H-atom Reaction Kinetics in Solid Parahydrogen Followed by Rapid Scan FTIR David T. Anderson Department of Chemistry, University of Wyoming Laramie, WY Paper: WH02, 2:05 to 2:20 pm WH02. Mini-Symposium: Spectroscopy in kinetics and dynamics 

H-atom reactions in solid parahydrogen (pH 2 ) pH Å H + H 2 → H 2 + H = H-atom para-H 4.3 K k Diff = 4.1 x10 -1 dm 3 mol -1 sec -1 H K k Diff = 7.0 x10 9 dm 3 mol -1 sec -1 NO HNO H-atom reactions should be diffusion limited

Takamasa Momose Chemistry, UBC T = 5.2 K experiments performed at only one temperature! before photo 155 min H + NO → HNO

There are two reactive H + NO surfaces U. Bozkaya, J.M. Turney, Y. Yamaguchi, H.F. Schaefer III, JCP 136, (2012). T = 5 K = 3.5 cm -1

H+NO reaction in solid pH 2 : Experimental protocol atmosphere vacuum FTIR beam radiation shield optical substrate pH 2 crystal pre-cooled pH 2 gas dopant gas UV beam M.E. Fajardo and S. Tam, J. Chem. Phys. 108, (1998). Deposit crystal at <2.5 K (rapid vapor deposition) Photolyze sample (193 nm, 240  J cm -2 pulse -1, 250 Hz) Repeated FTIR scans (3.6 min acquisition times, average 16 scans at 0.04 cm -1 resolution) Liquid helium bath cryostat

Measure reaction kinetics - agreement T=5.2 K 2 mJ/pulse, 40 Hz 48,000 pulses photo = 20 min [NO] 0 = 10 ppm 0.06 mJ/pulse, 250 Hz 90,000 pulses photo = 6 min [NO] 0 = 44 ppm Momose group (2003)Anderson group (2013) measured kinetics consistent with previous work! 1 HNO T=4.3 K k = 1.2x10 -2 min -1 k = 1.7x10 -2 min -1

Advantage of FTIR detection – broad coverage

Observe production of 1 HNO and 3 NOH T=4.3 K (MR02101)(MR02084)(MR02069) [NO] 0 = 18 ppm 48,000 pulses 0.36 mJ pulse -1 [NO] 0 = 38 ppm 48,000 pulses 0.39 mJ pulse -1 [NO] 0 = 44 ppm 90,000 pulses 0.06 mJ pulse -1 T=4.3 K yields are comparable despite the large barrier for 3 NOH production (consistent with diffusion limited kinetics! More later)

Rate constant increases with temperature T = 1.76 K T = 4.33 K [NO] 0 = 18 ppm 48,000 pulses 0.36 mJ/pulse 250 Hz [NO] 0 = 17 ppm 48,000 pulses 0.39 mJ/pulse 250 Hz

H-atom diffusion rate depends on temperature Normal hydrogen (75:25 oH 2 :pH 2 ) 99.9% parahydrogen JETP Letter 36, (1982).J. Chem. Phys. 116, (2002).

However, kinetics are NOT pseudo-first order! H∙ + NO ↔ H---NO → HNO kDkD k uni k rxn k rxn >>k uni rate = k D [H·][NO] diffusion limited

Investigate another H-atom reaction a S. P. Walch, JCP 98, (1993). b K. S. Bradley, P. McCabe, G.C. Schatz, S. P. Walch, JCP 102, (1995). H + N 2 O → N 2 + OH  H 298 = -21,820 cm -1 (a)(b)(a) (b)

Observe product peaks grow with time at 1.8 K 150, Hz = 10 min 0.08 mJ/pulse, 1.80 K, [ 15 N 2 18 O] 0 = 58 ppm

Now on to tunneling kinetics photo H + N 2 O → cis-HNNO  H = cm -1 cis-HNNO → trans-HNNO  H = cm -1 T = 1.78 K para-H 2 O ortho-H 2 O photo T = 1.78 K

First-order consecutive reactions (two-steps) A 1 → A 2 A 2 → A 3 k1k1 k2k2 k 1 ≈ k 2 H∙ + N 2 O → cis-HNNO cis-HNNO → trans-HNNO k1k1 k2k2 trans and cis data fit well to textbook expressions but, cannot fit both data sets to one set of parameters?

Now it starts to get crazy! reaction occurs at 1.8 K, but not at 4.3 K (minor) reaction starts 6 hours after photolysis by lowering the temperature! what are the reaction kinetics at intermediate temperatures??? F. M. Mutunga, S. E. Follett, and DTA, JCP 139, (2013).

Reaction starts abruptly at temperatures T ≤ 2.4 K 150, Hz = 10 min 0.1 mJ/pulse, 4.32 K, [ 15 N 2 18 O] 0 = 64 ppm 2.39 K 2.47 K cis-HNNO trans-HNNO k=2.93(14)x10 -3 min -1 TBTB

Similar kinetic behavior observed for other reactions H + CH 3 OH → H 2 + CH 2 OHH + HCOOH → H 2 + HOCO photo 1 photo 2 photo 3 [CH 2 OH] reactions only proceed at low temperature!

1st2nd3rd4th  E < 0  E > T  E>0 1st2nd3rd4th Quantum diffusion to a stationary reagent (impurity) attraction  E<0  E > T repulsion  E ≈ 2.4 K probability for hopping no longer depends on T, and irreversible capture occurs probability for hopping has an activation nature and increases linearly with T nearest neighbor site A. E. Meyerovich, “Low temperature clustering of o-H 2 impurities in p-H 2 crystals,” Physica B 165&166, (1990). Yu Kagan, “Quantum diffusion and recombination of atoms in a crystal at low-temperatures,” JETP Lett. 36, (1982).

Temperature controls the chemistry  E ≈ 2.4 K Long-range H-atom quantum diffusion qualitatively changes in the range 1.8 – 4.3 K Extremely small energy shifts (1 cal/mol) “control” reactions with 10 kcal/mol barriers Intermolecular forces dictate the kinetic behavior for a particular reagent H + N 2 O → cis-HNNO

Mahmut Ruzi MS 2012 UW Graduate Student The people who do the work and funding This research was sponsored in part by the Chemistry Division of the US National Science Foundation (CHE ). Fredrick M. Mutunga 2 nd year UW Graduate Student Shelby E. Follett 1 st year UW Graduate Student