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Low-temperature hydrogenation and deuteration of solid aromatic hydrocarbon by tunneling

Tetsuya Hama, Hirokazu Ueta, Akira Kouchi, and Naoki WatanabeInstitute of Low Temperature Science, Hokkaido University

From clouds to

protoplanetary

disks: the astrochemical link

4-8 October 2015, Hans

Harnack

Haus

Molecular clouds are chemically rich, despite their low temp.

At low temperature (10-50 K), thermal reactions rarely occur.Actually, however, many solid molecules: H2O, NH3, H2CO, and CH

3OH…

Two keys for cold chemistry: (1) Dust surface reactions, (2) Tunneling

e.g., H

2

CO formation

CO  HCO (radical)

 H2COH-addition to CO has a large activation barrier (2000 K), but it efficiently occurs by tunneling.

Hama and Watanabe (2013), Chem. Rev., 113, 8783.

Tunneling

Thermal

Aromatic hydrocarbons (benzene, PAHs) can be present in MCs.At low temp., C6

H6 can be adsorbed on the dust surface.Tunneling chemistry of solid aromatic hydrocarbons ?H-addition (aliphatic H) : Origin of the 3.4µm IR band.D-addition (aliphatic D) : D-enrichment in PAHs, and carbonaceous meteorites.

Aromatic → Aliphatic

Benzene

Cyclohexane

C

6

H

6 + H → C6H

7, Ea

= 2200 K, C

6

H

7

+ H → C

6

H

8

, Barrier-lessC6H8 + H → C6H9, Ea = 760 K,C6H9 + H → C6H10, Barrier-lessC6H10 + H → C6H11, Ea = 1300 K,C6H11 + H → C6H12, Barrier-less

This study: Hydrogenation (deuteration) of solid benzene (C6H6) at 20 K

Benzene (C

6H6): the barrier-less gas-phase reaction of the ethynyl radical and 1,3-butadiene:C2H + H2CCHCHCH2 → C6H6 + H. Jones et al. (2011) PNAS 108:452.Naphthalene from benzene Parker et al. (2012) PNAS 109:53.

Ultrahigh vacuum chamber

:Base pressure： 10

-10

torr

Al substrate

：

10 – 300 K

H atom: produced by dissociation of H

2 in a microwave discharge plasma. Cooled within an Al pipe at 100 K,to suppress any energetic processes. Flux of H or D atoms: 4-5×1014 atoms cm

-2 s-12 hr irradiation

→ a life-time of an interstellar cloud.

Experimental setup

H

or D atoms (120 K)

?

In-situ

infrared reflection absorption

spectroscopy

Benzene deposition8-10 monolayers(6×1015 molecules cm-2)

20 K

20 K

20 K

Wavenumber

/ cm-1(B) Difference spectra of C

6H

6 after exposure to H atoms at 20 K.

Negative: C

6

H

6

decrease. Positive: New aliphatic C-H str. (2900 cm-1)C

6H12 formation at 20 K, despite the activation energies (2200 K) !

 Quantum tunneling !

Experimental results

(A) IR spectra of

amorphous benzene (C

6

H

6

), and amorphous cyclohexane (C6H12) at 20 K.Hama et al. (2014), J. Phys. Chem. Lett., 5, 3843.

C

6

H

6

+ 6 D



C

6

H6D

6 (partially-deuterated

cyclohexane) formation ?

However, the reference spectrum of C

6

H

6

D

6

cannot be measured.

(not commercially available.)↓Temperature programmed desorption (TPD) mass spectrometryDifference spectra of C6H6 after exposure to D atoms at 20 K.Negative: C6H6 decrease. Positive: New aliphatic C-H str. (2900 cm-1) C-D str. (2200 cm-1) Experimental results

m/z

78, 82, 83, 84,

85,

86,

87,

88,

89, 90, 91, 92, 93,

94, 95,

96

C

6

H

6

(blank)

D-exposed C

6

H6 at 20 Km/z 78, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96Enlarged BCTemperature (K)

C6H6 (blank)

at 143 K

D-exposed C6H6at 143 Km/z Benzene D-cyclohexaneC6H

6

--

90

--

C

6

H

6

D

6

91

--

C

6

H

5

D

7

92

--

C

6

H

4

D

8

93

-- C

6

H

3

D

9

94

-- C

6

H

2

D

10

95

-- C

6

HD

11

96

-- C

6

D

12

More

deuterated

C

6

H

6

D

6

, C

6

H

5

D

7

…C

6

D

12

formations !

First, D addition to C

6

H

6

by tunneling

C

6

H6 → C6H6D6 Subsequently, H–D substitution of C6H6D6C6H6D6 → C6D12

Two-step H-D substitution by D atoms (Naoki’s talk)

H-transfer by D atom:

C

6

H

6

D

6

+D → C

6

H

5

D

6

(radical) +

H

D

barrier-less D atom addition: C

6

H

5

D

6

+D →

C

6

H

5

D

7

C

6

H

5

D

7

→

C

6

H

4

D

8

→

C

6

H

3

D

9

→

C

6

H

2

D

10

→

C

6

HD

11

→

C

6

D

12

C

6

H

6

decreases similarly upon H or D exposure

.

 Very small KIE !!

Quantum tunneling has very large KIE; H reacts with C

6H6 100 times faster than D !!Goumans and Kästner (2010), Angew

. Chemie. Int. Ed. 49, 7350.

Strange kinetic isotope effect on C6H6

consumption at 20 K.

C6H

6 consumption as a function of the atomic exposure time at 20-40 K.Large KIEs: H/D = 4–5 at 30–50 K (tunneling)○ H atom

△ D atom



C

6

H

6

1014 molecules cm-2H or D atom exposure time (min) 40 K

30 K

20 K

Very small KIE

H/D = 1.5

at 15–20 K

Hama et al. PNAS, 112, 7438 (2015).

“Quantum tunneling observed without its characteristic large kinetic isotope effects”.

Rate-limiting

adsorption

→ diffusion → tunneling Reaction-diffusion competition:Diffusion  reaction (tunneling)Diffusion : Temp. dependentTunneling: Temp. independentAt low temp., the total chemical kinetics is determined by surface diffusion.  Small KIEHighTunnelingTunneling

Temperature

Low

Short

interaction

Long

interaction

Diffusion

Fast

Slow

Rate-determining step

Tunneling

Diffusion

Summary and astrophysical implicationsHama et al. (2014), J. Phys. Chem. Lett

., 5, 3843.Hama et al. (2015), PNAS, 112, 7438. (1) H- or D-addition to benzene (C6H6) by tunneling. (C6

H12

, C

6

H

6

D

6 formations)In the cases of PAHs, the barriers for H or D-addition tend to be low, owing to the higher flexibility. PAHs (and carbonaceous dust) can be hydrogenated or

deuterated at < 50 K.(2) H–D substitution of C

6H

6

D

6

: C

6

H

6

D6 → C6D12Since the atomic D/H ratio in molecular clouds can be enhanced up to 10−2 to 10−1,the D-enrichment of hydrocarbons may occur.A new non-energetic D-enrichment process.(3) “Small-KIE tunneling” in “diffusion-limited reactions”.The large tunneling KIE would not strongly inhibit the deuteration.AcknowledgementsJSPS Grants-in-Aid 24224012,MEXT Grant-in-Aid 25108002.