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  • 1. Applications of Resonance Enhanced Raman Spectroscopy: Electronic Structure Probe of Metal-Sulfur Interactions in Oxo-MolybdenumEne-1,2-Dithiolate SystemsFrank E. InscoreThe University of Arizona C H3 (Tp*)MoO(bdt)OH3C N S NHBMoNH3CN SC H3 C H3N N H3CActive Site of Sulfite Oxidase

2. The Raman Spectroscopic TechniqueGeneral ConsiderationsResearch and Industrial ApplicationsStructure/ Solid State/ Biological ChemistryUtility of Raman SpectroscopyProblematic: Inherent Weak EffectProblematic: Fluorescence ComplicationsProblematic: Instrumental LimitationsDevelopment of New TechniquesFT-RS/ SERS/ RERS 3. Resonance Raman Spectroscopy Characterizing Structure/ Monitoring Reactivity in Catalytic SystemsChemical and Petroleum/ Energy Production Industries Catalyst Structure and Reactivity: Surface and In Situ StudiesHeterogeneous processes: Supported metal oxides (MoO/ WO) used as catalyst.Hydrodesulfurization catalyst: Removal of sulfur from petroleum feedstocks.Biological Systems Structure/ Function In Situ StudiesProtonation in Biomolecules: S-H/ S-S conversion.Mechanistic insight into Carcinogenesis: Blue/green particle in tumors; Cu-S bonding.Structural Insight in Metalloproteins. 4. Overview of Presentation Raman Applications What we are doing? Background Electronic Structure Why we are studying? Raman Instrumentation Resonance Raman Studies How we will probe? Implications for Mo EnzymesC H3 O H3CN SN HBMoN H3CN S C H3 C H 3 NNH3C 5. The Importance of Metal 1,2-Dithiolene ComplexesGeneral ConsiderationsWhy the Interest in Transition Metal-Sulfur Complexes? Industrial Applications/ Commercial Uses:Vulcanization Accelerators for Rubber Wear Additive Inhibitors in LubricantsCatalytic Inhibitors /Oxidation CatalystMode-Locking Additives in Nd Lasers Potential Biological Activity:Correlations with Biological Systems containing Metal to Sulfur Bonds.What Is an Ene-1,2-Dithiolate Ligand ? Four Prototypical Ene-1,2-Dithiolate Systems: -S H-S-S N-SS-SMS= -S-S-S N-SS-S HSRelevance to structure, bonding and function of Metalloenzyme active site centers 6. Pyranopterin Molybdenum and Tungsten Enzymes Background and Significance3 Mo Families based on structure and reactivity2 W Families similar to DMSO reductase familyOS O S er -OSO OMo S S S MoMoSS - C ysSO H2 S SSulfite Oxidase Xanthine Oxidase DMSO Reductase # X-ray crystallography revealsa common structural unit: Pyranopterin cofactorOS- H N S-HNH2N NN OOPO 2- H 3 7. The Resonance Raman Spectroscopic ProbeStructure/ Bonding in the Active Site of DMSO ReductaseSingle Metal Redox Center RO O (VI)XAS [Mo(VI,V,IV)]SS MMCD/ EPR [Mo(V)] SS(Mo-S)Electronic Absorption (Mo=O)xResonance RamanObserve enhanced isotopic sensitive Mo=O and Mo-S vibrations.Parallel model studies on both relevant and simpler systems needed. 8. Outstanding Issues in Pyranopterin Mo Enzyme CatalysisPrimary IssueWhat is Structural and Functional Role of the Pyranopterin Ene-1,2-Dithiolate Unit During Course of Catalysis? Research Goal Derive fundamental understanding at molecular level, into how the unique geometric and electronic structure of these enzyme active sites contribute to their reactivity.Research ObjectivesUtilize available physical characterization methods to determine thegeometric and electronic structure of small synthetic active site analogs.Derive key factors that define geometric/electronic structure relationshipsand correlate to the unique enzymatic spectroscopic features and theirelectronic contributions to structure-bonding/ function. 9. Chemical Evolution of Mo and W Dithiolene SystemsThe Reductionist ApproachCH3H3C OO-1, -2O -1NSRO HB NS SS SN N MMMH3CS S SS S CH3-1 NCH3 OR NS S-1,0M H3CS S S S CH3 M SSH3CS -2 N N Mo SOO HB S SH3C N N S MS S CH 0, +1N 3CH3N S MS -2H3C OOCH3 SSROMH3C 0, -1 SYN N SHB NMo OH3C N NS SS 0, -1, -2-1 S SR SSM CH3 M S Y NCH3SS NH3C 10. Minimal Structural Models/ Effective Spectroscopic Models Simple model; Mo coordinated by Ene-1,2-Dithiolate and terminal Oxo. Isolated Oxo-Mo-Dithiolate Center; Controlled six coordinate environment. Possess Mo(V) paramagnetic centers; Amenable to EPR/ MCD probes. C H3EH3C N N S(S-S) HBMNS H3 CN Cl -S-S C H3 - S-S N CH C H 3 N 3 -S-S-S -S N N(Tp*)ME(S-S) Cl H3CProbe fundamental properties of Oxo-Mo mono-ene1,2-dithiolate complexes:Metal (M = Mo, W), axial (E = O, S, NO) and dithiolate (S-S) coordination effects. 11. MCD and Electronic Absorption SpectroscopyLow Temperature Solid-State (PDMSO Mull) Studies0.81000Absorption (5K) MCD (5K /7T)0.70.6500 MCD Intensity (mdeg)0.5Absorbance0.400.30.2 12 -500 (3, 4)5 6 70.1 0 -10008000 12000 160002000024000 28000 32000 Energy (wavenumbers)Complimentary selection rules:resolve electronic transitions in spectra 12. Band Assignments from Combined Spectroscopic Approachy az2Solution Electronic Absorption (DCE) y ax2-y24 6400y xz y ayz a7 36756004800 y axy1 25Epsilon (M cm )-14000 6 y aopy aop-132002400y aip 51600y aip 8001 2 (3, 4)axy + aip axy + aop ayz + aopO0MS 8000 12000 1600020000 24000 28000 32000= 900Energy (wavenumbers)OMS > 90 0 13. Resonance Raman Scattering Enhancement of the Raman signalSensitive and selective probe of structure/ bonding Vibrational frequencies: sensitive to inner coordination environment. Intensitiy: selective enhancement associated with absorbing metal center. Resonance FC - A TermNormal RamanRayleigh RamanOHT - B Term EE1o oM S O SIR M S E0 SSelectivity based on resonant electronic transition and excited state distortion. Intensity depends on energy and intensity of electronic absorption band. Enhancement result of coupling with electronic excited state. 14. Raman Experimental Instrumentation and TechniquesDesign and MethodologyGoal: Obtain Low-frequency vibrational information regarding M-S bonding. ComputerSystem ControllerInterfaceTitanium Sapphire LaserArgon Ion LaserCCDSample Krypton Ion LaserIllumination/ CollectionPre Monochromator Optics SPEX 1877SPEX 1405Triplemate 15. Collection GeometryComputer SystemController InterfaceArgon Ion Laser 90 degree geometry CCD Sample Illumination/ Collection Optics SPEX 1877135 degree back scattering geometry Triplemate 16. The Resonance Raman Experiment 17. Laser Enhanced Raman Spectroscopy 18. Sample Illumination and Collection Optics 19. Sample Handling, Detection and Dispersal SystemSamples Problematic: Photo Decomposition/ Thermal Degradation? 20. Vibrational Raman SpectroscopyC H3 (Tp*)MoO(bdt) in NaCl/ Na2SO4 O8500 H 3C140K 528.7 nm ~40 mWN S 8000NHB Mo 7500 3 NH3C N S70001 C H3 C H 6500 6Raman Intensity (cps) N 36000N5500 300 400 500600700 800 9001000 1100 (Tp*)MoO(bdt) in BenzeneH 3C2000293K 514.5 nm ~75 mW1900 3 vibrational bands observed 180017001600 6 = 362 cm-1 1 = 393 cm-1 3 = 932 cm-1 1500 3 6 114001300 300400500 600 700 800 90010001100 Raman-shift (wavenumbers)Identify normal modes coupled to electronic transitions 21. Solution Raman Depolarization Studies2000Depolarization Ratio (Tp*)MoO(bdt) in Benzene1900293K 496.5 nm ~75 mW1800 = I/ I 1700 Parallel polarization0 3/4 1600 I 150031400 6 1Raman Intensity (cps)1300 300 400 500 600700 800 9001000 1100 Totally symmetric (polarized)Non-totally symmetric (depolarized) 200019001800 Perpendicular polarizationRatio indicates 3 modes are totally symmetric 17006(A = 362 cm-1)1600 I15001(A = 393 cm-1)1400 6 13 3(A = 932 cm-1 ) 1300 300 400 500 600700 800 9001000 1100 Raman-shift (wavenumbers) 22. Vibrational Analysis (Tp*)MoO(bdt) in Benzene2000CH3O H3COzzS NN S1900Mo N N Mo S HBSRaman Intensity (cps) H3C1800 OOCH3 CH N31700NM y (zy)My B N N SS1600 Sx S 362 cm-1 H C 3932 cm-1 x N -11500 ( A ) 393 cm ( A ) 6 3 ( A )11400 O O O1300 300 400500600700 800 9001000 1100 M M M-1 S S SS SS Raman-shift (cm ) 1 ( A ) 2 ( A ) 3 ( A )Key Points:3 bands observed polarized (A symmetry) O O O M M M Intensity enhancement patterns consistent S S SS SS With M-S/ M=O vibrational assignments 4 ( A ) 5 ( A ) ( A ) 6Resonance Raman spectroscopy probes:Differences in bonding between ground and excited states via distortions along specific normal modes. 23. Solid-State Excitation ProfilesKey Points:Observe large differential enhancement of Mo=OTransitions probed are orthogonal (in-plane vs out-of-plane)O(Tp*)MoO(bdt): 8K PDMSO mull EA; 100K RR NaCl/ Na2SO4 MS S3( A )Sop Mo dxz,yz OSip Mo dxy S M S 1( A ) O M S S6( A )Conclusions:Sip Mo dxy CT probes covalent contributions to ground-state S Mo CT probes electronic contributions to redox potentials 24. Implications for Catalytic Reactivity in EnzymesLowest energy (intense) CT must be Sip Mo dxy This CT transition probes covalency contributions to ET pathway.M=O aligns redox orbital for facile ET via unique 3-center 2-electron bond. OSO32-SO42-OOOH 2 SS yxya Mo (VI)Mo (IV) S S cysS S cysxyaH+, e-H2OS-Moxy3-center H+, e-pseudo- antibonding O OH S-Moxy3-centerSpseudo- bonding Mo (V) a ipS S cysCriteria for efficient ETyipa Reason Nature has chosen ene-1,2-dithiolate and M=O groupsGood M-L overlap/ Minimize ROE 25. ConclusionsResonance Raman Important Probe of Ground and Excited State StructureState of the Art Equipment Necessary for probing M-S Bonding. Contributions of M-L Bonding to Electronic Structure Elucidated by RR Especially when Combined with other Spectroscopic Techniques.RR Spectroscopy Important Tool for Characterizing Enzyme Active Siteswhen Interpreted within Context of Well-Defined Small Molecular Models.Protocols Developed can be Applied to more Complicated Systems. 26. Acknowledgements and Funding* HeI Prof. John H. EnemarkHOMOPseudo anti-bonding Mo dxyh = 579 nmEnemark Research GroupHOMO -1&-2 HeII SipUniversity of ArizonaPseudo bonding *HOMO-3 &-4 * * 10.5 10 9.5 9 8.5 8 7.5 7 6.5 HOMO-5 Ionization Energy (eV)Prof. Martin L. KirkNational Institutes of HealthNational Science Foundation Kirk Research GroupUniversity of New MexicoPetroleum Research FundSandia National Laboratories C16 O C14C15 S2