1. 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
Section: MK13, 4:51 to 5:06 pm
MK. Matrix Isolation (and droplets)

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

3. UV photolysis of N2O doped pH2 solid
N2O photodissociation dynamics are well known
At 193 nm ( 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, (1993).

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

5. 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)

6. 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
( 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

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

8. 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

9. 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, (2013).

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

12. 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, (2013).

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

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

15. 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)

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

17. In the v1 + v3 region … IP
OOP

18. 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

19. 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 & CHEM ).