new insight into the h2cn radical and its isomers from … · 2015-03-30 · new insight into the...

New insight into the H 2 CN radical and its isomers from photoionization experiments Fabian Holzmeier , a) Melanie Lang, a) Patrick Hemberger, b) Ingo Fischer a) a) Institute of Physical and Theoretical Chemistry, University of Würzburg, Germany b) Molecular Dynamics Group, Paul Scherrer Institut (PSI), Villigen, Switzerland 1. Motivation Nitrogen-containing radicals, like methylene amidogen H 2 CN, play an important role in the combustion of biofuels and reactions in the interstellar space. For H 2 CN there are four stable isomers: H 2 CN, cis-HCNH, trans-HCNH, and H 2 NC. We investigated these species by threshold photoelectron spectroscopy (TPES) and gained important information for further experiments (e.g. online monitoring of radical con- centrations in flames 1 ). 1 Detection of reactive radicals in combustion chemistry (Talk on Friday 10:40 a.m. by Tina Kasper) 2. Experiment 3. Potential energy surface of H 2 CN 4. Mass-selected TPE-spectrum and Franck-Condon-simulations 6. Summary Level of theory: MP2/cc-pVTZ 4 stable isomers for both neutral and cationic triplet species ● only one stable isomer on cationic singlet energy surface ● C 2v -symmetric transistion state on singlet surface Acknowledgment Financial support: Deutsche Forschungsgemeinschaft (FI 575/7-2) Swiss Federal Office for Energy (101969/152433) Computations were performed at the Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften (LRZ Munich). ● Threshold photoelectron spectrum originates from contributions of all four H 2 CN isomers ● Franck-Condon simulations help assign the structures to the various isomers ● Excitation energies for transitions to the cationic triplet surface could be determined ● For trans-HCNH and cis-HCNH the assignment might be off by one vibrational mode 5. H 2 CN + singlet activated complex ● feature at 10.43 eV so far unexplained ● D. Neumark: transition states of neutral reactions can be observed by PES ● assignment to cationic activated complex of singlet-H 2 CN + ● computations confirm this assignment Activated complex belongs to an isomerization of lin-HCNH + : C N H H C N H H C N H H ● generation of the H 2 CN radical and its isomers by flash pyrolysis from methylhydrazine ● for all isomers excitation energies for transitions from the neutral molecules to the triplet cation were determined by mass-selected threshold photoelectron spectroscopy ● Observation of a transition to the [H 2 CN + ] ‡ activated complex on the singlet potential energy surface Pyrolysis source for radical generation carrier gas (Ar) electrically heated SiC-tube faceplate with 150 µm pinhole iPEPICO (imaging Photoelectron Photoion Coincidence) H 2 CN H 2 CN + +e - VUV synchrotron radiation precursor: methyl hydrazine N N H H CH 3 H Swiss Light Source + TOF mass spectrum electron image mass-selected threshold photoelctron spectrum

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Page 1: New insight into the H2CN radical and its isomers from … · 2015-03-30 · New insight into the H2CN radical and its isomers from photoionization experiments Fabian Holzmeier,a)

New insight into the H2CN radical and its isomers from photoionization experiments

Fabian Holzmeier,a) Melanie Lang,a) Patrick Hemberger,b) Ingo Fischer a)

a) Institute of Physical and Theoretical Chemistry, University of Würzburg, Germany b) Molecular Dynamics Group, Paul Scherrer Institut (PSI), Villigen, Switzerland

1. MotivationNitrogen-containing radicals, like methylene amidogen H2CN, play an important role in the combustion of biofuels and reactions in the interstellar space.For H2CN there are four stable isomers: H2CN, cis-HCNH, trans-HCNH, and H2NC.We investigated these species by threshold photoelectron spectroscopy (TPES) and gained important information for further experiments (e.g. online monitoring of radical con-centrations in flames1).

1 Detection of reactive radicals in combustion chemistry (Talk on Friday 10:40 a.m. by Tina Kasper)

2. Experiment

3. Potential energy surface of H2CN

4. Mass-selected TPE-spectrum and Franck-Condon-simulations

6. Summary

Level of theory: MP2/cc-pVTZ 4 stable isomers for both neutral and cationic triplet species● only one stable isomer on cationic singlet energy surface● C2v-symmetric transistion state on singlet surface

AcknowledgmentFinancial support: Deutsche Forschungsgemeinschaft (FI 575/7-2)

Swiss Federal Office for Energy (101969/152433)

Computations were performed at the Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften (LRZ Munich).

●Threshold photoelectron spectrum originates from contributions of all four H2CN isomers

●Franck-Condon simulations help assign the structures to the various isomers

●Excitation energies for transitions to the cat-ionic triplet surface could be determined

●For trans-HCNH and cis-HCNH the assignment might be off by one vibrational mode

5. H2CN+ singlet activated complex● feature at 10.43 eV so far unexplained●D. Neumark: transition states of neutral reactions can be observed by PES

●assignment to cationic activated complex of singlet-H2CN+

●computations confirm this assignment

Activated complex belongs to an isomerization of lin-HCNH+:

C N HH C NH

HC N HH

●generation of the H2CN radical and its isomers by flash pyrolysis from methylhydrazine

● for all isomers excitation energies for transitions from the neutral molecules to the triplet cation were determined by mass-selected threshold photoelectron spectroscopy

●Observation of a transition to the [H2CN+]‡ activated complex on the singlet potential energy surface

Pyrolysis source for radical generation

carrier gas (Ar)

electrically heatedSiC-tube

faceplate with 150 µm pinhole

iPEPICO (imaging Photoelectron Photoion Coincidence)

H2CN H2CN+ + e-

VUV synchrotronradiation

precursor: methyl hydrazine N NH

H CH3

Swiss Light Source

TOF mass spectrum electron image