Photoinitiation of intra-cluster electron scavenging: An IR study of the

CH3NO2·(H2O)6 anion

Kristin Breen, Timothy Guasco, and Mark Johnson Department of Chemistry, Yale University, New Haven,

CT, 06511.

Presented at the 65th International Symposium on Molecular Spectroscopy

June 21-25, 2009

Outline

• Background info (electron scavenging, nitromethane and hydrated electron facts)

• Motivation (Nagata’s results)

• Ar-mediated condensation – trapping reaction intermediates

• Spectral results for two isomeric forms of nitromethane-water anion

• Conclusions

Electron scavenging

H2O → ?hH2O →hH2O·+ + e¯

H2O*H2O →h

H2O·+ + e¯

H2O*

+ H2O → e¯(aq)

• Attaches easily to other species

• Fast solvation dynamics: ~300 – 450 fs

• Reactions involving e¯(aq) are typically diffusion limited

Hydrated electrons: e¯(aq)

Can we capture a reaction intermediate with this labile species???

Water Cluster anions

• Model for hydrated electron, eaq¯

• Diffuse excess electron

• Can react via charge transfer to form hydrated valence ions

(H2O)6¯

Farhataziz, Rodgers, M. A. J., Eds.; Radiation Chemistry; VCH Publishers: New York, 1987

Background info…Nitromethane (CH3NO2)

• Simplest of the nitro-containing organic molecules

• Large dipole moment = 3.46D

• AEA of anion = 2100 cm-1

Compton, et al., J. Chem. Phys. 105 (9) 1996Weber, et al. J. Chem. Phys., 115, (23), 2001

NM + e¯(aq) → NM¯ rate = 2.2 x 1010 M-1s-1

• Representative of e¯ scavenging rxn

Wallace et al., Radiation Research, 54, 49-62, 1973

barrier to e¯ attachment

• Barrier to electron attachment due to the pyramidal distortion required for nitrate formation

Goal: isolating an intermediate

NM + (H2O)6¯→ [NM···(H2O)6]¯→ NM¯·(H2O)6-n + nH2O + heat

Our analogue to e¯(aq) target

Can we isolate this intermediate that would otherwise be forced to the valence ion product due

to condensation energy?

Valence ion product

Due to condensation

energy

• for n < 15, the binary collision is dominated by associative electron detachment

(H2O)n¯ + D2O → [(H2O)n(D2O)¯]* → (H2O)n(D2O) + e¯

• happens because AEA is less than

H of condensation

Nagata’s strategy• Took (H2O)6¯ with multiple Ar attached in hopes of reproducing our D2O expts

McCunn et al., Phys. Chem. Chem. Phys., 10, 3118–3123, 2008.R. Nakanishi and T. Nagata, J. Chem. Phys. 130, 224309 (2009)

• We proved that you could use Ar-mediated condensation to produce desired anion

(H2O)6¯·Ar12 + D2O → D2O·(H2O)6¯·Ar6 + 6 Ar

Ar-mediated condenstaion to trap in tiny

barrier!

D2O + Wn¯→

D2O·Wn¯

e¯→

D2O + Wn¯·Ark→

D2O·Wn¯·Arj

(k – j)Ar→

Evaporative Condensation

NM + (H2O)6¯·Arn>6 → [NM···(H2O)6·Arn>6 ]¯

Reactants in →

Without Ar rxn goes directly to product ion side

Ar-mediated condensation traps

reactive intermediate

NM system

Evaporative Condensation

NM + (H2O)6¯·Arn>6 → [NM···(H2O)6·Arn>6 ]¯

NM¯·(H2O)6·Arvalence ion

product

[NM·(H2O)6]¯·Ardiffuse electron reactive

intermediate

?

Nagata – a tale of two isomers

Electron Binding Energy (eV)

Hydrated NM¯ anionsElectronic properties

similar to (H2O)6¯

Reactive intermediate species

Valence ion

R. Nakanishi and T. Nagata, J. Chem. Phys. 130, 224309 (2009)

[CH3NO2·(H2O)6]¯[CH3NO2·(H2O)6]¯

(H2O)6¯

Hammer et al., J. Chem. Phys. 109, 7896 (2005)

Ea

Ea

En

erg

y

Reaction Coordinate

different binding

energies!

Electron binding energies distinguishes two chemical

compositions

Valence product ion

Reactive intermediate

Potential Energy Landscape

[CH3NO2· (H2O)6]¯

CH3NO2¯· (H2O)6·Ar

h

3 (H2O)loss

CH3NO2¯·(H2O)3 + 3 H2O

-1 H2O

h

-2 H2O

transition state for intra-cluster electron capture

(bonds breaking and small KE of leaving

waters)

Water loss channels not

accessible due to photon

energy

Solvent coordinate

Ar predissociation

EA

CH3NO2¯· (H2O)6

Experimental Set-up

Reflectron

1 keV Electron Gun

Supersonic Expansion

Ar / H2O

Reflectron Detector

hν

Ion Optics

Bleed Valve

CH3NO2

MassGate

Valence ion product

[CH3NO2· (H2O)6]¯

CH3NO2¯· (H2O)6·Ar

hCH3NO2¯·(H2O)3 + 3 H2O

-1 H2O

h

-2 H2O

Solvent coordinate

Ar predissociation

EA

Results – Valence ions

2600 2800 3000 3200 3400 3600 3800

Pre

dis

soci

atio

n Y

ield

Photon Energy (cm-1)

n = 2

n = 3

n = 4

n = 5

n = 6

CH3NO2¯· (H2O)n· Ar

a)

b)

c)

d)

e)

CH stretches

Water regionCH3NO2¯· (H2O)6·Ar

-1 H2O

h Ar predissociation

Isolating reactive intermediate using differential loss channels

[CH3NO2· (H2O)6]¯

CH3NO2¯· (H2O)6·Ar

h

3 (H2O)loss

CH3NO2¯·(H2O)3 + 3 H2O

-1 H2O

h

-2 H2O

transition state for intra-cluster electron capture

Solvent coordinate

EA

evaporative calorimetry

Hrxn

Hwater

≈ 3

[CH3(NO2) · (H2O)6]¯

Loss of 3 H2O

1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 3800

Photon Energy (cm-1)

x 7

H-bonds in water

CH stretchH2O bend and NO stretch

Isolation of reactive intermediate spectrum

[CH3(NO2) · (H2O)6]¯

Loss of 3 H2O

1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 3800

Photon Energy (cm-1)

x 7

H-bonds in water

CH stretchH2O bend and NO stretch

Isolation of reactive intermediate spectrum

CH3NO2¯· (H2O)6· Ar

[CH3(NO2) · (H2O)6]¯

Loss of 3 H2O

1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 3800

Photon Energy (cm-1)

x 7

H-bonds in water

CH stretchH2O bend and NO stretch

Isolation of reactive intermediate spectrum

CH3NO2¯· (H2O)6· Ar

Excess electron binding site intact

3000 3200 3400 3600 3800Photon Energy (cm-1)

a)

b)

62O)(H sym OHAA

62O)(H asym OHAA

62O)(H stretch OHAAD

62O)(H stretch OHAD

a) (H2O)6¯· Ar7

Loss of 7 Ar

b) [CH3NO2· (H2O)6]¯

Loss of 3 H2O

Structures: Jordan Group

[CH3(NO2) · (H2O)6]¯

Loss of 3 H2O

1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 3800

Photon Energy (cm-1)

x 7

H-bonds in water

CH stretchH2O bend and NO stretch

CH stretches support neutral NM

2750 2850 2950 3050 3150Photon Energy (cm-1)

23NOCH sym-CH

23'

NOCH

sym-CH

23NOCH asym-CH

23NOCH sym-CH

23NOCH sym-CH

23NOCH asym-CH

2750 2850 2950 3050 3150Photon Energy (cm-1)

CH3NO2¯

[CH3NO2· (H2O)6]¯

Weber et. al., J. Chem. Phys., 115, (23), 2001Gorse et. al.,J. Phys. Chem., 97, 4262,1993

CH3NO2¯· (H2O)6· Ar

[CH3(NO2) · (H2O)6]¯

Loss of 3 H2O

1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 3800

Photon Energy (cm-1)

x 7

H-bonds in water

CH stretchH2O bend and NO stretch

1200 1300 1400 1500 1600 1700Photon Energy (cm-1)

62O)(H bendAA

23NOCH sym-NO

23NOCH asym-NO

62O)(H bendAD

62O)(H bendDD

NO stretches confirm neutral NM

[CH3NO2· (H2O)6]¯

(H2O)6¯· Ar7

Conclusions• Two isomeric forms of [CH3NO2·(H2O)6]¯ exist: valence anion and higher energy species

• Valence anions (NM·H2On¯) shows expected CH stretches, IHB, and OH stretches, with some extra features in n = 5 case

• Ar trapping indeed prepares diffuse electron reactive intermediate

• We observe intra-cluster conversion from diffuse intermediate to valence ion

• Likely mechanism is IVR followed by solvent rearrangement to mediate charge transfer event

• Theoretical studies suggest that the reactive isomer occurs with the NM molecule attached to the backside of the water network via accepting H-bonds

• This is an excellent system for future studies involving real time kinetics using fpes

[CH3NO2· (H2O)6]¯

CH3NO2¯· (H2O)6·Ar

h

3 (H2O)loss

CH3NO2¯·(H2O)3 + 3 H2O

-1 H2O

h

-2 H2O

Ar predissociation

EA

AcknowledgementsMark Johnson

Timothy GuascoRachael RelphBen ElliottGeorge GardenierMike KamrathHelen GerardiChristopher LeavittArron WolkAndrew DeBlase

Nagata GroupJordan Group