H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Tunneling process is facile because theinitial and final states are isoenergetic

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

time / min

0 100 200 300 400 500

conc

entra

tion

/ ppm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

cis

transphoto

T = 1.81(2) K

Motivated by recent experiments of Mutunga et al. investigating HNNO formation in N2O-doped solid pH2:

H atoms produced in situ via photolysis of precursor species in the matrix

Mutunga et al., J. Chem. Phys. vol. 139, article 151104 (2013).

time / min

0 100 200 300 400 500

conc

entra

tion

/ ppm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

cis

transphoto

T = 1.81(2) K H atoms produced in situ via photolysis of precursor species in the matrix

Mutunga et al., J. Chem. Phys. vol. 139, article 151104 (2013).

What happens at different matrix temperatures?

Mutunga et al., J. Chem. Phys. vol. 139, article 151104 (2013).

HNNO formation appears to “turn on” abruptly when the matrix temperature drops below T ≈ 2.4 K!

Why? Trapping of H atoms in “pre-reactive” sites?

ener

gy /c

m-1

-25000

-20000

-15000

-10000

-5000

0

5000

H + NNO

+6120 cm-1

cis-HNNOtrans-HNNO

N2 + OH

H + ONN

+3360 cm-1

?

2A'

Figure by Anderson, based on Bradley et al., J. Chem. Phys. vol. 102, p. 6696 (1995).

Formation of HNNO proceeds via tunneling through an energetic barrier . . .

ener

gy /c

m-1

-25000

-20000

-15000

-10000

-5000

0

5000

H + NNO

+6120 cm-1

cis-HNNOtrans-HNNO

N2 + OH

H + ONN

+3360 cm-1

?

2A'

Figure by Anderson, based on Bradley et al., J. Chem. Phys. vol. 102, p. 6696 (1995).

Formation of HNNO proceeds via tunneling through an energetic barrier . . . preceded by a van der Waals complex:

H • • • NNOvan der Waals

complex

So we are interested in understanding the energetics of various H + N2O pre-reactive complexes, stabilized by the

solid pH2 matrix environment:

N=N=O

N=N=O

So we are interested in understanding the energetics of various H + N2O pre-reactive complexes, stabilized by the

solid pH2 matrix environment:

Ar

But first, let’s start with a simpler model system: Ar-doped solid pH2 . . .

A quantitative approach requires us to account for the constituents’ large-amplitude zero point motions:

Simulations carried out using QSATS code: Hinde, Comput. Phys. Commun. vol. 182,p. 2339 (2011).

Ar

H

pH2

Ar

Evaluate the energies of various H • • • Ar pairs, using “infinitely separated” H and Ar atoms as a reference:

Ar

Evaluate the energies of various H • • • Ar pairs, using “infinitely separated” H and Ar atoms as a reference:

Ar

Evaluate the energies of various H • • • Ar pairs, using “infinitely separated” H and Ar atoms as a reference:

Evaluate the energies of various H • • • Ar pairs, using “infinitely separated” H and Ar atoms as a reference:

Site NominalRelative

distance (a0)

energy (K)

4th NN 12.4011.6 ± 0.1

3rd NN 11.6911.8 ± 0.1

2nd NN 10.13 12.9 ± 0.2NN 17.16

17.5 ± 1.3 ???

To understand these findings, let’s look at the pair interactions:

DistanceR (a0)

Ar–H

pH2–H

pH2–pH2

Pot

entia

l ene

rgy

V(R

) (K

)

Ar–pH2

It’s highly unfavorable to displace a pH2 molecule from the

Ar dopant’s nearest-neighbor solvation shell!

DistanceR (a0)

Ar–pH2

Ar–H

pH2–H

pH2–pH2

Pot

entia

l ene

rgy

V(R

) (K

)

Take-home message: we can’t ignore the host matrix environment when evaluating the energies of the various H • • • N2O pre-reactive complexes.

N=N=O

Steeringimpurity-doped

H-atom diffusion throughsolid parahydrogen

Robert Hinde, Dept. of Chemistry, Univ. of Tennessee

Thanks to:

David Andersongroup (U of Wyoming)

NSF, UTK ($$$)