Photophysical properties of protonated aromatic hydrocarbons

1
Photophysical properties ofprotonated aromatic
hydrocarbons

• Vadym Kapinus
• Department of Chemistry
• Blake group

2
PAHs in Space

• Polycyclic aromatic hydrocarbons (PAHs) are the
  most abundant free organic molecules in
  interstellar medium (ISM), as a class.
• As evidence, the Unidentified IR emission bands
  (UIRs) are most likely produced by PAHs.

Sample UIR spectrum
3
Astronomical Spectra
4
PAHs and DIBs

• Diffuse interstellar bands (DIBs) unassigned
  absorption bands from diffuse interstellar
  clouds. Discovered in 1920s.
• PAHs are possible carriers of DIBs.

• In diffuse clouds PAHs would be ionized and may
  protonate easily.
• Protonated PAHs are closed shell ions and have
  similar to neutrals electronic structure.
• Their electronic transitions are red-shifted with
  respect to neutral PAHs. May expect even smaller
  molecules to absorb in DIB l range.

5
Importance in Chemistry and Biology

• Protonated aromatic hydrocarbons are close in
  structure to intermediates in aromatic
  electrophilic substitution reactions.

C6H6 E - gt C6H6E - gt C6H5E H E
Cl, Br, NO3, SO3H, etc.

• DNA base pairs in cells form bridges through
  proton transfer.
• UV spectroscopy of protonated aromatic molecules
  may shed more light on cell radiation damage.

6
Goals
Tasks to perform

• Determine possible structures of protonated PAHs.
• Find out how protonated PAHs interact with UV
  visible radiation.
• Measure electronic spectra of protonated PAHs.
• Due to experimental considerations will work on
  photodissociation spectra.

Main questions
Quick answers
1. Do protonated PAHs exist in interstellar
medium? 2. If yes, do they produce DIBs?
Most likely, Yes Most likely, No
7
Protonated Benzene Structure

• Ab initio structures for protonated benzene CH2
  benzenium (1),
• bridged benzonium (2) and ring (3) isomers.

C6H7
Protonated benzene DFT structure. Proton binds
to carbon atom !
8
Protonated Benzene Experiments

• C6H7 UV dissociation in
• FT-ICR mass spectrometer
• (Freiser, Beauchamp 1976)

C6H7 Ar cluster IR dissociation (Solca,
Dopfer 2002)
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Calculations Details

• GAUSSIAN 98W - calculations
• GaussView - visualization
• PC configuration Intel Pentium 4 2.72GHz CPU,
• 1Gb PC1066 RDRAM, 30Gb on HD, Microsoft Windows
  XP SP1

• Geometries and vibrational frequencies calculated
  with density functional theory
• B3LYP 6-311G(2d,2p) - benzene
• B3LYP 6-311G(d,p) - naphthalene
• B3LYP 6-31G(d) - anthracene, phenanthrene,
  pyrene
• Excited electronic states calculated with
  configuration interaction singles method
• CIS 6-311G(2d,2p) on B3LYP
  6-311G(d,p) geometries

10
PC Choice for GAUSSIAN 98

• Tested with GAUSSIAN 98W, Revision-A.9
• Test systems
• P4 Intel Pentium 4 2.66 GHz _at_ 2.72GHz (2.26
  OC) , 1Gb RAM, HD ATA-5 (133Mb/s), Windows XP SP1
• AXP AMD Athlon XP 2800 _at_ 2.112GHz (0 OC), 1Gb
  RAM, HD SATA (150Mb/s), Windows 2000 SP4
• A64 AMD Athlon 64 3000 _at_ 2.1GHz (5 OC), 1Gb
  RAM, HD ATA-5 (133Mb/s), Windows XP SP1

P4 AXP A64
CPU clock, MHz 2720 2112 2100
In-memory job, s 86286 106568 -
High disk swap job, s 5484 7102 5665

• Conclusion get Pentium 4 system, as fast as you
  can afford !

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Protonated Naphthalene, Anthracene Structures

• Protonated naphthalene C10H9

Protonated anthracene C14H11
(1)
(1)
(2)
(2)
(9)
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Protonated Phenanthrene, Pyrene Structures

• Protonated phenanthrene C14H11

Protonated pyrene C16H11
(1)
(1)
(2)
(3)
(2)
(4)
(9)
(4)
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Proton Affinities

• Calculated proton affinities (in kCal/mol) are
  in good agreement with experimental values.
  Tests if the theory level and basis set are good
  enough.

Experimental Calculated H site
Benzene 179.3 182.59 Any
Naphthalene 191.9 196.21 1
Naphthalene 191.9 193.32 2
Anthracene 209.7 204.28 1
Anthracene 209.7 200.89 2
Anthracene 209.7 213.02 9
Phenanthrene 197.3 199.04 1
Phenanthrene 197.3 199.39 2
Phenanthrene 197.3 197.85 3
Phenanthrene 197.3 200.02 4
Phenanthrene 197.3 199.85 9
Pyrene 207.7 212.26 1
Pyrene 207.7 197.98 2
Pyrene 207.7 201.89 4
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Energy Landscape Protonated Benzene
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Energy Landscape Protonated Naphthalene
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Energy Landscape Protonated Anthracene
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Vibrational Spectrum Changes

• Typical changes in IR vibrational spectrum
  more IR active modes, new CH2 modes appear at
  2780 - 2900 cm-1

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Ground electronic state results

• Proton binds to carbon atoms. Bridged structures
  are transition states.
• Protonated aromatic molecules are still planar.
• Aromaticity breaks at the CH2 site. That ring
  looks like a cyclodiene.
• C-H bond length for sp3 carbon is longer than sp2
  1.100Å vs. 1.085Å
• C-H vibrations are around 2780-2900cm-1 for sp3,
  less than 10cm-1 apart. For sp2 they are in
  3000-3100cm-1 range and very weak or not IR
  active. This may contribute to the long
  wavelength shoulder in 3.3mm UIR feature.
• Dissociation energies are in 2-3eV (400-600nm)
  range.
• Lowest dissociation channels are loss of H atom
  or H2 molecule. When loosing 2H or H2, will
  again form closed shell species.
• With sufficient internal energy, can isomerize
  without dissociation.
• In diffuse clouds PAHs would be most definitely
  in protonated form,
• if they can survive in the radiation field!

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Benzene Molecular Orbitals

• C6H6 (benzene)

C6H7 (protonated benzene)
20
Excited electronic states results. S0 S1
transition, nm
Unlike neutral PAHs, small protonated PAHs have
their S0 S1 transitions well into DIB
wavelengths range.
Calc Red Shift f
Benzene
C6H6 262.56 0.0000
C6H7 349.50 86.94 0.1741
Naphthalene
C10H8 312.30 0.0825
1-C10H9 382.53 70.22 0.40904
2-C10H9 438.90 126.60 0.1714
Anthracene
C14H10 361.17 0.1399
1-C14H11 443.09 81.92 0.3459
2-C14H11 490.41 129.24 0.1862
9-C14H11 376.46 15.29 0.7120
Calc Red Shift f
Phenanthrene
C14H10 341.16 0.0318
1-C14H11 493.38 152.22 0.1984
2-C14H11 461.04 119.88 0.3068
3-C14H11 498.03 156.87 0.1205
4-C14H11 477.43 136.27 0.4661
9-C14H11 479.60 138.44 0.2854
Pyrene
C16H10 367.57 0.3658
1-C16H11 440.54 72.97 0.3338
2-C16H11 566.34 198.77 0.1499
4-C16H11 496.65 129.08 0.1464
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DIBs and Predicted S0-S1 Transitions

• Variety of isomers increases chances of
  coincidence with DIBs.
• This may be used for more certain DIB assignment.

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Experimental Setup Discharge Source

• Possible protonation mechanisms in H2
• H2 e- - gt H2 2e-
• H2 H2 - gt H3 H
• H3 PAH - gt PAH-H H2
• or
• PAH e- - gt PAH 2e-
• PAH H2 - gt PAH-H H

P(H2) 1-2 atm
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Experimental Setup Time-of-Flight Mass
Spectrometer

• Discharge plasma is guided to skimmer.
• Ions are extracted into TOF MS by pulser.
• Separated ions are intercepted by laser in
• front of reflectron.
• Ions are turned around by reflectron to
• reflectron detector.
• Neutrals go through reflectron to
• linear detector.

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Experimental Setup Tunable Light Sources

• Old BBO type II OPO

Pumped by Coherent Infinity 40-100 NdYAG laser
New mixed BBO type I and II prism OPO
Pumped by SpectraPhysics GCR-16S NdYAG laser _at_
10Hz
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OPO Operational Principle
OPO conversion phase matching conditions

• Light vectors in nonlinear crystal

wp ws wi kp ks ki
Beam polarizations for OPO
Xtl Pump Signal Idler
BBO I e o o
BBO II e o e
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Mixed Cavity OPO

• Using BBO type II as bandwidth filter, BBO type I
  as amplifier.
• Different crystal types eliminate a need for a
  waveplate.
• Improved beam profile. Beam divergency 3 mrad.
• Extended generation range to degeneracy point,
  with good pulse energy.

_at_ 2m
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Data Acquisition System

• The experiment is controlled by a PC (PIII,
  533MHz, 392Mb RAM, Windows 2000 SP4) via a
  developed BGSpecT software package .
• TOF MS traces are acquired by GaGe CS85G
  oscilloscope card, laser pulse energy is measured
  with pyroelectric detector via GaGe CS1450 card.
  Each TOF MS trace is analyzed for the presence of
  certain level of ion signal. Good waveforms are
  then averaged.
• OPO crystal positions are controlled with Newport
  850F microstepper motors via Precision
  MicroControl DCX PC100 card.
• Time delays for lasers, pulsed valves and
  discharge are controlled by Stanford Research
  Systems DG535 digital delay/pulse generators.
• Pulsed valves are operated at 0.91Hz (10/11) due
  to slow pumping speed.

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Blake Group Spectroscopy Tools/Software

• Simultaneously controls multiple devices of the
  same kind.
• Controls devices via GPIB, RS232 interfaces, PCI
  and ISA plug-in cards.
• Simultaneously controls multiple GPIB boards.
• Smart oscilloscope waveform acquisition.
• Wavelength source wavelength conversions.
• Master/Slave locking of delay lines from pulse
  delay generator.
• Huge number of supported oscilloscopes.
• Easy process of spectra acquisition.
• Fast. Runs easily on 100MHz Pentium system.
• Flexible to configure.
• User friendly interface. Partial Windows XP
  themes support.

www.its.caltech.edu/vadym/BGSpecT_exe.zip
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Discharge Products TOF MS Spectra

• Combined for H2 discharge with
• different PAHs and without

Protonation evidence
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Photodissociation TOF MS Spectra

• Photodissociation with high energy excimer laser
  pulses (193, 248 nm) is rather efficient.
• Main dissociation channel loss of H2 molecule
  (or 2 H atoms).
• No dissociation by low pulse energy visible (415
  - 600 nm) and UV (208 290 nm) wavelengths .

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Photodissociation Protonated Anthracene

• Dissociation is clearly multiphoton

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Photodissociation Protonated Pyrene

• Similarly to protonated anthracene - multiphoton

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Protonated Anthracene Photostability Estimate

• Protonated anthracene dissociation is 3-photon at
  both 193 and 248 nm.
• Need 13-15 eV to fall it apart. This is much
  higher than predicted 2.6 eV.
• IVR is responsible for such behavior.

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Photostability of Protonated PAHs

• Protonated PAHs do not dissociate from visible
  photons.
• Even in the UV l range dissociation is
  multiphoton.
• Needed photon energy is much higher than
  predicted.
• IVR is likely responsible for the photostability.
• Good news for ISM !
• May absorb UV/visible photons and then within
  milliseconds cool off by emitting in IR. Can
  cycle for long time.
• Bad news for spectroscopy.
• Need to use a different method to record
  spectra.
• Will work on cluster dissociation.

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Cluster Source

• Most atoms and molecules in the discharge turn
  PAH protonation off.
• Need to mix in the third molecule at the
  discharge exit.
• Use H2 as a carrier gas in both pulsed valves.
• Clusters dont form with rare gases.
• Works with water.

P1(H2) 1 atm P2(H2) 2.3 atm
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Protonated Anthracene Clusters with Water

• Can produce large quantities of C14H11 (H2O)n
  clusters.
• Cluster spectrum should be red-shifted by 2 nm.

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Cluster Geometry Electrostatic Nature

• C6H6 H2O cluster
• interaction between water dipole and benzene
  quadrupole moments.
• Can bind only at the top or bottom.

• C14H11 H2O cluster
• charge - dipole interaction.
• Charge makes water O atom face protonated
  anthracene. Can bind only from the side. Will
  bind to a site with largest charge or dipole
  CH2

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Cluster Photodissociation Spectrum

• The visible spectrum does not have narrow
  features
• Observed two bands at 445.8 and 470.7 nm,
  FWHM19.6 nm

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Implications

• Observed bands are broad. Likely, due to high
  vibrational density of states, clusters being
  warm.
• Temperature in diffuse interstellar clouds is
  100-200K. Clusters are not hotter than clouds.
• Vibrational density of states in protonated PAHs
  is higher than in neutrals and cations. Mainly,
  due to the ability of H atom to jump from one C
  atom to another. This feature is unique to
  protonated PAHs.
• Since clusters are in similar to ISM conditions
  and absorption bands are much wider than DIBs,
  small protonated PAHs are not DIB carriers ! The
  isomerization process should be present in larger
  protonated PAHs as well, and so should produce
  broad absorption features.

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Summary

• Aromatic hydrocarbons are protonated effectively.
• Ground state DFT calculations were performed for
  different protonated PAHs. Loss of H atom or H2
  molecule were identified as energetically lowest
  dissociation channels. Isomerization is possible
  with enough vibrational energy.
• CH vibrations of sp3 carbon were calculated. They
  may account for the red wing in 3.3mm feature in
  UIRs.
• Calculated S0-S1 transitions for protonated PAHs
  are in the DIB wavelengths range even for small
  PAHs.
• Protonated PAHs are very photostable for 1-photon
  absorption. This makes them even more viable
  candidates for ISM.
• Direct UV/visible dissociation spectra are
  impossible to record.
• Visible spectra of protonated anthracene-water
  clusters were measured. Spectral features are
  very broad. Protonated PAHs are unlikely to be
  DIB contributors if this broadening is intrinsic
  to the protonated PAHs.
• Sources for PAH protonation and clustering with
  water were designed.
• Mixed BBO type I and II OPO with the prism cavity
  was developed.
• Flexible data collection software was designed.

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Conclusions

• 1. Do protonated PAHs exist in interstellar
  medium?
• Yes. Most likely.
• PAHs are UIR carriers. In diffuse clouds PAHs
  will protonate.
• Protonated PAHs are very photostable.
• 2. If yes, do they produce DIBs?
• No.
• Electronic absorption bands are wider than DIBs,
  although, they are in DIB range.

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Acknowlegements

• Geoffrey A. Blake - adviser
• Sheng Wu - OPOs
• Blake group
• Funding
• NASA
• NSF