1. Thermal Cracking of Ethanethiol and Dimethyl Sulfide
Jared Whitman
Middlebury College
Department of Chemistry and Biochemistry

2. Refining Improvements
Large scale
refining industry
Modifications to infrastructure must first be modeled using engineering simulations
Simulations require accurate initial decomposition chemistry
Sulfur is found in fuels and can be up to 5% per mass. Must be removed to PPM. Current removal is expensive and sill releases SO2.
Implementing completely new techniques is unlike
Simulations that predict the products of a given technique or reaction and thus these are highly dependent on initial decomposition chemistry
Knowing the chemistry is crucial!
Vandeputte, A.G., Reyniers, M.F., Marin, G.B. Phys. Chem. Chem. Phys., 2013, 14,

3. Modeling of Desulfurization
Accurate modeling of desulfurization requires complete understanding of thermal decomposition mechanisms
Relevant Organosulfurs
Dimethyl Sulfide (CH3SCH3)
Diethyl Sulfide (CH3CH2SCH2CH3)
Dimethyl Disulfide (CH3SSCH3)
Ethanethiol (CH3CH2SH)
Tert-butylthiol ((CH3)3CSH)
To simplify, in this example research has shown that DD results in Ethylene and H2S. But there is no evidence to show transition.
To look at this we decided to focus on Organosulfurs that are very relevant to the majority of compounds found in fuels, but are smaller and have less possible decompositions.
Small organic sulfur contaminants in fossil fuels and biofuel.
1= sulfides (Ds, DS), 2 = disulfides (DD), 3 = thiols (E, T) Δ ?
Diethyl Disulfide
Ethylene
Hydrogen Sulfide

4. Ethanethiol
Lack of experimental information on initial decomposition products of thiols.
Lack of assigned vibrational frequencies in literature.
We are starting with ethanethiol because of its relatively small size (which will lead to fewer dissociation products) and the apparent lack up-to-date experimental spectra
First off the structure is interesting and should allow for interesting results.
There are many predictions and proposed mechanisms, but nothing concrete.
Without any evidence we had to investigate possible pathways and come up with a procedure to identify the intermediates.

5. Mechanism for Ethanethiol
74 kcal/mol
85 kcal/mol
This mechanism shows two possible mechanisms for ethanethiol.
After the unimolecular decomposition, secondary reactions can occur that in the end will lead to the same results. C2H4 and H2S.
Sehon, A.H., Darwent, B., The thermal decomposition of mercaptans, J. Am. Chem. Soc. 1954, 76,
No reactive intermediates have been observed

6. Mechanism for Ethanethiol
As I will discuss shortly, thermally cracking using MIIR ethanthiol attempts to limit the number of collisions. Hopefully allowing us to get radical intermediates which we can then identify.
Sehon, A.H., Darwent, B., The thermal decomposition of mercaptans, J. Am. Chem. Soc. 1954, 76,

7. Mechanism for Ethanethiol
Sehon, A.H., Darwent, B., The thermal decomposition of mercaptans, J. Am. Chem. Soc. 1954, 76,

8. Hyperthermal Nozzle 1 mm SiC Tube @ 300-1700K Sulfur In Ar.
[R] ≅ 10 pulse-1
1 mm SiC Tube
@ K
Sulfur In Ar.
τ (nozzle)≅ μs
[R] ≅ 10 pulse-1
[radicals] ≅ 1013 pulse-1
τ (nozzle) ≅ 75 µsec
τ (nozzle)≅ μs
[R] ≅ 10 pulse-1
Sulfur Compound In Ar behind pulsed Valve.
1 % Sulfur Compound in Ar/Ne behind pulsed Valve.
Powerful tool allows us to observe radical intermediate using commercial spectroscopy techniques.
bread and butter apparatus
Thermal Cracking- (nozzle function) similar to pyrolysis in which intense heat cause the C-S bond to break and then through the use of pressure, vacuums, move the molecules at supersonic speeds to avoid collisions.
C-S bond breaks because it has the lowest bond dissociation, enthalpy Kcal mol-1
Supersonic Jet of decomposition products
in Ar/Ne into 10-6 torr
Sulfur In Ar.

9. Matrix Isolation FTIR Hyperthermal nozzle doses sample mixture
Matrix is formed on 4 K CsI window
Commercial FTIR probes “isolated” species with infrared radiation
Powerful tool allows us to observe radical intermediate using commercial spectroscopy techniques
bread and butter apparatus
Thermal Cracking- (nozzle function) similar to pyrolysis in which intense heat cause the C-S bond to break and then through the use of pressure, vacuums, move the molecules at supersonic speeds to avoid collisions.
C-S bond breaks because it has the lowest bond dissociation, enthalpy Kcal mol-1

10. Photoionization Mass Spectrometry (PIMS)
1 mm SiC 300–1700 K
[radicals] ≅ 1013 pulse-1
τ (nozzle) ≅ 75 µsec
Supersonic Jet of Radicals/Ar into 10-6 Torr
1% Sulfur Compound
In Ar/He carrier gas
Time-of-flight (TOF) detector
Hyperthermal nozzle pulses sample mixture
Beam is ionized by eV laser pulse
Ions are injected into time-of-flight mass spectrometer
118.2 nm VUV Photoionization Laser ( eV)

11. Ethanethiol
Two conformers, Gauche and Trans, with 42 vibrational modes.
Gauche
Trans
The interesting thing about Ethanethiol, is that similar to ethanol it has to conformers. Previous research have stated that ethanethiol exist in a 3:1 G to T ratio. In addition, all of the gauche conformations have been assigned in past research. Trans, however, only has one mode previously assigned.
Conformers vary by the Hydrogen Atom bounded to the Sulfur. As you can see it rotates between these two locations.
21 modes for each conformation!!!!!!
21 Gauche assigned, only 1 Trans.
Measured CH
Microwave specra done by Margaules in 1998 ( intrestign asymetric rotar problem)
Energy differncen approx: 100 wavembumbers (cm -1) or 0.3 kca/mol
3:1 Ratio determined from relationship between intensities. (microwave experiment)
Beers law ABC, we assume A and B are the same and that concentration changes. Therefore higher intensity has direct relationship to concentration.
Absorbance = e L c (e=molar extinction coefficient L=Path length of cell holder)
A α c
since A is directly proportionate to Intensity, C is also directly proportionate to Intensity.
3:1 Gauche to Trans ratio
Only one Trans mode (SH-str) has been assigned.

12. Vibrational Frequencies
ab initio calculations.
Compared to experimental values.
Ethanethiol parent peaks assigned.
Having tables in power points is not always appealing. But in this case I just want to draw your attention to one mode.
As you can see only one trans value had been assigned before we started.
Since we were confident in our experimental data we were able to combine the Experimental data with the computational data and were able to assign 19 of the unknown trans frequencies.
The final frequency was unable to be obtained because it has a frequency below the range of our machinery.

13. Vibrational Frequencies
ab initio calculations.
Compared to experimental values.
Ethanethiol parent peaks assigned.
This is the only previously assigned mode. Dan Anderson and Maggie Phillips other students in the lab preformed multiple ab inition calculations through comparison with one of our ethanethiol spectra we were able to tentatively assign all but 2.
Since we were confident in our experimental data we were able to combine the Experimental data with the computational data and were able to assign 19 of the unknown trans frequencies.
The final frequency was unable to be obtained because it has a frequency below the range of our machinery.

14. Vibrational Frequencies
ab initio calculations.
Compared to experimental values.
Ethanethiol parent peaks assigned.
These values are in the process of being looked at further through …. By the other students iun the lab. However, with these tentatativly assigned frequencies we know have a better understanding of the initial layout of ethanethiol and can move on to heated spectra.
As you can see only one trans value had been assigned before we started.
Since we were confident in our experimental data we were able to combine the Experimental data with the computational data and were able to assign 19 of the unknown trans frequencies.
The final frequency was unable to be obtained because it has a frequency below the range of our machinery.

15. Ethanethiol at 300k
Here is a short over view of a few of the peaks, because I am not going to how you all 21 modes.
But I just wanted to give you a glimpse at a few of the peaks to see the resolution of the peaks and a few of the modes we tentatively assigned.
B is the mode location of the only previous assigned Trans.
*If needed mention how Gauche and Trans can alter which has the higher frequency for each mode. EXPLAINATION----

16. 2640 - 2560 cm-1 Vaida’s Experimental Assignment
Vibrational frequency (cm-1)
Gauche
2591
Trans
2599
Black = Experimental
Sticks = O3LYP/aug-cc-pV(T + d)Z
This is the spectra of the only previously assigned trans mode. So looking at Vaida’s experimental assignments, we can see our values were very close.
In addition, up here is also the theoretically calculated values for more comparison.
We were also able to assign a peak to each conformer. In addition we were able to compare the values to our computational values.
Although the experimental values are shifted slightly up field, the difference between the trans and gauch peaks is the same for both experiment and computation.
Scaling FACTOR: from Gauche experiment data ?

17. 1140 – 1040 cm-1 Black = Experimental Sticks = O3LYP/aug-cc-pV(T + d)Z
Once again to show you a new mode the same sort of comparison allowed us tentative assignments this and other modes.
*If needed mention how Gauche and Trans can alter which has the higher frequency for each mode. EXPLAINATION----
GOOD AGGREment

18. PIMS “Rapid” Identification of Thermal Cracking Temperature
Complimentary to Matrix Isolation IR
In order to find the correct heat to test Ethanethiol at PIMS was used.
PIMS is a very good beginning step because a test runs very quickly compared to MIIR and can test the sample at multiple temperatures.
In addition, mass to charge ration although not definitive in identifying molecules can give an idea at what mass an object might be at a specific temp.

19. Results from PIMS m/z value Compound 62 Ethanethiol (CH3CH2SH) 34
Hydrogen Sulfide (H2S) 33
SH Radical (SH)
Thermally cracking suggested possible presence of Sulfur Radical (SH*). However, also presence of SH2 which means some secondary reactions might still be occuring.
Also determined a temperature to focus on, 1300K.

20. m/z value
Compound 46
Thioformaldehyde : CH2= S 34 33 28 15
Hydrogen Sulfide: H2S
SH Radical: SH
Ethylene: H2C=CH2
Methyl Radical: CH3

21. Matrix Isolated Infrared Spectrometer Results From Thermal Cracking
Thermally crack Ethanethiol and obtain a spectrum.
Then compare original Ethanethiol vibrational frequencies, spectrum at 300K, and Ethylene spectrum at 300K to heated Ethanethiol spectrum, 1300K.
Now that parent peaks have been assigned, we will be able to identify unknown peaks in the IR as possible radical intermediates.
In order to be fully confident on this matter, however, we also needed to run an IR scan of Ethene at RT.

22. Preliminary Results
SH radical very well known in the gas phase not the matrix.
SH radical in matrix has much more rotation ability which accounts for the big shift.
Without preforming more experiments. For example with deuterated sample we can not claim that this is SH radical even though it is where we other researchers believe it should be. Past researchers have a wide range however of the possible locations ranging from So to sum up more work needs to be done.
On the right,

23. Mechanism for Ethanethiol+
Observed in PIMS
Observed in IR
CH3
CH2S
CH2CH2
H2S SH
CH2CH2
H2S SH

24. Summary/Current Work
Tentatively Assigned most vibrational frequencies to Ethanethiol.
Detect CH2SH and CH3 in matrix
Experiment with 50% CH3CH2SH and CD3CD2SD
Detect pyrolysis products for CH3SCH3 in IR

25. Acknowledgments Jessica Kong William Melhado Danny Anderson
Maggie Phillips
Thomas Cowell
AnGayle (AJ) Vasiliou
Thank Past and Present members of the Vasiliou Lab

26. Mechanism for Dimethyl Sulfide

27. Mechanism for Dimethyl Sulfide

28. Mechanism for Dimethyl Sulfide
Further decomposition of CH3S to SCH2.

29. Dimethyl Sulfide (check on time)

30. Dimethyl Sulfide (check on time)
CH3SCH3

31. Dimethyl Sulfide (check on time)
CH3SCH3
SCH3

32. Dimethyl Sulfide (check on time)
CH3SCH3
SCH2
SCH3

33. Dimethyl Sulfide (check on time)
CH3SCH3
SCH2
SCH3

34. Bibliography
Sehon, A.H., Darwent, B., The thermal decomposition of mercaptans, J. Am. Chem. Soc. 1954, 76,
Kong, J. Detecting the initial thermal decomposition products of organosulfurs. Department of chemistry and biochemistry, Middlebury College, 2015.
Melhado, W. A Molecular View of Decompositional Chemistry: Ethanethiol and Dimethyl Sulfide. Department of Chemistry and Biochemistry, Middlebury College, 2016.
Anderson, D. Theoretical Analysis of the Vibrational Structure of Ethanethiol by Gaussian Calculations. Department of Chemistry and Biochemistry, Middlebury College, 2016.
Vasiliou, A. Sulfur Chemistry: Molecular Mechanisms. Department of Chemistry and Biochemistry, Middlebury College, 2016.
Grossman, M. J. et al. “Microbial Desulfurization of a Crude Oil Middle-Distillate Fraction: Analysis of the Extent of Sulfur Removal and the Effect  of Removal on Remaining Sulfur.” Applied and Environmental Microbiology 65.1 (1999): 181–188. Print.

35. Maybe add at end SD spectrum observe shift neon spectra of SH radical
Dissociation SH with 315 nm light
Why no H2S

36. Additional content if needed

37. Fuels and Contaminants
Fossil Fuels
Biofuels
Organic sulfur contaminants originate as sulfate salts
Contain 0.1 to 5% sulfur by weight
Sulfur contaminants are preserved in biomass conversion
Contain <0.2% sulfur by weight
No matter the level of contamination the Sulfur MUST be removed due to its toxicity
However it really needs to be to the parts per billion
Sulfur needs to be removed because it degrades catalysts in the refining process and has harmful health effects
Sulfur contaminates deactivate catalysts in refining processes
Complete sulfur removal is necessary
Orr, W. et al. Geochem. of Sulfur in Fossil Fuels. 1990; Michaelides, E. Alt. Ener. Sour. 2012

38. Hydrodesulfurization
Desulfurization Techniques
Hydrodesulfurization
Most common desulfurization technique in refining industry
Crude oil is subjected to high pressure and temperature H2(g)
Expensive catalysts (Ni, Al, Mo) are required
Dirtier fuels require higher pressures and temps (and more cats?) to achieve same levels of acceptable sulfur levels in refined fuels
To reiterate sulfur contaminant ion is a major problem and these species are removed by desulfurization techniques
But…
Increasingly ‘sour’ fuels → increasingly energetically and fiscally expensive
Hydrodesulfurization has reached plateau
Timko, M. T.; Ghoniem, A. F.; Green, W. H. J. Supercrit. Fluids 2015, 96, 114–123.; Srivastava, V. RSC Adv 2012, 2 (3), 759–783.

39. Previous Decomposition Research
Ethanethiol
Dimethyl sulfide
CH3–CH2–SH
H3C–S–CH3
Thermal decomposition mechanism proposed
Experimental limitations
Center first two, then bold some shit
No one observed reactive intermediates
----- Meeting Notes (1/25/16 17:13) -----
reactive intermediates-eg. radical
these are small, imagine a bigger more complex system
Only final product observed: H2S
Only final products observed: H2S, CH4, C2H4, CS2, and C2H5SH
Laid solid groundwork, but…
No reactive intermediates were observed
Sehon, A. H.; Darwent, B. J. Am. Chem. Soc. 1954, 76 (19), 4806–4810. Shum, L. G. S.; Benson, S. W. Int. J. Chem. Kinet. 1985, 17 (7), 749–761.

40. Thermochemistry
Thermochemical data used to support proposed decomposition mechanisms
Upon pyrolysis, bond with the lowest bond dissociation enthalpy will homolytically cleave first
Highlight Bonds of interest
These should produce
We predict them to be this (supported by the literature)
----- Meeting Notes (1/25/16 17:13) -----
unimolecular decomposition model
Boivin, L. et al. Can. J. Chem. 1955; Oyeyemi, V. Chem. Phys. Chem. 2011; Chen, P. et al. J. Phys. Chem. 1986; Zhang, X. et al. Rev. Sci. Instrum

41. EtSH in Ar at 300 K
Ar at 300K
EtSH in Ar at 300 K
Ar at 300K
gauche
trans

42. Methods for Computation
Hartree-Fock (HF)
Assumes each electron only feels an average charge distribution due to other electrons
Møller–Plesset Perturbation Theory (MPx)
Treats electron correlation as small x-order perturbations to the Hartree-Fock description
Density Functional Theory (DFT)
Uses electron density instead of complicated many-electron wavefunctions
Coupled-Cluster
Expresses wavefunctions as an exponential products

43. Methods for Computation
Low accuracy
“Cheap”
Hartree-Fock (HF)
Møller–Plesset Perturbation Theory (MPx)
Density Functional Theory (DFT)
Coupled-Cluster
RHF
MP2
MOLLER-PLESSET PERTERBATION THEORY
-Further tweaking within each one
-HF: assumes that each electron only feels an average charge distribution due to other electrons (an approximation)
-MPx: treats electron correlation as a “small” x-order perturbations to the Hartree-Fock description
-More accurate than HF but cost goes way up (O(N5))
-DFT: uses the density instead of complicated many-electron wavefunctions
-Includes electron correlation and is only slightly more costly than HF
-Often very high accuracy (comparable to coupled-cluster)
-Not a convergent family of methods (does not converge to the infinite basis set limit/”true” value)
-CC: expresses the wavefunction as an exponential product
-Has higher-order correction terms built in as products of lower-order terms
-Very costly but highly accurate
BLYP, B3PW91, B3LYP, O3LYP
High accuracy
“Expensive”

44. Density functional theory
Uses electron density instead of complicated many-electron wavefunctions
Includes electron correlation and is only slightly more costly than Hartree-Fock
Often has very high accuracy (sometimes comparable to coupled-cluster methods)
Not a convergent family of methods
Does not converge to the infinite basis set limit

45. Schrödinger equation Born-Oppenheimer approximation
Nucleus
Born-Oppenheimer approximation
PES = property of a molecular formula
Schrödinger equation of nuclear motion
Eigenvalues give vibrational frequencies
Electron
-HAMILTONIAN – depends on only the positions and atomic numbers of the nuclei and the total number of electrons
-DFT = electron density can be integrated to find the number of electrons. Nuclei = point charges = local maxima in the electron density, derivative gives atomic number  form the Hamiltonian operator, solve the Schrödinger equation, and determine the wavefunctions and energy eigenvalues
-EIGENVALUE = ENERGY
-BARRIER TO SOLUTION = WE DON’T KNOW THE EXACT FORM OF V
-The Born-Oppenheimer approximation is the assumption that, since protons and neutrons are roughly 1800 times more massive than electrons, the motion of electrons can be considered as relative to fixed nuclei
-Under this assumption, we can define a PES as an inherent property of a molecular formula
-Allowing us to solve Schrödinger equations for nuclear motion
-[Hamiltonian operator] (laplacian)
-N is the number of atoms in the molecule, m is the atomic mass, V is the potential energy (from the PES) of the given geometry, q, and Ψn is the nuclear wavefunction
-Solving Schrödinger equations for nuclear motion allows us to associate the 3N – 6 components of q that correspond to molecular vibrations with sets of wavefunctions and eigenvalues that represent the IR vibrational frequencies of the ‘normal modes’ of the molecule

46. O3LYP CCSD(T) frequencies for SH stretch (literature)
Gauche = 2591 cm-1
Trans = 2599 cm-1
O3LYP frequencies for SH stretch
Gauche = 2592 cm-1
Trans = 2595 cm-1
Experimental frequencies for SH stretch (MIIR)
Gauche = 2596 cm-1
Trans = 2598 cm-1
-COUPLED CLUSTER = EXPENSIVE
-FREQUENCIES = COMPARABLE
-OUR CALCULATED = LESS EXPENSIVE (COMMUNITY MOVING IN THIS DIRECTION)
-HIGHTLIGHT WHICH IS THEIRS/OURS (MORE IMPORTANCE TO OURS)
-DIDN’T PUBLISH THE REST (WASN’T AS GOOD?), SHOWED ALL THE B3LYP
-Under 4 wavenumbers off of CCSD(T) AND experiment
Miller CCSD(T) frequencies for SH
Significantly more expensive method
First complete experimental identification of the trans conformer of ethanethiol
Not just based off of one mode (SH str.)
2591 (2599)
Miller, B. J.; Howard, D. L.; Lane, J. R.; Kjaergaard, H. G.; Dunn, M. E.; Vaida, V. SH-Stretching Vibrational Spectra of Ethanethiol and Tert-Butylthiol.
J. Phys. Chem. A 2009, 113 (26), 7576–7583.