 4.27
Spectroscopic constants
Experimental values (cm-1)
Literature Values12 (cm-
1)
Percent Error (%)
G”(0) 138.15 107.11 29.0
v”e 224.93 214.53 4.85
vex”e 2.14 0.6130 249
vey”e -0.5177 0.0754 73303.88
D”o 11731.229 12440.2 5.70
E(I*) 7927.4 7602.98 4.27
B-State Morse Potentials
Spectroscopic constants Experimental Values Literature Values12 Percent Error (%)
D’e 3968.578 cm-1 4381.2 cm-1 9.42
R’e Used Lit. Value 3.0267 Å n/a
Te 15828.15 cm-1 15769.1 cm-1 0.374
v’e 84.603 cm-1 125.67 cm-1 32.7
Morse Potential plots FCintensity
Discussion
Morse potentials show longer eq. bond length for X-state than B-state ----Not good
Data shows less transitions than literature predicts
largest Franck-Condon factor from the emission spectrum at v” = 5, at 544 nm
The results from our calculations are more reliable than FCIntensity values equilibrium bond length of the X-State is less than
that of the B-State
Questions?