MUCCC10-EricksonBeerninkSwenson

Investigating excited state  dynamics in 7‐azaindole Nathan Erickson, Molly Beernink, and Nathaniel Swenson Background I 7AI Dimer • Previous studies have shown that 7‐azaindole (7AI)  readily forms H‐bonded dimers in solution1 • The N‐‐‐H‐N bonds in 7AI dimer are simple models the  of adenine‐thymine base pair interaction of DNA • The 7AI dimer and DNA base pairs have higher than  expected Gibbs energies of association (non‐ negative).2 – other significant factors that contribute to the stability of  these systems.  (1) Ingham, K.; El-Bayoumi, C M J Am Chem Soc 1971, 93, 5023 (2) Kyogoku, Y.; Lord, R C.; Rich, A J Am Chem Soc 1967, 89, 496 Example of DNA Base pairs H-Bonding Excited state double proton (ESDPT) • This is a possible mechanism for photo‐damage of DNA • Gas phase experiments have given insight into time scales – A serial transition of the protons in the excited state – First electron shuttles in 650 fsec step1  • Solvated system experiments have shown evidence of both  parallel and serial transition mechanisms • We are further investigating transition mechanisms in various  solvent systems through resonance Raman Douhal, Kim, and Zewail, Nature, 1995, 378, 260 Goals • Solvent dependent geometry and  energetics • Solvent dependent excited state  dynamics • Resonance Raman and simulations:  are we there yet? Computational Overview • • • • 7AI dimer geometry Implicit, explicit, and mixed model Gibbs energy of association  Resonance Raman spectral simulation – Compared with experimental spectra – Correlated with dynamic modes of prevalent peaks  to search for evidence of ESDPT – Generated step‐wise electron transition models 7‐azaindole dimer geometry B3LYP/6‐31G(d) CPCM Image: VMD Continuum Solvation Energetic comparison B3LYP/6-31G(d) CPCM implicit solvation Gibbs Energy (Hartree) Solvent monomer dimer kcal/mole ∆G water ‐379.78923 ‐759.56718 7.1 methanol ‐379.78872 ‐759.56648 6.7 acetonitrile ‐379.78882 ‐759.56742 6.4 Laser Raman Spectroscopy Raman vs resonance Raman Raman Resonance Raman UV-Vis spec showing virtual level absorption 300 Quantum Electronic Diagram 400 Raman Rayleigh Wavelength [nm] Laser Virtual Level 500 600 700 800 Ground State Resonance enhancement: ~105 Chromophore selective Sensitive to local structure Sensitive to excited state dynamics (100’s fsec) Experimental Setup A very simple guide to how our setup works: 355 or 532nm light from Nd:YAG laser H2 Raman Shifter Dispersal Prism Wavelength Selection Sample Light Collection SPECTRA! Nuts and bolts of spectral simulation I k ∝ ω L (ω L − ωk ) Δ k ωk 2 Intensity of spectral line associated with kth vibration Change in geometry (reflected in gradient) between ground and excited state along kth Frequency of the laser (L) vibrational mode and the kth vibration (k) 10 Computational Spectral Simulation Theory Resonance Raman Intensity Calculation Short time wave-packet propagation approximation Intensity of the kth vibrational band: I k ∝ ω L (ω L − ωk ) Δ k ωk 2 I k ∝ ω L (ω L − ωk ) Δ k Φ (ω L ) − Φ (ω L − ωk ) Scaled quantum mechanical force constants (SQM) are added to the final calculated frequencies to better correlate with experimental data ~15 cm-1 vibrational frequency accuracy Baker, Jarzecki, Pulay, J Phys Chem A., 102, (1998) Jarzecki and Spiro, J Phys Chem A., 109 (2005) 11 Resonance Raman spectral Simulation:  Three Computational Steps: 1.) Ground State: B3LYP/6-31G(d) frequency and optimization Vibrational modes for subsequent calculations generated 2.) Excited State (resonant state): CIS/6-31G(d) force (gradient) using the optimized geometry from calculation #1 3.) HF/6-31G(d) frequency to correct the gradient predicted in calculation #1 The vibrational modes are then scaled by Quantum mechanical force constants based on internal coordinates 12 Web Interface for Spectral  Simulation Three steps: 13 Simulated dimer RR spectrum Mode 29 808 cm-1 Mode 48 1145 cm-1 Mode 62 1469 cm-1 14 Mode 29 Largest RR enhancement Large component along ESDPT coordinate Strong experimental RR enhancement at similar wavenumber Ultrafast ESDPT dynamics sensitivity 15 Simulation comparison 16 Resonance Raman of 7AI: Experiment  Meets Theory 223 nm excitation wavelength 7AI solvated in Methanol 17 Explicit solvation 18 Implicit and Mixed solvation vs Experimental  19 Probing excited state dynamics • Strategy: – Compute excited state gradient on a grid of proton  positions for dimer – Simulate corresponding spectra – Compare to experimental with different solvents – What is timescale for dynamics? – Time snapshot for experiment? 20 The transfer positions are in a ratio of 0-0 indicating the starting position and 10-n indicating a fully transferred proton(s) *Please wait for the animation to start, no clicks necessary Parallel Serial Possible Proton Transfer mechanisms 21 Simulation Grid Created from computations of implicitly positioning the protons between the N’s of the 7AI Dimer ( relative proton position on the right side of the figure) 22 Conclusions • Dimerization of 7AI is unfavorable in aqueous solution – Computation: + ∆G values – Experiment al spectra do not match dimer simulations • Evidence of solvent interactions with 7AI monomers – Hydrogen bonding is favorable for the solvents we studied – Can correlate simulated RR peaks of monomer and solvent to  experimental spectra • Mechanism dynamics were investigated in step placement of  protons • Mixed Solvation and Implicit simulations are very similar 23 Future Directions • Analyze isotopic RR spectral data  • Time domain laser‐induced fluorescence  experimentation of system • TDDFT calculations on 7AI system 24 Acknowledgements • • • • • • • • Dr. Jonathan Smith Michael Kamrath, Krista Cruse Midwest Undergraduate Computational Chemistry Consortium NSF‐MRI ACS‐PRF NSF‐CCLI Gustavus Adolphus College Chemistry Department Sigma Xi local chapter 25