1. 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

2. 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

3. Fundamental Chemical Kinetics
10 cm-1
30000 cm-1

4. 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

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

6. 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 = kcal/mol
OSU 2004
UMP2/6-311G(d,p)

7. 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 / g(d,p)
Delta A apparent from pos of q branch
B reduction ok
Dipole?
Measure Perp band to get magnitude of A
ns = cm-1
B = cm-1
A = cm-1
= 2.58 D
De = 780 cm-1
DA = A” – A’

8. 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”

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

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

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

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

13. Electron Difference Densities
Iso-surface

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

15. CH3 --HCN Experiment n0 = 3265.67 cm-1 B = 0.025 cm-1 DA = 0.04 cm-1
G = cm-1
UMP2 / g(d,p)
ns = cm-1
B = cm-1
A = cm-1
DA = cm-1
= 3.42 D
De = 418 cm-1
8.926 kV/cm
DA = A” – A’

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

17. C2H5 --HCN Experiment n0 = 3260.29 cm-1 A = 0.25 cm-1
(B+C)/2 = cm-1
DJ = 2.8 x 10-5 cm-1
= 3.7 ± 0.2 D
G = cm-1
UMP2 / g(d,p)
2.58 kV/cm
ns = cm-1
A = cm-1
B = cm-1
C = cm-1
= 3.64 D
De = 637 cm-1

18. C3H5 --HCN Experiment n0 = 3260.13 cm-1 A = 0.09 cm-1
(B+C)/2 = cm-1
DJ = 1.6 x 10-5
= 3.2 ± 0.2 D
G = cm-1
UMP2 / g(d,p)
5.79 kV/cm
ns = cm-1
A = cm-1
B = cm-1
C = cm-1
= 3.59 D
De = 793 cm-1

19. Linewidths?
V-V resonance

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

21. 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

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

24. CxHy --- HCN,HF (cm-1) HCN-CH3 HF-CH3 HCN-C2H5 HF-C2H5 HCN-C3H5A 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
-40.74
-50.91
-36.38
-51.07
--- 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

26. 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

27. Rotational Dynamics

28. Periodically Poled LiNbO3 CW OPO
2.0 – 5.0 mm
3 Watts
Pump
Idler nm
20 Watts
Signal

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