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..

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

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

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

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.

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.

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

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

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

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

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) ≈ 12-15 eV

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

Can make some progress on assignment on the basis of PUC

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.

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.

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!

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.

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!