Photophysical properties ofprotonated aromatic hydrocarbons

Vadym KapinusDepartment of Chemistry

Blake group

Sample UIR spectrum

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.

Astronomical Spectra

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 range.

Importance in Chemistry and Biology

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

C6H6 + E+ - > C6H6E+ - > 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.

Goals

1. Do protonated PAHs exist in interstellar medium?2. If yes, do they produce DIBs?

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

Most likely, YesMost likely, No

Protonated Benzene Structure

Ab initio structures for protonated benzene – CH2 benzenium (1),

bridged benzonium (2) and ring (3) isomers.

Protonated benzene DFT structure.

Proton binds to carbon atom !

C6H7+

Protonated Benzene Experiments

C6H7+ UV dissociation in

FT-ICR mass spectrometer(Freiser, Beauchamp 1976)

C6H7+ · Ar cluster

IR dissociation(Solca, Dopfer 2002)

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 6-311++G(2d,2p) - benzene

B3LYP 6-311++G(d,p) - naphthalene

B3LYP 6-31+G(d) - anthracene, phenanthrene, pyrene• Excited electronic states calculated with configuration interaction singles

method

CIS 6-311++G(2d,2p) on B3LYP 6-311++G(d,p) geometries

PC Choice for GAUSSIAN 98

• Tested with GAUSSIAN 98W, Revision-A.9

• Test systems : – P4: Intel Pentium 4 2.66 GHz @ 2.72GHz (2.26% OC) , 1Gb RAM, HD ATA-5

(133Mb/s), Windows XP SP1

– AXP: AMD Athlon XP 2800+ @ 2.112GHz (0% OC), 1Gb RAM, HD SATA (150Mb/s), Windows 2000 SP4

– A64: AMD Athlon 64 3000+ @ 2.1GHz (5% OC), 1Gb RAM, HD ATA-5 (133Mb/s), Windows XP SP1

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

P4 AXP A64

CPU clock, MHz 2720 2112 2100

In-memory job, s 86286 106568 -

High disk swap job, s 5484 7102 5665

Protonated Naphthalene, Anthracene Structures

Protonated naphthalene C10H9+ Protonated anthracene C14H11

+

(1) (1)

(2) (2)

(9)

Protonated Phenanthrene, Pyrene Structures

Protonated phenanthrene C14H11+ Protonated pyrene C16H11

+

(1)

(1)

(2)(2)

(9)

(3)

(4)

(4)

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

193.32 2

Anthracene 209.7 204.28 1

200.89 2

213.02 9

Phenanthrene 197.3 199.04 1

199.39 2

197.85 3

200.02 4

199.85 9

Pyrene 207.7 212.26 1

197.98 2

201.89 4

Energy Landscape – Protonated Benzene

Energy Landscape – Protonated Naphthalene

Energy Landscape – Protonated Anthracene

Vibrational Spectrum Changes

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

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 3.3m 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!

Benzene Molecular Orbitals

C6H6 (benzene) C6H7+ (protonated benzene)

Excited electronic states results. S0 – S1 transition, nm

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

Unlike neutral PAHs, small protonated PAHs have their S0 – S1 transitions well into DIB wavelengths range.

DIBs and Predicted S0-S1 Transitions

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

Experimental Setup – Discharge Source

Possible protonation mechanisms in H2:

H2 + e- - > H2+ + 2e-

H2+ + H2 - > H3

+ + H

H3+ + PAH - > PAH-H+ + H2

or

PAH + e- - > PAH+ + 2e-

PAH+ + H2 - > PAH-H+ + H

P(H2) = 1-2 atm

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.

Experimental Setup – Tunable Light Sources

Old BBO type II OPO

New mixed BBO type

I and II prism OPO

Pumped by SpectraPhysics GCR-16S Nd:YAG laser @ 10Hz

Pumped by Coherent Infinity 40-100 Nd:YAG laser

OPO Operational Principle

Light vectors in nonlinear crystal

Xtl Pump Signal Idler

BBO I e o o

BBO II e o e

p = s + i

kp = ks + ki

OPO conversion – phase matching conditions

Beam polarizations for OPO

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.

@ 2m

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.

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

Discharge Products – TOF MS Spectra

Combined for H2 discharge with

different PAHs and withoutProtonation evidence

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 .

Photodissociation – Protonated Anthracene

Dissociation is clearly multiphoton

Photodissociation – Protonated Pyrene

Similarly to protonated anthracene - multiphoton

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.

Photostability of Protonated PAHs

• Protonated PAHs do not dissociate from visible photons.• Even in the UV 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.

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 don’t form with rare gases.

• Works with water.

P1(H2) = 1 atm

P2(H2) = 2.3 atm

Protonated Anthracene – Clusters with Water

• Can produce large quantities of C14H11+ · (H2O)n clusters.

• Cluster spectrum should be red-shifted by ~2 nm.

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

Cluster Photodissociation Spectrum

• The visible spectrum does not have narrow features

• Observed two bands at 445.8 and 470.7 nm, FWHM=19.6 nm

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.

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 3.3m 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.

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.

Acknowlegements

• Geoffrey A. Blake - adviser

• Sheng Wu - OPOs

• Blake group

• Funding

– NASA

– NSF