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?