Computational and Spectroscopic Investigation of Solution Phase Excited State Dynamics in 7 azaindole

Transcription

1 Computational and Spectroscopic Investigation of Solution Phase Excited State Dynamics in 7 azaindole Nathan Erickson, Molly Beernink, and Nathaniel Swenson Midwest Undergraduate Computational Chemistry Symposium February, 2009

2 Background 7AI Dimer Previous studies have shown that 7 azaindole (7AI) readily forms H bonded dimers in solution 1 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 (nonnegative). 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, (2) Kyogoku, Y.; Lord, R. C.; Rich, A. J. Am. Chem. Soc. 1967, 89, 496. Example of DNA Base pairs H-Bonding 2

3 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 step 1 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. 1. Douhal, Kim, and Zewail, Nature, 1995, 378,

4 Goals Solvent dependent geometry and energetics Solvent dependent excited state dynamics Resonance Raman and simulations: Window into excited state dynamics 7AI dimer double excited state proton transfer parallel or serial? Often reflects dynamics of mixture of species in solution 4

5 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 5

6 7 azaindole dimer geometry B3LYP/6 31G(d) CPCM Image: VMD 6

7 Continuum Solvation Energetic comparison B3LYP/6-31G(d) CPCM implicit solvation Gibbs Energy (Hartree) kcal/mole Solvent monomer dimer G water methanol acetonitrile

8 Laser Raman Spectroscopy Raman vs. resonance Raman Raman Resonance Raman UV-Vis spec showing virtual level absorption 300 Quantum Electronic Diagram Virtual Level 400 Ground State Laser Rayleigh Raman Wavelength [nm] Resonance enhancement: 1. ~ Chromophore selective 3. Sensitive to local structure 4. Sensitive to excited state dynamics (100 s fsec) 8

9 or 532nm light from Nd:YAG laser 2. H 2 Raman Shifter 3. Dispersal Prism Experimental Setup A very simple guide to how our setup works: 5. Wavelength Selection 6. Sample 7. Light Collection 8. SPECTRA! 9

10 Nuts and bolts of spectral simulation I k ω L ( ω ω ) Δ ω L k k k Intensity of spectral line associated with kth vibration Frequency of the laser (L) and the kth vibration (k). Change in geometry (reflected in gradient) between ground and excited state along kth vibrational mode. 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 84 vibrational modes in 7AI-dimer 10

11 Computational Spectral Simulation Theory Resonance Raman Intensity Calculation Short time wave-packet propagation approximation I k Intensity of the k th vibrational band: ω ( ω ω ) Δ ω L L k k k I k ω L ( ω ) Φ( ω ω ) ( ω ω ) Δ Φ L k k L L k 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

12 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

13 Resonance Raman Spectrum 0.001M 7AI in Methanol 13

14 Experimental vs. Computational 14

15 Mode 73 Largest RR enhancement Large component along ESDPT coordinate Strong experimental RR enhancement at similar wavenumber Ultrafast ESDPT dynamics sensitivity 15

16 Explicit solvation 7AI methanol normal mode along proton transfer coordinate. Contributes a portion of observed RR signal in methanol 7AI methanol geometry 16

17 Theoretical treatment of explicit solvation 7AI with 1 explicit methanol methanol - 7AI H-bond distance Computed N-H stretch (cm-1) 1.85 HF/aug cc pvdz O3LYP/aug cc pvdz B3LYP/aug cc pvdz MP2/aug cc pvdz H Bond (Å) N H stretch (cm 1)

18 Implicit solvation hydrogen bonding (+ continuum) 7AI Methanol H Bond distance (Å) HF/6 31G(d) O3LYP/cc pvdz B3LYP/6 31G(d) B3LYP/cc pvdz MP2/6 31G(d) BLYP/cc pvdz 18

19 Solvation Theories Compared 19

20 Simulated RR of species in solution vs. experimental RR

21 Basis Set comparison Simulation of single methanol with 7AI monomer 21

22 Deuteration study 22

23 Solvent Comparison 23

24 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? Serial or parallel proton transfer mechanism 24

25 Possible Proton Transfer mechanisms Serial Parallel 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. 25

26 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) 26

27 Conclusions Energetics Dimerization of 7AI is unfavorable in solution Computation: + G values with BSSE Evidence of solvent interactions with 7AI monomers Hydrogen bonding is favorable for the solvents we studied Dyanamics Modes which show greatest enhancement correspond to initial proton transfer dynamics along parallel trajectory, (synchronous). 27

28 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 28