1. 70th International Symposium on Molecular Spectroscopy
Infrared Laser Stark Spectroscopy of the OHCH3OH Complex Isolated in Superfluid Helium Droplets
70th International Symposium on Molecular Spectroscopy
Miller Prize Lecture
Christopher M. Leavitt
Department of Chemistry, University of Georgia Athens, Georgia, USA
Honor…. Etc..

2. Nature Chemistry, 2013, 5, 745.
This latter behaviour observed in the reaction of OH and Methanol by Heard and co-workers

3. Typical of reactions between closed-shell reactants
I.R. Sims, Nature Chemistry, 2013, 5, 734.
What we teach our freshman, Arrhenius behaviour, slope related to positive activation energy; typical of reactions between closed-shell reactants
Typical of reactions between closed-shell reactants

4. I.R. Sims, Nature Chemistry, 2013, 5, 734.
Barriers in entrance channels typical of reactions between one closed-shell reactant and one open-shell reactant. If the complex in the entrance channel is strongly bound, exotic temperature behaviour is observed, which is a combo of previous two examples. Rate goes through a minimum and then increases dramatically at lower temperatures. Stabilization of the entrance channel complex and tunneling through the barrier.
Typical of reactions between one closed-shell reactant and one open-shell reactant

5. Two hydrogen abstraction paths, methyl (1a) versus hydroxyl (1b)
Two hydrogen abstraction paths, methyl (1a) versus hydroxyl (1b). Computations predict strong H-bonded complex (De=-20.5 kJ/mol) in the entrance channel.
Nature Chemistry, 2013, 5, 745.

6. At room temperature, methyl H-abstraction dominates, and rate constant drops as temperature drops. At very low temperature, however, not only is the rate constant 100 times larger than at room temperature, but now the higher barrier hydroxyl H-abstraction path dominates the branching ratio. This is rationalized as due to the stabilization of the entrance channel complex and QMT through the narrower TS-H barrier.
Nature Chemistry, 2013, 5, 745.

7. Marsha Lester’s group has shown many examples of beautiful spectroscopy experiments of hydroxyl radical containing entrance channel complexes.

8. T = 0.4 K pick-up cells Dissipation via He atom evaporation
Recently we have been applying the helium nanodroplet technique to stabilize complexes containing the hydroxyl radical. Sequential pick-up and rapid He atom evaporation stabilizes the entrance channel-type complexes. Say something here about basic idea behind droplet formation and pick-up.
Dissipation via He atom
evaporation

9. Hydroxyl Radical Production via Flash Vacuum Pyrolysis of TBHP
Water cooled copper electrodes
Gate Valve
Ta filament / Quartz tube
Air
Vacuum
O-ring seal
Rough Pump
Droplet beam
Hydroxyl radicals are made this way… X
∙OH + (CH3)2CO + ∙CH3

10. or OH CH3OH Detect laser-induced depletion of ionization cross-section
cw-OPO (idler 3m)
Droplet Beam
Stark Spectroscopy. Mass spec detection.
Elaser
EStark
Elaser
EStark or
M = 0
M = ±1

11. Helium Droplet Detection via Mass Spectrometry
Neat droplet beam
He+ + CH3OH
{CH3OH}+*
(CH3O)+ + H
Add CH3OH
Take them through the mass spectra
Add TBHP
Pyrolyze TBHP
IP difference (He – molecule) ≈ eV

12. m/z=31 depletion
1000 K
300 K
Take them through the depletion technique, with/without pyro. New peaks when pyro present. 31

13. Can make some progress on assignment on the basis of PUC

14. Assignments. Red-shifted band at 3445 is consistent with h-bonded OH stretch of a complex in which OH is the h-bond donor to the hydroxyl group of MeOH. Mention that ab initio freq shift is ~155 cm-1, whereas the experimental shift is 123 cm-1. Red box… let’s have a closer look at this peak.

15. Stark Spectroscopy ⊥ ∥ Elaser EStark Elaser EStark M = ±1 M = 0
⊥ ∥
Elaser
EStark
Elaser
EStark
M = ±1
M = 0
Beautiful Stark Spectizums. Perp vs Parallel polarization. Perp polarization spectra are very sensitive to a-component of the dipole moment.

16. Vibrational Averaging
Wide-amplitude motion discovered. 15 cm-1 barrier (Cs transition state) between equivalent C1 minima. Experimental dipole moment in excellent agreement with vibrationally averaged dipole moment. Effective Cs structure!

17. Summary
A hydrogen bonded complex between the hydroxyl radical and methanol was formed in He droplets via the sequential pick-up of the monomers.
The red shifted H-bonded stretch exhibits partially resolved rotational structure, which has been probed with Stark spectroscopy.
The experimental rotational contours measured at several electric field strengths are in excellent agreement with predictions from ab initio theory that take into account a wide-amplitude bending motion in the complex.
The vibrationally averaged geometry of the OH∙∙MeOH complex is Cs symmetry, with the OH radical hydrogen bonded to the hydroxyl group of MeOH. Nevertheless, the potential energy surface for OH bending relative to the COH plane in MeOH is rather flat, with only a 15 cm1 barrier between equivalent C1 minima.
You might want to trim these statements. But… left them like this so you could think about what to say here. Emphasize that this is first direct detection of the postulated entrance channel complex. In agreement with theory, but Cs symmetry with floppy motion.

18. Acknowledgments University of Georgia Tao Liang Joseph T. Brice
Gary E. Douberly
Universidad Nacional de Córdoba
Federico J. Hernandez
Gustavo A. Pino
Support: U.S. National Science Foundation
Thanks!