Investigating excited state dynamics in 7-azaindole

Nathan Erickson, Molly Beernink, and Nathaniel Swenson

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Background I• 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 (non-negative).2 – other significant factors that contribute to the stability of

these systems.

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(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.

7AI Dimer

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.

31. 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?

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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

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7-azaindole dimer geometryB3LYP/6-31G(d) CPCM

6Image: VMD

Continuum Solvation

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Energetic comparison B3LYP/6-31G(d) CPCM implicit solvation

Gibbs Energy (Hartree) kcal/mole

Solvent monomer dimer ∆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

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Raman

Wavelength [nm

]

300

400

500

600

700

800

Resonance Raman

Virtual Level

Ground State

Ray

leig

h

Ram

an Resonance enhancement:1. ~105

2. Chromophore selective3. Sensitive to local structure4. Sensitive to excited state

dynamics (100’s fsec)

Lase

r

Raman vs. resonance Raman

UV-Vis spec showing virtual level absorption

Quantum Electronic Diagram

Experimental Setup

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1. 355 or 532nm light from Nd:YAG laser

2. H2 Raman Shifter

3. Dispersal Prism

5. Wavelength Selection

6. Sample

7. Light Collection

8. SPECTRA!

A very simple guide to how our setup works:

Nuts and bolts ofspectral simulation

223)( kkkLLkI

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Intensity of spectral line associated with kth vibration

Change in geometry (reflected in gradient) between ground and excited state along kth vibrational mode.

{Frequency of the laser (L) and the kth vibration (k).

Computational Spectral Simulation Theory

11Jarzecki and Spiro, J. Phys. Chem. A., 109 (2005)

Resonance Raman Intensity CalculationShort time wave-packet propagation approximation

223)( kLLkkLLkI

223)( kkkLLkI

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)

Intensity of the kth vibrational band:

Resonance Raman spectral Simulation:

Three Computational Steps:

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The vibrational modes are then scaled by Quantum mechanical force constants based on internal coordinates.

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.

Web Interface for Spectral Simulation

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3

1

2

Three steps:

Simulated dimer RR spectrum

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Mode 29808 cm-1

Mode 481145 cm-1

Mode 621469 cm-1

Mode 29Largest RR enhancement

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Large component along ESDPT coordinateStrong experimental RR enhancement at similar wavenumberUltrafast ESDPT dynamics sensitivity

Simulation comparison

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Resonance Raman of 7AI: Experiment Meets Theory

223 nm excitation wavelength7AI solvated in Methanol

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Explicit solvation

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Implicit and Mixed solvation vs Experimental

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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?

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Possible Proton Transfer mechanisms

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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.

Serial

Parallel

Simulation Grid

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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)

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

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Future Directions

• Analyze isotopic RR spectral data • Time domain laser-induced fluorescence

experimentation of system• TDDFT calculations on 7AI system

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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

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