Probing the Reaction Mechanisms of Hydrocarbon Radicals with Infrared Spectroscopy and Helium Nanodroplets Jeremy M. Merritt and Roger E. Miller Department of Chemistry University of North Carolina at Chapel Hill

Hydrocarbon Radicals in Chemistry and Physics Combustion RH + O2  R• + HO2• R• + O2  ROO• Atmospheric Kinetics X• + CxHy  HX + CxHy-1• X• + CnH2n  CnH2nX• Highly Exothermic Negative activation energies reported Negative temperature dependence of the rate constant Quantum tunneling Intermediate complexes influence the dynamics Kinetics is at the heart

Fundamental Chemical Kinetics 10 cm-1 30000 cm-1

Experimental Apparatus Br2, Cl2, DTBP, C2H5I, etc… Emphasize pick up thermally fragile molecule + radical produced by high temp pyrolysis -V Cryostat Skimmer +V Multi-pass/ Stark Cells to pump Bolometer

Pendular Survey Scans 1200 K Pyrolysis Source HCN – C2H5 HCN + C2H5I Spectra are in general clean, efficient production which is necessary HCN only

Hydrogen Abstraction Reactions F+ CH4 Exit Channel Complex Entrance Channel Complex Data presented before Fundamental reaction Importance of entrance and exit channels, our focus Entrance channel decides branching Exit determines final states HF+ CH3 fHexp = -32.0 kcal/mol OSU 2004 UMP2/6-311G(d,p)

CH3 --HF Experiment n0 = 3796.80 cm-1 B = 0.066 cm-1 DA = 0.03 cm-1 G = 0.13 cm-1 15.42 kV/cm UMP2 / 6-311++g(d,p) Delta A apparent from pos of q branch B reduction ok Dipole? Measure Perp band to get magnitude of A ns = 3830.64 cm-1 B = 0.193 cm-1 A = 4.791 cm-1 = 2.58 D De = 780 cm-1 DA = A” – A’

Vibrational Averaging DA = A” – A’ DA = 0.01 cm-1 DAexp = 0.03 cm-1 As HF bends CH3 rocks around making the I smaller; A bigger but this is not DA. In excited state HF bends less due to dipole, making A’ smaller than A”

F + C2H6  HF + C2H5 More exothermic than CH3 MP2/6-311G(d,p) level of theory at 0 K and including ZPE’s

C2H5 --HF Experiment n0 = 3774.45 cm-1 A = 0.30 cm-1 (B”+C”)/2 = 0.058 cm-1 (B’+C’)/2 = 0.061 cm-1 DJ” = 1.5 x 10-4 cm-1 DJ’= 2.2 x 10-4 cm-1 = 2.7 ± 0.2 D G = 0.038 cm-1 3.86 kV/cm UMP2 / 6-311++g(d,p) Hybridization effecting interaction strength ns = 3753.37 cm-1 A = 0.782 cm-1 B = 0.129 cm-1 C = 0.116 cm-1 = 2.86 D De = 1109 cm-1

F + C3H6  HF + C3H5 1-Propenyl radical Cyclopropane Propene Isopropenyl radical Cyclopropyl radical Allyl

C3H5 --HF Experiment n0 = 3810.10 cm-1 A = 0.095 cm-1 (B+C)/2 = 0.040 cm-1 DJ = 1.0 x 10-4 = 2.4 ± 0.2 D G = 0.080 cm-1 UMP2 / 6-311++g(d,p) 5.10 kV/cm Underlying impurity ns = 3843.30 cm-1 A = 0.293 cm-1 B = 0.117 cm-1 C = 0.092 cm-1 = 2.52 D De = 1092 cm-1

Electron Difference Densities Iso-surface

CN + CH4  HCN + CH3 fHexp = -20.4 kcal/mol UMP2(Full) 6-31g(d) CN psuedo halogen fHexp = -20.4 kcal/mol UMP2(Full) 6-31g(d)

CH3 --HCN Experiment n0 = 3265.67 cm-1 B = 0.025 cm-1 DA = 0.04 cm-1 G = 0.040 cm-1 UMP2 / 6-311++g(d,p) ns = 3287.99 cm-1 B = 0.086 cm-1 A = 4.785 cm-1 DA = 0.037 cm-1 = 3.42 D De = 418 cm-1 8.926 kV/cm DA = A” – A’

CN + C2H6  HCN + C2H5 UMP2/6-311G(d,p) CN + HCN + V. Sreedhara Rao and A.K. Chandra: Chem. Phys. 192, 247 (1995).

C2H5 --HCN Experiment n0 = 3260.29 cm-1 A = 0.25 cm-1 (B+C)/2 = 0.019 cm-1 DJ = 2.8 x 10-5 cm-1 = 3.7 ± 0.2 D G = 0.035 cm-1 UMP2 / 6-311++g(d,p) 2.58 kV/cm ns = 3272.46 cm-1 A = 0.674 cm-1 B = 0.065 cm-1 C = 0.061 cm-1 = 3.64 D De = 637 cm-1

C3H5 --HCN Experiment n0 = 3260.13 cm-1 A = 0.09 cm-1 (B+C)/2 = 0.015 cm-1 DJ = 1.6 x 10-5 = 3.2 ± 0.2 D G = 0.030 cm-1 UMP2 / 6-311++g(d,p) 5.79 kV/cm ns = 3276.61 cm-1 A = 0.295 cm-1 B = 0.055 cm-1 C = 0.049 cm-1 = 3.59 D De = 793 cm-1

Linewidths? V-V resonance

Reaction Dynamics in Clusters No impact parameter averaging Recover products downstream IVR? Predissociation vs Prereaction Illustration by G. Douberly

Conclusions Produced clean sources of hydrocarbon radicals for doping in helium nanodroplets Exit channel complexes of Methyl, Ethyl, and Allyl radicals with HCN and HF have been observed with rotational resolution Vibrational Averaging provides important clues to the anisotropy of the PES Benchmarks for Theory Stabilization of Entrance Channel X-CxHy complexes behind reaction barriers should be possible in some cases Reaction Dynamics in Molecular clusters: Predissociation vs. Prereaction

Professor James T. Dobbins Sr. Memorial Fund Acknowledgements Professor James T. Dobbins Sr. Memorial Fund Svemir Rudic The Miller Group

CxHy --- HCN,HF (cm-1) HCN-CH3 HF-CH3 HCN-C2H5 HF-C2H5 HCN-C3H5 A RF 4.785 4.785* -- 0.674 0.25 2.70 0.782 0.30 2.61 0.295 0.090 3.28 0.293 0.095 3.08 (B+C)/2 0.086 0.025 3.44 0.193 0.066 2.92 0.063 0.019 3.32 0.122 0.059 2.07 0.052 0.016 3.25 0.104 0.040 2.60 Dn Exp pVTZ -23.21 -45.53 -41.74 -136.32 -162.39 -191.71 -40.74 -50.91 -218.30 -184.74 -272.11 -36.38 -51.07 -122.90 -149.11 --- m 3.42 3.0 2.58 3.1 3.64 3.7 2.86 2.7 3.59 3.2 2.52 2.4 De (CPC) 418 556 780 1058 637 1109 1451 793 1092 1445 G 0.130 0.035 0.038 0.030 0.080 A consts and Linewidths scale with BE correctly Rot const reductions tell us he-dopant potential

Non-thermal Population of States C3v 3,3 A ns = 4A  2E 3,2 E 3,1 E 2,2 E 2,1 E 3,0 A 1,1 E Q branch verifies C3v 2,0 A Despite large A rot. constant Significant population of K=1 states allows a strong Q-branch 1,0 A 0,0 A [J,K] OSU 2004

Rotational Dynamics

Periodically Poled LiNbO3 CW OPO 2.0 – 5.0 mm 3 Watts Pump Idler 1025-1035nm 20 Watts Signal

Pendular Survey Scans E + When mE > B: 2B - hn Q(0) 2F HCN only 3F