Higher Electronic Excited States of Jet-CooledAromatic Hydrocarbon Radicals: 1-phenylpropargyl

(C9H7), 1-naphthylmethyl (C11H9), 2-naphthylmethyl(C11H9) and 9-anthracenylmethyl (C15H11)

The University of Sydney School of Chemistry

Gerard Dean O’Connor

Klaas Nauta

and Timothy W. SchmidtMolecular Photonics Group | UNSW

The Diffuse Interstellar Bands (DIBs)

Star Interstellar Medium

Earth

What are the carriers of the DIBs?

Amorphous Carbon Fullerenes Carbon Chains & Carbenes

Polycyclic Aromatic Hydrocarbons (PAHs)

H2Cn

Testing the PAH hypothesis: gas phase spectroscopy

Resonantly stabilized radicals and their cations

ultraviolet spectra

ultraviolet spectra

ultraviolet spectra

visiblespectra

ultraviolet spectra

hn hn

visiblespectra

visiblespectra

Reilly, N.J; Kokkin, D.L; Masakazu N; Klaas N; Kable, S.H and Schmidt, T.W. (2008). "Spectroscopic Observation of the Resonance-Stabilized 1-Phenylpropargyl Radical J. Am. Chem. Soc. 130 3137-3142. DOI:10.1021/ja078342t

Chalyavi, N; Troy, T.P; Nakajima, M; Gibson, B.A; Nauta, K; Sharp, R.G; Kable, S.H and Schmidt, T.W. (2011) “Excitation and emission spectra of jet-cooled naphthylmethyl radicals.” J. Phys. Chem A, 115 (27), 7959-7965, 2011. DOI: 10.1021/jp203638h

O’Connor, G.D; Bacskay, G.B; Woodhouse, G.V.G;Troy T.P; Nauta, K; Schmidt, T.W. (2013) “Excitation Spectra of Large Jet-Cooled Polycyclic Aromatic Hydrocarbon Radicals: 9-Anthracenylmethyl (C15H11) and 1-Pyrenylmethyl (C17H11)” The Journal of Physical Chemistry A,117(50)13899-13907DOI: 10.1021/jp4088833

D1 ← D0 excitation spectra of jet cooled aromatic radicals in the gas phase

Trends in Larger Resonance Stabilised Radicals

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• D1 ← D0 transitions weak (oscillator strength f~10-4)

• Larger molecules D1 ← D0 spectra non-origin dominated

1-PyMe R2C2PI Excitation Spectrum

7

13417cm-1

Assigned 1-PyMe Excitation Spectrum, low frequency modes

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80% intensity of origin

9

D0

D1

Dn

Transition Moments

John A. Pople

H.C. Longuet-Higgins

Longuet-Higgins, H.; Pople, J. Proc. Phys. Soc. Lond. A 1955, 68, 591–600

f~10-4 f >10-2

Detector

Source

Extractionof ions

Time-Of-Flight MS

Resonant 2-Colour 2-PhotonIonization (R2C2PI)

Ionisation continuum

l1

+e-

l2

Detector

Source

Extractionof ions

Time-Of-Flight MS

Double Resonant Depleation

Ionisation continuum

ld

+e-

D0

D1

D3

1-phenylpropargyl calculated transitions

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CASPT2 [6311G(d)] electrons in 9 orbital active space

D3 ← D0

Vertical excitation energy = 33858 cm-1 Oscillator Strength = 0.016

G.V.G. Woodhouse: Undergraduate Honours Thesis, 2012

D3 ← D0 excitation spectrum of 1-phenylpropargyl radical

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FWHM=149 cm-1

Lifetime = 36 fs

Too high in energy to be a DIB

Matrix results of the naphthylmethyl radicals

D1 ← D0 1&2-npme

Nagy, Fulara, and Maier. J. Am. Chem. Soc. 2011, 133, 19796–19806.

Calculated (by us) to be weak (CASPT2 and TD-B3LYP)

We calculate both to be strong (CASPT2 and TD-B3LYP) and assign both to D3 ← D0

D2 ← D0 2-npme

D3 ← D0 1&2-npme

D3 ← D0 excitation spectra of naphthylmethyl radicals

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FWHM=290 cm-1

FWHM=292 cm-1

Lifetime = 18 fs

Broadest DIB FWHM = 155 cm-1

Ionisation lifetime scan of 9-anthracenylmethyl

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6.42eV photon energy similar to IE

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Signal independent of lifetime

Trends in TD-B3LYP vertical excitation energy

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Benzyl data from: :Porter, G; Wright, FJ; (1955) “PRIMARY PHOTOCHEMICAL PROCESSES IN AROMATIC MOLECULES .3. ABSORPTION SPECTRA OF BENZYL, ANILINO, PHENOXY AND RELATED FREE RADICALS” TRANSACTIONS OF THE FARADAY SOCIETY 51(11) 1469-1474

9-AnMe Vertical Excitation Energies B3LYP/TD-DFT 6311++G(d,p)

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• B3LYP TD-DFT [6311++G(d,p)] (corrected) – 23874(cm-1)• M06 TD-DFT [6311++G(d,p)] (corrected) – 24229(cm-1)

D3 ← D0 excitation spectrum of 9-anthracenylmethyl radical

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FWHM=326 cm-1

Lifetime = 16 fs

Trends in TD-B3LYP vertical excitation energy

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Trend, the energy gap law in action

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Trends in observed in polycyclic aromatic hydrocarbon resonance stabilised radicals (PAH

RSRs)

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• odd-alternate hydrocarbon radicals D1 ← D0 extremely weak (f ≈10-4), with most intensity going to higher energy transition

• transitions with intensity greater than f ≈10-2 have been calculated and observed

• Strong visible transitions of PAH RSRs will be significantly broader then any observed DIBs

• As these are the transitions most likely to be observed, PAH RSR can be largely dismissed as potential DIB carriers

Acknowledgements

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Supervisor: Prof Tim Schmidt

Co-Workers:Gabrielle Woodhouse

Prof Scott Kable

Dr Klaas NautaDr Tyler Troy

Laser Spectroscopy Group

UNSW Molecular photonics group