FIRST HIGH RESOLUTION INFRARED SPECTROSCOPY OF GAS PHASE CYCLOPENTYL RADICAL: STRUCTURAL AND DYNAMICAL INSIGHTS FROM THE LONE CH STRETCH Melanie A. Roberts, Richard S. Walters, David J. Nesbitt Department of Chemistry and Biochemistry, JILA University of Colorado at Boulder National Institute of Standards and Technology Funding: DOE

Motivation Alternative sources of oil Heavy oil and tar sand deposits  Canada  Venezuela  China Contain cycloalkanes  Cyclopentanes  Cyclohexanes  Aromatics Open pit mine, photo from Wired magazine

Combustion Chemistry Spectroscopy of combustion intermediates creates opportunities of in situ probing of complex mixtures First step in combustion is hydrogen abstraction to create an alkyl radical, R˙ RH + OH → R˙ R˙ + R˙ → larger hydrocarbons → soot R˙ + O 2 → RO 2 RO 2 → QOOH QOOH → QO + OH E. Sharp, Thesis OSU

Radical Source Synthesis of hydrocarbon radicals by dissociative attachment (RX + e - → R + X - ) Langevin rate process: k ≈ 5 x cm 3 /s ⇨ fast! Localized discharge (~ 1 μs transit time) High radical densities at slit (~ /cm 3 ) Sub-Doppler molecular linewidths (~50 MHz in Ne/He expansion) Flow C 5 H 9 I + Ne/He through slit

Experimental Setup  Quantum shot noise limited sensitivity: A min ≈ 1.5 x  Servoloop locked optical transfer cavities for high frequency precision (~1-10 MHz)  Absolute frequency calibration from sub-doppler methane reference lines  Quantum shot noise limited sensitivity: A min ≈ 1.5 x  Servoloop locked optical transfer cavities for high frequency precision (~1-10 MHz)  Absolute frequency calibration from sub-doppler methane reference lines

Cyclopentyl Radical CH Vibrations Focus on highest frequency vibration  Eliminate precursor (C 5 H 9 I) interference  Reduce congestion from other CH stretches in C 5 H 9 ˙ Predicted Vibrational Frequencies [Scaled B3LYP/6311++G(3df,3pd)]

Hybridization Carbon atoms in C 5 H 9 I are all sp 3 hybridized Lone CH carbon atom in C 5 H 9 ˙ is sp 2 hybridized and the unpaired e - is in a p-orbital sp 2 hybridization  Stiffer bond  Higher energy vibration

Lone CH Stretch B-type band  ΔK a = odd  ΔK c = odd A B C Principle Axes

Predicted Rotational Structure Ab initio rotational constants [B3LYP/6311++G(3df,3pd)]  cm -1  cm -1  cm -1 Near oblate top (A ≈ B > C) Asymmetry parameter κ = (2B – A – C) (A – C)  κ = -1 prolate top  κ = 1 oblate top κ ≈ 0.74

8 08 → → → → → → → → C Predicted P-Branch Structure Asymmetry splitting not resolvable, except at low J ~ 2 Spacing between lines based upon:  A’ = A”, B’ = B”, C’ = C”  Oblate top energy levels E J = BJ(J+1) + (C – B)K → → → → → → → → C Labels: N K a K c

ΔJ = ΔK Progressions: Sample P-Branch Data J” = K c ” J” = K c ” + 1 J” = K c ”+ 2

Preliminary Fit: P-Branch View Only the band origin, ½ (A+B), and C is fit - for both upper and lower states 8 08 → → → → → → → → → → → → 6 24

Summary of Lone CH Band Rotationally resolved spectrum Preliminary fit results: v o = (11) cm -1 ½ (A”+B”) = (48) cm -1 C” = (5) cm -1 No spectral information to determine asymmetry splitting, (A – B) Uncertainty in J, (+/- 1)  Another band, with unambiguous J assignments  4-line combination differences 210J210J Another vibrational state Lone CH stretch Ground state

γ-C Asymmetric Stretch Region Predicted Vibrational Frequencies [Scaled B3LYP/6311++G(3df,3pd)] C-type bandB-type band

C-Type Band Predictions Predictions based upon Ab Initio rotational constants and assuming a small (~ 0.1%) change in rotational constants between upper and lower states:  Strong Q-branch  Relatively weak P-, R-branches Spacing between most intense p-branch lines is ~ 2B Asymmetry splitting still not resolvable for most intense peaks, but… Less intense lines are sensitive to asymmetry splitting 4 40 ← ← ← ← 6 61

C-Type Band Data Q Branch 4 40 ← ← ← ← ← ← ← ← 5 51

Predicted vs. Observed Structure J = 4 ← J = 5 line ½ (A + B) and C are the same for both simulations Asymmetry splitting (A – B) is smaller than ab initio calculations predict Experimental observations indicate that cyclopentyl radical is more like an oblate top than the equilibrium geometry indicates κ = 0.96 κ = 0.74

Large Amplitude Dynamics Pseudorotation  Superposition of near degenerate vibrations  Low energy path for confirmation rotating around ring Commonly seen in 5 – membered rings: Cyclopentane Tetrahydrofuran 1,3-DioxolaneCyclopentyl Radical. O OO Meyer, et al. J. Chem. Phys. 111, 7871 (1999)

Future Work Incorporate interaction of pseudorotation and rotation into Hamiltonian Calculate potential as a function of the pseudorotation coordinate. Search for vibrational bands coming out of the lowest pseudorotational states Other cyclic hydrocarbon radicals  Phenyl radical  Cyclohexyl radical

Acknowledgements David Nesbitt Erin Sharp Nesbitt Group Funding: DOE