The ethyl radical in superfluid helium nanodroplets: Rovibrational spectroscopy and ab initio calculations 1 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

2 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 G 12 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, (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 AIRVACUUM Gate Valve Water-cooled Cu electrodes Valve ∆ Experimental Setup Pick-up Chamber Stark Chamber Mass Spectrometer x Droplet beam 90° Ta wire coiled around quartz tube 700 K 3 97% 30 bar 17 K

∆ 700 K 4

Gas phase (sub)band origins a,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… 5 a 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, (2006).

6 ΔKa ΔJ Ka” (J”) Γ v’ = a 1 ’, a 1 ”, a 2 ” = a-, b-, c-type, respectively a-type: Δm = 0, ΔK a = 0, ΔK c = ±1 b-type: Δm = 0, ΔK a = ±1, ΔK c = ±1 c-type: Δm = 0, ΔK a = ±1, ΔK c = 0 where m is the torsional quantum number

E laser E Stark E laser E Stark or  M J = 0  M J = ±1 7 He Droplet Source Pick-up cells Stark Spectroscopy Pick-up Chamber Stark Chamber Mass Spectrometer 30 bar 17 K

8 He Droplet Source Pick-up cells Stark Spectroscopy Pick-up Chamber Stark Chamber Mass Spectrometer 30 bar 17 K CCSD(T)/cc-pVTZ μ a = 0.23 D μ b = 0.13 D μ c = 0.00 D Vib. Averaged μ a = 0.23 D μ b = 0.01 D μ c = 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, (2005). 9 T. Häber, A. C. Blair, D. J. Nesbitt, and M. D. Schuder, J. Chem. Phys. 124, (2006). b-type asym-CH 2 b-type asym CH 3 c-type sym CH 3 a-type lone CH

10 T. J. Sears, P. M. Johnson, P. Jin, and S. Oatis, J. Chem. Phys. 104, 781 (1996). ∆V 6 corresponds to the computed frequency difference between staggered and eclipsed configs. Table units are cm -1.

11 Modew theory a v theory a Gas d,e He f ∆V 6 d v theory -v He v1v (13.4)3034 (49.7) v2v (21.3)2929 (15.3) v3v (23.5)2809 (56.6) v4v (1.8)1455 (6.5) v5v (3.3)1440 (1.4) v6v (0.9)1368 (0.7) v7v (0.0)1047 (0.0) v8v8 988 (0.4)983 (0.7) v9v9 469 (50.6)489 (39.7)528 v (12.6)3097 (11.1) v (19.6)2959 (0.4) v (4.5)1448 (3.9) v (1.8)1171 (25.1) v (1.2)806 (1.4) v (0.1) 2v (0.3) v 4 + v (0.1) v62v (0.3) 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, (2006). f This work. *Format: frequency (intensity) *Units: cm -1 (km mol -1 )

12 Modew theory a v theory a Gas d,e He f ∆V 6 d v theory -v He v1v (13.4)3034 (49.7) v2v (21.3)2929 (15.3) v3v (23.5)2809 (56.6) v4v (1.8)1455 (6.5) v5v (3.3)1440 (1.4) v6v (0.9)1368 (0.7) v7v (0.0)1047 (0.0) v8v8 988 (0.4)983 (0.7) v9v9 469 (50.6)489 (39.7)528 v (12.6)3097 (11.1) v (19.6)2959 (0.4) v (4.5)1448 (3.9) v (1.8)1171 (25.1) v (1.2)806 (1.4) v (0.1) 2v (0.3) v 4 + v (0.1) v62v (0.3) 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, (2006). f This work. *Format: frequency (intensity) *Units: cm -1 (km mol -1 )

13 Summary We have located 3 new bands and have utilized theory to assign these to overtones and combination bands (2ν 12, ν 4 + ν 6, 2ν 6 ) of the ethyl radical. G 12 permutation inversion group theory successfully simulates transition intensities. We have utilized Stark spectroscopy to measure the a-component of the permanent electric dipole moment (μ a = 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