H-atom Reaction Kinetics in Solid Parahydrogen Followed by Rapid Scan FTIR

David T. Anderson

Department of Chemistry, University of Wyoming

Laramie, WY 82071-3838

danderso@uwyo.edu

Paper: WH02, 2:05 to 2:20 pmWH02. Mini-Symposium: Spectroscopy in kinetics and dynamics

Y

H-atom reactions in solid parahydrogen (pH2)

pH2

3.8 Å

H + H2 → H2 + H

= H-atom

para-H2 @ 4.3 K kDiff = 4.1 x10-1 dm3 mol-1 sec-1

H2O @ 300 K kDiff = 7.0 x109 dm3 mol-1 sec-1

NOHNO

• H-atom reactions should be diffusion limited

Takamasa MomoseChemistry, 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, 164303 (2012).

T = 5 K = 3.5 cm-1

H+NO reaction in solid pH2: Experimental protocol

atmosphere

vacuum

FTIR beam

radiationshield

opticalsubstrate

pH2

crystal

pre-cooledpH2 gas

dopantgas

UVbeam

M.E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237-4241 (1998).

Deposit crystal at <2.5 K(rapid vapor deposition)

Photolyze sample(193 nm, 240 mJ 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 Hz48,000 pulsesphoto = 20 min[NO]0 = 10 ppm

0.06 mJ/pulse, 250 Hz90,000 pulsesphoto = 6 min[NO]0 = 44 ppm

Momose group (2003) Anderson group (2013)

• measured kinetics consistent with previous work!

1HNO

T=4.3 K

k = 1.2x10-2 min-1 k = 1.7x10-2 min-1

Advantage of FTIR detection – broad coverage

wavenumber / cm-1

1000 1500 2000 2500 3000

log 1

0(I 0

/I)

/ ab

s

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NO

before

photo

5.7 hrs

NH3

N2O

CO2

pH2

HNO

N2O

NOH

H2O

HNONH3

HNO HNO

NOH

Observe production of 1HNO and 3NOH

T=4.3 K

(MR02101)(MR02084)(MR02069)

[NO]0 = 18 ppm48,000 pulses 0.36 mJ pulse-1

[NO]0 = 38 ppm48,000 pulses 0.39 mJ pulse-1

[NO]0 = 44 ppm90,000 pulses 0.06 mJ pulse-1

T=4.3 K T=4.3 K

• yields are comparable despite the large barrier for 3NOH production (consistent with diffusion limited kinetics! More later)

Rate constant increases with temperature

T = 1.76 K T = 4.33 K

[NO]0 = 18 ppm48,000 pulses0.36 mJ/pulse

250 Hz

[NO]0 = 17 ppm48,000 pulses0.39 mJ/pulse

250 Hz

H-atom diffusion rate depends on temperature

Normal hydrogen(75:25 oH2:pH2)

99.9% parahydrogen

JETP Letter 36, 472-475 (1982). J. Chem. Phys. 116, 1109-1119 (2002).

However, kinetics are NOT pseudo-first order!

*HNOk

]H[]H][NO[]HNO[ *

HNOHNO kkdt

d H∙ + NO ↔ H---NO → HNO

kD

kuni

krxn

]NO][H[]HNO[

rxnuni

Drxn

kk

kk

dt

d

krxn>>kuni rate = kD[H·][NO]

diffusion limited

Investigate another H-atom reaction

ene

rgy

/cm

-1

-25000

-20000

-15000

-10000

-5000

0

5000

H + NNO

+6120 cm-1

cis-HNNOtrans-HNNO

N2 + OH

H + ONN

+3360 cm-1

?

2A'

aS. P. Walch, JCP 98, 1170-1177 (1993).bK. S. Bradley, P. McCabe, G.C. Schatz, S. P. Walch, JCP 102, 6696-6705 (1995).

H + N2O → N2 + OH DH298 = -21,820 cm-1

(a) (b)(a)(b)

Observe product peaks grow with time at 1.8 K

wavenumber / cm-1

1560 1580 1600 1620 1640

log 1

0(I

0/I)

/ ab

s

0.0

0.2

0.4

0.6

H218O

111-000

2 bend

H15N15N18O

oH2-H218O

cis

trans

2 NN str

before

photo

8.4 hrs

T = 1.78 K

150,000 pulses @ 250 Hz = 10 min0.08 mJ/pulse, 1.80 K, [15N2

18O]0 = 58 ppm

time / min

0 100 200 300 400 500

inte

gra

ted

inte

nsity

/ c

m-1

0.0

0.1

0.2

0.3

0.4

H2O v2 R(0)

H2O v2 P(1)

photo#1 vs Col 13

time / min

0 100 200 300 400 500

inte

gra

ted

inte

nsi

ty /

cm

-1

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

cis-HNNO

trans-HNNO

Now on to tunneling kinetics

photo

H + N2O → cis-HNNO DH = -7065 cm-1

cis-HNNO → trans-HNNO DH = -1994 cm-1

T = 1.78 K T = 1.78 K

para-H2O

ortho-H2O

photo

T = 1.78 K

First-order consecutive reactions (two-steps)

A1 → A2

A2 → A3

k1

k2k1 ≈ k2

H∙ + N2O → cis-HNNO

cis-HNNO → trans-HNNO

k1

k2

time / min

0 100 200 300 400 500

conc

entr

atio

n /

ppm

0.0

0.2

0.4

0.6

0.8

1.0

A2

A3

A1

time / min

0 100 200 300 400 500

conc

entr

atio

n /

ppm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

cis

transphoto

T = 1.81(2) K

• 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???

time / min

0 100 200 300 400 500

conc

entr

atio

n /

ppm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

cis

transphoto

T = 1.81(2) K

time / min

0 100 200 300 400 500

conc

entr

atio

n /

ppm

0.0

0.1

0.2

0.3

0.4

cis

T = 4.33(2) K

trans

photo

T = 1.71(2) K

F. M. Mutunga, S. E. Follett, and DTA, JCP 139, 15104 (2013).

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

157.6 min 1.65 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

151.4 min 1.65 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

145.2 min 1.70 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

139.0 min 1.73 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

132.8 min 1.81 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

126.6 min 1.89 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

120.4 min 2.08 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

114.2 min 2.16 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

108.0 min 2.39 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

101.8 min 2.81 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

95.6 min 2.88 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

89.3 min 2.99 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

83.1 min 3.09 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

76.9 min 3.13 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

70.7 min 3.21 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

64.5 min 3.22 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

58.3 min 3.27 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

52.1 min 3.33 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

45.9 min 3.43 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

39.7 min 3.51 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

33.5 min 3.63 K

wavenumber / cm-1

1570 1572 1574 1576 1578 1580 1582 1584

log 10

(I0/

I) /

abs

0.00

0.05

0.10

0.15

27.3 min 3.98 K

Reaction starts abruptly at temperatures T ≤ 2.4 K

150,000 pulses @ 250 Hz = 10 min0.1 mJ/pulse, 4.32 K, [15N2

18O]0 = 64 ppm

tem

pe

ratu

re /

K

1

2

3

4

5

1

2

3

4

5

time / min

0 100 200 300 400 500

conc

en

tra

tion

/ p

pm

0.0

0.2

0.4

0.6

0.8

0.0

0.2

0.4

0.6

0.8

2.39 K 2.47 K

cis-HNNO

trans-HNNOk=2.93(14)x10-3 min-1

TB

Similar kinetic behavior observed for other reactions

time / min

0 60 120 180 240 300 360 420 480

con

cen

tra

tion

/ p

pm

0

1

2

3

4

5

4.33 K 1.74 K

1.74 K

H + CH3OH → H2 + CH2OH H + HCOOH → H2 + HOCO

photo 1photo 2

photo 3[CH2OH]

• reactions only proceed at low temperature!

1st2nd3rd4th

dE < 0dE > T

dE>0

1st2nd3rd4th

Quantum diffusion to a stationary reagent (impurity)

attraction

dE<0

dE < 0dE > T

repulsion

dE ≈ 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-H2 impurities in p-H2 crystals,” Physica B 165&166, 809-810 (1990).

Yu Kagan, “Quantum diffusion and recombination of atoms in a crystal at low-temperatures,” JETP Lett. 36, 253-256 (1982).

Temperature controls the chemistry

dE ≈ 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 + N2O → cis-HNNO

Mahmut RuziMS 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 08-48330).

Fredrick M. Mutunga2nd year

UW Graduate Student

Shelby E. Follett1st year

UW Graduate Student