Using IR Spectroscopy to Probe the Temperature Dependence of the H + N2O Reaction in Parahydrogen Crystals Fredrick Mutunga and David T. Anderson Department of Chemistry, University of Wyoming Laramie, WY 82071-3838 danderso@uwyo.edu fredrick.mutunga@gmail.com Section: MK13, 4:51 to 5:06 pm MK. Matrix Isolation (and droplets)

PhD Public Talk June 16, 2016 UW Commencement May 14, 2016 “Posing” 2013

UV photolysis of N2O doped pH2 solid N2O photodissociation dynamics are well known At 193 nm (6.42405 eV): N2O → N2 + O(1D) In solid pH2 O + pH2 → OH + H OH + pH2 → H2O + H hv N2O is a good source of H atoms T. F. Hanisco and A. C. Kummel, J. Phys. Chem . 97, 7242-7246 (1993).

H atom reactions in N2O doped pH2 solid H + pH2 → pH2 + H (H atom diffusion) H + H → H2 (H atom termination channel) H + N2O → cis-HNNO → trans-HNNO 1.8-4.3 K pH2 H + N2O cis-HNNO H-atom diffusion is the rate determining step T. Kumada, Phys. Rev. B 68, 052301 (2003).

Our experimental setup – Photolysis scheme FTIR beam in the range of 700 to 5000 cm-1 To InSb or MCT detector CH3NH2 gas flow pH2 gas flow CH3NH2 doped pH2 crystal 193 nm laser beam BaF2 optical Substrate (T = 1.6 – 4.3 K)

Setup to make and characterize doped pH2 crystals Specifications sample-in-a-vacuum liquid helium cryostat (1.6 – 4.3 K) variable temperature ortho/para converter (10 - 80 K) 180 l s-1 turbo pump (<10-4 torr during deposition) IR diagnostics – Bruker IFS120 (0.006 cm-1) IR cryostat turbo pump pH2 dopant UV

Observe product peaks grow with time at 1.8 K 150,000 pulses @ 250 Hz = 10 min 0.08 mJ/pulse, 1.80 K, [15N218O]0 = 58 ppm

First-order consecutive reactions (two-steps) H∙ + N2O → cis-HNNO cis-HNNO → trans-HNNO k1 k2 A1 → A2 A2 → A3 k1 k2 k1 ≈ k2 trans and cis data fit well to textbook expressions

Now it starts to get crazy! T = 4.31 K T = 1.71 K reaction occurs at 1.7 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, J. Chem. Phys. 139, 15104 (2013).

Can we map out the transition temperature? 150,000 pulses @ 250 Hz = 10 min 0.08 mJ/pulse, 4.30 K, [15N218O]0 = 60 ppm

Here is our initial hypothesis H. + N2O H---N2O cis-HNNO trans-HNNO H∙ + N2O ↔ H---NNO → HNNO kD kuni krxn At high temp (T > = 2.5 K) krxn << kuni krxn >> kuni At low temp ( T < 2.5 K) F. M. Mutunga, S. E. Follett, and DTA, J. Chem. Phys. 139, 15104 (2013).

However, kinetics are NOT pseudo-first order! kD krxn H∙ + N2O ↔ H---NNO → HNNO kuni krxn>>kuni rate = kD[H·][NNO] diffusion limited

Furthermore … compare with k = 5.23 x 10-3 min-1 @ T = 1.8 K Reaction is much faster at 4.3 K only that the product yield is low!

What does N2O spectroscopy tell us? N2O trapping site 1 N2O trapping site 2 Immed. after photo 3 hrs 6 hrs 6.5 hrs 8 hrs T = 4.31 K T = 2.16 K T = 1.71 K 2v1 overtone band There are two possible N2O trapping sites: Site 1 is significantly populated at all our expt. temperatures while site 2 is only considerably populated at low temperatures (T < 2.5 K)

Correlation N2O quantum states and product yield cis trans site 1 site 2 Immed. 3 hrs 6 hrs 6.5 hrs 8 hrs

In the v1 + v3 region … IP OOP

Conclusions Reaction occurs at T < 2.5 K but very little above it. Kinetic argument is not sufficient to explain the temperature dependence to the reaction. IR spectroscopy shows two possible N2O trapping sites There is a very strong relationship between the population of the identified trapping sites and the observed temperature dependence to the reaction

The people who do the work and funding Aaron Undergrad. Student Wes REU Student Morgan Grad. Student Shelby Grad. Student Fred Mutunga This research was sponsored in part by the Chemistry Division of the US National Science Foundation (CHE 08-48330 & CHEM 13-62497).