Long Range Proton-Migration in VUV Photoionization of Acetone Clusters

Ken-ichi Hanaue, Yoshiyuki Matsuda, Ayako Yamada, Keisuke Ohta, Naohiko Mikami, and Asuka Fujii

65th International Symposium on Molecular SpectroscopyJune 24, 2010 Ohio State University

Department of Chemistry, Graduate School of Science,Tohoku University, Japan

Cluster cations of protic molecules

IR spectroscopy based on VUV photoionization detection has been applied to cluster cations of protic molecules.

JCP 131, 184304 (2009) 　 PCCP 11, 1279 (2009).

(NH3)2+ (CH3OH)2

+ (CH3COOH)2+

JCP 129, 094306 (2008).

　 VUV photoionization of protic molecular dimers

　 intracluster H+- transfer between NH and OH

Can a proton of CH (alkyl group) be transferred?

Roles of solvents in VUV photoionization of solvated clusters?

This study

Understanding of VUV one-photon ionization dynamics of acetone-H2O

○　 IR spectroscopy based on VUV photoionization detection of neutral and cationic clusters.

○　 Reaction route search with the GRRM (Global Reaction Route Mapping) method

?

VUV photoionization

acetone-H2O

acetone: aprotic molecule

neutral cation

Experiment and calculation

IR spectroscopy of neutral clusters IR spectroscopy of cluster cations

ion signal

IRVUV

S0

IE

dissociationt

V’

IRV’

S0

IE

dissociation

ion signalt

VUV

By changing the relative delaytime between of the IR and VUVpulses, IR spectroscopy of both neutral (precursor) and cationic (product) clusters in the VUV photoionization can be carried out.

IR spectroscopy based on VUV (118 nm) photoionization detection

Experiment

CalculationGRRM (Global Reaction Route Mapping) method (Maeda & Ohno, 2004)

search of reaction routes following photoionization

2600 2800 3000 3200 3400 3600 3800

IR spectra of neutral acetone and acetone-Ar cluster cation

2800 　 3000 3200 3400 3600 Wavenumber / cm-1

Wavenumber / cm-1

acetone

acetone+-Ar

Scaled by 0.931MP2/6-31++G**

MP2/6-31++G**

Scaled by 0.931

IR-VUV-PIS

IR

VUV

IE

S0

V’

ion signal

VUV @ 118 nm

VUV @ 128 nmneutral

cation

OH

keto16.2 kcal/mol

enol0 kcal/mol

neutral

cation

acetone-H2O

(acetone-H2O)+

2800 3000 3200 3400 3600

Wavenumber / cm-1

2600 2800 3000 3200 3400 3600 3800Wavenumber / cm-1

H-bonded free OH

low IR power

CH

OH

31H-bonded OH

IR spectra of acetone-H2O and (acetone-H2O)+

MP2/6-31++G** scaled by 0.94

scaled by 0.948MP2/6-31++G**

cyclic-type

the alkyl proton migrates !

118 nm photo-ionization detection

118 nm photoionization

two possible mechanisms

photo-ionization

proton-relay

catch & release

Proton migration mechanism

enol form with the proton migration

cation

keto form

VUV photoionzation

proton-migration induced by photoionization

neutral

DD

DD

neutral acetone-D2O

acetone-D2O cation

2600 2800 3000 3200 3400 3600 3800Wavenumber / cm-1

2800 3000 3200 3400 3600 3800Wavenumber / cm-1

free OD

CH

H-bonded OH

no free OH

proton-migration

catch & release

IR spectra of neutral and cationic acetone-D2O clusters

catch & release

Proton-migration induced by photoionization

Proton-migration in ionization process

Calculated isomerization path in photoionization with the GRRM methodPBE1PBE/6-311+G(2d,2p)

catalytic action of the water molecule

enol Ienol II

Intracluster proton-transfer from CH occurs upon the photoionization.

IR spectroscopy based on VUV photoionization detection and reaction route search with the GRRM method were carried out to study VUV one-photon ionization dynamics of the acetone-water cluster.

enol type cationneutral proton-migration

Summary

catch & release

VUV

The water molecule acts as the “catch & release” catalyst in the proton-migration.

acetone-water

ionization

apparatus

pulsedvalve

rare gascell

tim e of flightm ass spectrom eter

MCPdetectorVUV

IR (DFG)

IR (OPO)1st. Q-m ass 2nd. Q-m ass

octopoleion guide

channeltrondetector

rare gascell

VUV

fragment ion

Ion

Inte

nsity

IR frequency

parent ion

Ion

Inte

nsity

spectroscopy of neutral and cations

spectroscopy of cations

Setup ITOF & DFG

Setup IITandem Q-mass & IR-OPO

dip spectrum

enhancement spectrum

IR frequency

Mass spectrum of photoionization @118 nm

12 13 14 15 TOF / s

Mass spectrum of acetone-H2O @118 nm

(acetone-H2O)+

(acetone)+

no higher clusters

2600 2800 3000 3200 3400 3600Wavenumber / cm-1 Scaled by 0.948 MP2/6-31++G**

1 3

free OH

CHx10

x10

x10

H-bonded OH

CH H-bonded OH

CH

CH

CH

13

1 3

13

H-bonded OH

absorption

(a)

(b)

(c)

(d)

(e)

(a) H+-migrated 0 kcal/mol

(b) H+-migrated II 0.7 kcal/mol

(c) Charge-shared I　　 26.4 kcal/mol

(d) Charge-shared II　　 29.2 kcal/mol

(e) H-transferred　　 33.2 kcal/mol

IR spectrum of acetone-water cluster cation

IRPDS-VUV-PI@118 nm