Photoinitiation of intra-cluster electron scavenging: An IR study of the CH 3 NO 2 ·(H 2 O) 6 anion Kristin Breen, Timothy Guasco, and Mark Johnson Department of Chemistry, Yale University, New Haven, CT, Presented at the 65 th 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 H 2 O → ? h H 2 O → h H 2 O· + + e¯ H 2 O* H 2 O → h H 2 O· + + e¯ H 2 O* + H 2 O → 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, e aq ¯ Diffuse excess electron Can react via charge transfer to form hydrated valence ions (H 2 O) 6 ¯ Farhataziz, Rodgers, M. A. J., Eds.; Radiation Chemistry; VCH Publishers: New York, 1987

Background info… Nitromethane (CH 3 NO 2 ) 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) 1996 Weber, et al. J. Chem. Phys., 115, (23), 2001 NM + e¯ (aq) → NM¯ rate = 2.2 x M -1 s -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 + (H 2 O) 6 ¯→ [NM···(H 2 O) 6 ]¯→ NM¯·(H 2 O) 6-n + nH 2 O + 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 (H 2 O) n ¯ + D 2 O → [(H 2 O) n (D 2 O)¯]* → (H 2 O) n (D 2 O) + e¯ happens because AEA is less than  H of condensation Nagata’s strategy Took (H 2 O) 6 ¯ with multiple Ar attached in hopes of reproducing our D 2 O expts McCunn et al., Phys. Chem. Chem. Phys., 10, 3118–3123, R. Nakanishi and T. Nagata, J. Chem. Phys. 130, (2009) We proved that you could use Ar-mediated condensation to produce desired anion (H 2 O) 6 ¯·Ar 12 + D 2 O → D 2 O·(H 2 O) 6 ¯·Ar Ar Ar-mediated condenstaion to trap in tiny barrier! D 2 O + W n ¯→ D 2 O·W n ¯ e¯→ D 2 O + W n ¯·Ar k → D 2 O·W n ¯·Ar j (k – j)Ar→

Evaporative Condensation NM + (H 2 O) 6 ¯·Ar n>6 → [NM···(H 2 O) 6 ·Ar n>6 ]¯ Reactants in → Without Ar rxn goes directly to product ion side Ar-mediated condensation traps reactive intermediate NM system

Evaporative Condensation NM + (H 2 O) 6 ¯·Ar n>6 → [NM···(H 2 O) 6 ·Ar n>6 ]¯ NM¯·(H 2 O) 6 ·Ar valence ion product [NM·(H 2 O) 6 ]¯·Ar diffuse electron reactive intermediate ?

Nagata – a tale of two isomers Electron Binding Energy (eV) Hydrated NM¯ anions Electronic properties similar to (H 2 O) 6 ¯ Reactive intermediate species Valence ion R. Nakanishi and T. Nagata, J. Chem. Phys. 130, (2009) [CH 3 NO 2 ·(H 2 O) 6 ]¯ (H 2 O) 6 ¯ Hammer et al., J. Chem. Phys. 109, 7896 (2005) EaEa EaEa Energy Reaction Coordinate different binding energies! Electron binding energies distinguishes two chemical compositions Valence product ion Reactive intermediate

Potential Energy Landscape [CH 3 NO 2 · (H 2 O) 6 ]¯ CH 3 NO 2 ¯· (H 2 O) 6 ·Ar h 3 (H 2 O) loss CH 3 NO 2 ¯·(H 2 O) H 2 O -1 H 2 O h -2 H 2 O 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 EAEA CH 3 NO 2 ¯· (H 2 O) 6

Experimental Set-up Reflectron 1 keV Electron Gun Supersonic Expansion Ar / H 2 O Reflectron Detector hνhν Ion Optics Bleed Valve CH 3 NO 2 Mass Gate

Valence ion product [CH 3 NO 2 · (H 2 O) 6 ]¯ CH 3 NO 2 ¯· (H 2 O) 6 ·Ar h CH 3 NO 2 ¯·(H 2 O) H 2 O -1 H 2 O h -2 H 2 O Solvent coordinate Ar predissociation EAEA

Results – Valence ions CH stretches Water region CH 3 NO 2 ¯· (H 2 O) 6 ·Ar -1 H 2 O h Ar predissociation

Isolating reactive intermediate using differential loss channels [CH 3 NO 2 · (H 2 O) 6 ]¯ CH 3 NO 2 ¯· (H 2 O) 6 ·Ar h 3 (H 2 O) loss CH 3 NO 2 ¯·(H 2 O) H 2 O -1 H 2 O h -2 H 2 O transition state for intra-cluster electron capture Solvent coordinate EAEA evaporative calorimetry  H rxn  H water ≈ 3

[CH 3 (NO 2 ) · (H 2 O) 6 ]¯ Loss of 3 H 2 O Photon Energy (cm -1 ) x 7 H-bonds in water CH stretch H 2 O bend and NO stretch Isolation of reactive intermediate spectrum

[CH 3 (NO 2 ) · (H 2 O) 6 ]¯ Loss of 3 H 2 O Photon Energy (cm -1 ) x 7 H-bonds in water CH stretch H 2 O bend and NO stretch Isolation of reactive intermediate spectrum CH 3 NO 2 ¯· (H 2 O) 6 · Ar

[CH 3 (NO 2 ) · (H 2 O) 6 ]¯ Loss of 3 H 2 O Photon Energy (cm -1 ) x 7 H-bonds in water CH stretch H 2 O bend and NO stretch Isolation of reactive intermediate spectrum CH 3 NO 2 ¯· (H 2 O) 6 · Ar

Excess electron binding site intact Photon Energy (cm -1 ) a) b) a)(H 2 O) 6 ¯· Ar 7 Loss of 7 Ar b) [CH 3 NO 2 · (H 2 O) 6 ]¯ Loss of 3 H 2 O Structures: Jordan Group

[CH 3 (NO 2 ) · (H 2 O) 6 ]¯ Loss of 3 H 2 O Photon Energy (cm -1 ) x 7 H-bonds in water CH stretch H 2 O bend and NO stretch

CH stretches support neutral NM Photon Energy (cm -1 ) Photon Energy (cm -1 ) CH 3 NO 2 ¯ [CH 3 NO 2 · (H 2 O) 6 ]¯ Weber et. al., J. Chem. Phys., 115, (23), 2001 Gorse et. al.,J. Phys. Chem., 97, 4262,1993 CH 3 NO 2 ¯· (H 2 O) 6 · Ar

[CH 3 (NO 2 ) · (H 2 O) 6 ]¯ Loss of 3 H 2 O Photon Energy (cm -1 ) x 7 H-bonds in water CH stretch H 2 O bend and NO stretch

Photon Energy (cm -1 ) NO stretches confirm neutral NM [CH 3 NO 2 · (H 2 O) 6 ]¯ (H 2 O) 6 ¯· Ar 7

Conclusions Two isomeric forms of [CH 3 NO 2 ·(H 2 O) 6 ]¯ exist: valence anion and higher energy species Valence anions (NM·H 2 O n ¯) 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 [CH 3 NO 2 · (H 2 O) 6 ]¯ CH 3 NO 2 ¯· (H 2 O) 6 ·Ar h 3 (H 2 O) loss CH 3 NO 2 ¯·(H 2 O) H 2 O -1 H 2 O h -2 H 2 O Ar predissociation EAEA

Acknowledgements Mark Johnson Timothy Guasco Rachael Relph Ben Elliott George Gardenier Mike Kamrath Helen Gerardi Christopher Leavitt Arron Wolk Andrew DeBlase Nagata Group Jordan Group