Modeling of S-N Bond Breaking in an Aromatic Sulfilimine. By Jacob Brunsvold & Katrina Hanson Advisor: Stacey Stoffregen

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1 Modeling of S-N Bond Breaking in an Aromatic Sulfilimine By Jacob Brunsvold & Katrina Hanson Advisor: Stacey Stoffregen

2 Outline! Background Photochemical Reaction! Introduction to Photochemistry and Quantum Yield! Wet Chemistry Background! Similar Photochemical Reactions! Computational Analogs! Proposed Mechanism for Sulfoxide Deoxygenation! Computational Details for Modeling Sulfilimine Dissociation! Hartree-Fock Results! CASSCF Results! Summary of Results and Conclusions

3 Deoxygenation of Aromatic Sulfoxides O S hν hν S + O( 3 P) Φ < 0.01 Upon photolysis, dibenzothiophene-s-oxide (DBTO) deoxygenates with a low quantum yield. Chemical trapping studies were consistent with formation of O( 3 P), suggesting involvement of 3 DBTO. An understanding of the reaction mechanism is currently being sought. 2p x 2p y 2p z 2s Ground State Electron Configuration J. Org. Chem., 2004, 69 (24), pp

4 Photochemistry Energy Singlet Products Singlet Excitations (light absorbed) Second Excited Singlet (S 2 ) Rxn First Excited Singlet (S 1 ) Fluorescence (light emitted) Intersystem Crossing (ISC) (heat emitted) After the molecule has been excited to the first or second lowest excited singlet state via light absorption, there are many potential pathways. Second Lowest Energy Triplet Excited State (T 2 ) Phosphorescence (light emitted) Lowest Energy Triplet Excited State (T 1 ) Rxn Triplet Products Ground State S 0

5 Quantum Yield hν φ = # of observed events # of photons supplied J. Org. Chem., 2008, 73 (12), pp

6 Quantum Yield hν φ DBT + O3P formation = J. Org. Chem., 2008, 73 (12), pp

7 Which excited state is the Precursor to O( 3 P)? Atomic oxygen, the observed product, is a triplet in the ground state. Since the molecule under investigation excites from a single ground state to a singlet excited state, reactivity must be accompanied by intersystem crossing. Difficulty in explaining product formation occurs because intersystem crossing to the spectroscopic triplet yields an excited state that is lower in energy than the product. The energy required to break the S-O bond once the spectroscopic triplet has been reached exceeds the energy potential of the molecule. S 1 T 1 S 0 ISC to the spectroscopic triplet = no deoxygenation

8 Deoxygenation Observed with Higher Quantum Yields An increased quantum yield was observed with the introduction of one or more halogen substituents to the benzene ring. J. Org. Chem., 2004, 69 (24), pp

9 Deoxygenation Observed with Higher Quantum Yields Replacing the sulfur with a more massive selenium corresponded to an even greater increase in quantum yield. Heavy atoms increase the probability of intersystem crossing events, supporting further the belief that an ISC event is part of the mechanism. J. Org. Chem., 2008, 73 (12), pp

10 Computational Modeling of Sulfoxide Deoxygenation! Deoxygenation was computationally modeled to identify a reaction reasonable mechanism. O S Thiophene-S-oxide! Thiophene-S-oxide was chosen for computational analysis because of its likeness to the aromatic sulfoxides previously studied while maintaining minimal size, reducing computational expense and increasing efficiency.! The ground and lowest four excited states were optimized at fixed S-O bond lengths using CASSCF and MRMP2 calculations to mimic deoxygenation. J. Org. Chem., 2008, 73 (12), pp

11 Energy Surface of Thiophene Oxide Deoxygenation 1A = Second Singlet State 1A = First Singlet State 3A = Second Triplet State 3A = First Triplet State 1A = Ground State (singlet) Photochem. Photobiol. Sci., 2014, (13),

12 Proposed Mechanism of Thiophene Oxide Deoxygenation Excitation into a singlet state, followed by intersystem crossing into the 3A phantom state, provides accessibility to triplet atomic oxygen and thiophene. Photochem. Photobiol. Sci., 2014, (13),

13 Photochemistry Proposed Mechanism for Aromatic Sulfoxide and Selenoxide Deoxygenation Note that the triplet state depicted in the mechanism is T2, a phantom triplet state, not the spectroscopic (T1) state. As previously mentioned, the spectroscopic state is too low in energy to explain the breaking of the S-O bond and the consequent formation of the triplet product.

14 Generation of Other Reactive Intermediates hν hν hν The same dissociation photochemistry is seen with aromatic sulfilimines and sulfonium ylides.

15 Molecule Under Investigation H N S 1H-1λ4-thiophen-1-imine The molecule above was chosen for computational analysis because of its likeness to the aromatic sulifilmines previously studied and will be used to ascertain whether a mechanism similar to that proposed for thiophene-s-oxide is reasonable for sulfilimines.

16 Computational Details! Programs : GAMESS and MacMolPlt! 6-31G(d,p) basis set! Cs symmetry maintained! Hessians confirmations output file

17 Energy Minimization of a Structure Energy Iteration

18 S-N Bond Constraints 1.58 Å 1.88 Å 3.58 Å

19 180 HF/6-31 G (d,p) Optimizations of NH-Thiophene Hartree-Fock Relative Energy Plot Energy Relative to Ground State (kcal/mol) NH-thiophene Ground State NH-thiophene Triplet Excited State S-N Bond Length (Angstroms)

20 CASSCF Calculations! Geometry NH-thiophene re-optimized using CASSCF method! Coordinates and orbitals of HF optimization were the starting point for the CASSCF calculations.! The initial optimization missing the C-Sσ* orbital! Orbitals reordered to include the missing orbital! Last calculation had all expected orbitals! Bond constrained calculations pending! Generating potential energy plot

21 Active Space N-S σ N-H σ C-S σ A C-S σ A S-LP N-LP N-LP C-C π A C-C π A N-S σ* N-H σ* C-S σ* A C-S σ* A C-C π* A C-C π* A The orbitals selected for the active space were included because they are expected to experience the most change during the bond breaking process.

22 Active Space Virtual Orbitals Active Space Active Space All electron configurations are examined for the orbitals in the active space. Those in the core are considered to be fully occupied, while those in the virtual orbitals are considered to be empty. Core Orbitals

23 CASSCF Optimizations of NH-Thiophene CASSCF Relative Energy Plot 5 Energy Relative to Ground State (kcal/mol) S-N Bond length (Angstroms) NH-thiophene Gound State

24 Summary of Results! Optimized ground state and one triplet excited state of NH-thiophene at HF/6-31G(d,p)! Using coordinates from HF calculations as starting point to generate the potential energy surface for NH-Thiophene at CASSCF/6-31G(d,p)! Will identify several of the lowest excited states and generate the potential energy curve using higher levels of computational theory and larger basis sets! Will compare results for sulfilimine dissociation with model of sulfoxide deoxygenation

25 Acknowledgements! Advisor: Dr. Stacey Stoffregen! Dr. McLaughlin! Funding: National Science Foundation! Midwest Undergraduate Computational Chemistry Cluster! The University of Wisconsin- River Falls Chemistry Department