The ethyl radical in superfluid helium nanodroplets:Rovibrational spectroscopy and ab initio calculations

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Department of Chemistry, University of Georgia Athens, Georgia, USA

Christopher P. Moradi, Paul L. Raston, Jay Agarwal, Justin M. Turney, Henry F. Schaefer, and Gary E. Douberly

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Prototype for studying hyperconjugation effects that influence molecular structure

• Shortening and strengthening of C—C bond (≈15% double bond character).a

• Lengthening and weakening of β-C—H bond.b

• Bending of methylene group away from planarity.c,d

Prototype for open-shell molecules with large-amplitude nuclear motions

• Torsional motions expected to affect reaction dynamics.e • Requires use of the G12 permutation-inversion symmetry group.e

• Transitions out of thermally excited torsional states complicate high-res spectra.

Doping ethyl radical in 0.4 K helium droplets alleviates spectral congestion.

Ethyl Radical Background

a S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000).b L. B. Harding, J. Am. Chem. Soc. 103, 7469, (1981).c P.M. Johnson and T. J. Sears. J. Chem. Phys. 111, 9222 (1999).d T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006).e T. J. Sears, P. M. Johnson, P. Jin, and S. Oatis, J. Chem. Phys. 104, 781 (1996).

He Droplet Source

Pick - up cells

Metering

Precursor AIR VACUUM

Gate Valve

Water-cooledCu electrodes

Valve

∆

Experimental Setup

Pick-upChamber

StarkChamber Mass Spectrometer

x

Droplet beamintersects @ 90°

Ta wire coiledaround quartz tube

700 K

3

97%

30 bar17 K

∆700 K

10 20 30 40 170 180

28

174

hot: ~700 Kx100

cold: ~300 K

m/z

di-tert amyl peroxideTnozzle=17 K

174

4327

284He droplet peaks(12, 16, 20, ...)

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• Gas phase (sub)band originsa,b in the survey scan are marked with black dashed lines.

• Asterisked peaks are due to side products of pyrolysis, e.g. acetone, ethane, ethylene, etc.

• Expanded views of the bands marked with black (rotationally resolved) and red arrows coming up next…

5a S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000).b T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006).

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ΔKa ΔJ Ka” (J”) Γv’ = a1’, a1”, a2” = a-, b-, c-type, respectively

a-type: Δm = 0, ΔKa = 0, ΔKc = ±1b-type: Δm = 0, ΔKa = ±1, ΔKc = ±1c-type: Δm = 0, ΔKa = ±1, ΔKc = 0

where m is the torsional quantum number.

12 4 6 2

Elaser

EStark

Elaser

EStark

or

MJ = 0 MJ = ±17

He Droplet Source

Pick - up cells

Stark Spectroscopy

Pick-upChamber

StarkChamber Mass Spectrometer

30 bar17 K

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He Droplet Source

Pick - up cells

Stark Spectroscopy

Pick-upChamber

StarkChamber Mass Spectrometer

30 bar17 K

CCSD(T)/cc-pVTZμa = 0.23 Dμb = 0.13 Dμc = 0.00 D

Vib. Averagedμa = 0.23 Dμb = 0.01 Dμc = 0.00 D

These bands are nicely rotationally resolved in the gas phase!

The cause of broadening is not known but is thought to be due to efficient helium assisted V-V relaxation.

Seen before in He-solvated ethylene.C. M. Lindsay, R. E. Miller, J. Chem. Phys. 122, 104306 (2005).

9T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006).

b-typeasym-CH2

b-typeasym CH3

c-typesym CH3

a-typelone CH

10T. J. Sears, P. M. Johnson, P. Jin, and S. Oatis, J. Chem. Phys. 104, 781 (1996).

∆V6 corresponds to the computed frequency difference betweenstaggered and eclipsed configs.

Table units are cm-1.

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Mode wtheorya vtheory

a Gasd,e Hef ∆V6d vtheory-vHe

v1 3158 (13.4) 3034 (49.7) 3037 3038 2.17 -4v2 3065 (21.3) 2929 (15.3) 2876 2877 -40.79 52v3 2984 (23.5) 2809 (56.6) 2854 2853 29.42 -44v4 1493 (1.8) 1455 (6.5)v5 1480 (3.3) 1440 (1.4)v6 1404 (0.9) 1368 (0.7)v7 1070 (0.0) 1047 (0.0)v8 988 (0.4) 983 (0.7)v9 469 (50.6) 489 (39.7) 528v10 3261 (12.6) 3097 (11.1) 3129 3129 3.20 -32v11 3109 (19.6) 2959 (0.4) 3000 3000 11.15 -41v12 1492 (4.5) 1448 (3.9)v13 1201 (1.8) 1171 (25.1)v14 809 (1.2) 806 (1.4)v15 128 (0.1)2v12 2893 (0.3) 2884 15.68 9

v4 + v6 2819 (0.1) 2817 8.19 22v6 2720 (0.3) 2721 3.10 -1

a This work. Computed at the VPT2/CCSD(T)/cc-pVTZ level of theory.b J. Pacansky and M. Dupuis, J. Am. Chem. Soc. 104, 415, (1982).c G. Chettur and A. Snelson, J. Phys. Chem. 91, 3483, (1987).d T. J. Sears, P. M. Johnson, and J. BeeBe-Wang, J. Chem. Phys. 111, 9213 (1999).e S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000).e T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006).f This work.

*Format: frequency (intensity)*Units: cm-1 (km mol-1)

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Mode wtheorya vtheory

a Gasd,e Hef ∆V6d vtheory-vHe

v1 3158 (13.4) 3034 (49.7) 3037 3038 2.17 -4v2 3065 (21.3) 2929 (15.3) 2876 2877 -40.79 52v3 2984 (23.5) 2809 (56.6) 2854 2853 29.42 -44v4 1493 (1.8) 1455 (6.5)v5 1480 (3.3) 1440 (1.4)v6 1404 (0.9) 1368 (0.7)v7 1070 (0.0) 1047 (0.0)v8 988 (0.4) 983 (0.7)v9 469 (50.6) 489 (39.7) 528v10 3261 (12.6) 3097 (11.1) 3129 3129 3.20 -32v11 3109 (19.6) 2959 (0.4) 3000 3000 11.15 -41v12 1492 (4.5) 1448 (3.9)v13 1201 (1.8) 1171 (25.1)v14 809 (1.2) 806 (1.4)v15 128 (0.1)2v12 2893 (0.3) 2884 15.68 9

v4 + v6 2819 (0.1) 2817 8.19 22v6 2720 (0.3) 2721 3.10 -1

a This work. Computed at the VPT2/CCSD(T)/cc-pVTZ level of theory.b J. Pacansky and M. Dupuis, J. Am. Chem. Soc. 104, 415, (1982).c G. Chettur and A. Snelson, J. Phys. Chem. 91, 3483, (1987).d T. J. Sears, P. M. Johnson, and J. BeeBe-Wang, J. Chem. Phys. 111, 9213 (1999).e S. Davis, D. Uy, and D. J. Nesbitt, J. Chem. Phys. 112, 1823 (2000).e T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, 054316 (2006).f This work.

*Format: frequency (intensity)*Units: cm-1 (km mol-1)

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SummaryWe have located 3 new bands and have utilized theory to assign these to overtones and combination bands (2ν12, ν4 + ν6, 2ν6) of the ethyl radical.

G12 permutation inversion group theory successfully simulates transition intensities.

We have utilized Stark spectroscopy to measure the a-component of the permanent electric dipole moment (μa = 0.28 (2) D).

Most of the a-type bands have baseline resolved rotational structure whereas b- and c-type bands are broadened; this has been seen before in ethylene, and although not understood completely, is attributed to He-assisted V-V relaxation.

Due to the combination of its complexity and computationally tractable size, the ethyl radical will be a useful benchmark for developing a better model for molecules with vibrational modes that strongly couple to torsion.

Acknowledgments