soon to begin …

soon to begin …

Michael Probst

Sassari, September 27, 2007

Electron – Molecule Reactions:Quantum Chemistry of Electron Attachment to Biomolecules

Institute of Ion Physics and Applied Physics, Innsbruck University, Technikerstraße 25, 6020 Innsbruck, Austria

Collaboration with:

Natcha Injan, Jumras Limtrakul

Stephan Denifl, Fabio Zappa, Ingo Mähr, Manuel Beikircher,Sylwia Ptasinska,Tilmann Märk and Paul Scheier

Aim:Application and understanding of electron-driven processes

Electron – Molecule Reactions:

electron-driven processes ...

... are dominant in many areas of basic and applied Science and Technology.

for example (1) ... Atmospheric

physics and planetary atmospheres

Radiation damage of DNA and cellular material

for example (2) - related to... Astrophysics -

reactions in space

Radiation treatment

for example (3) ...

Semiconductorplasmas

Nanotechnologyand surface engineering

Our topic is related to the 2nd...

Radiation damage of DNA and cellular material

somewhat

e

e

... but what does really happen ?

e

e

...but what does really happen ?

oven

hemisphericalelectron monochromator

quadrupolemass filter

channeltron SEM

Experimental setup ...

Experimental result:

Typical mechanism

An electron is captured in a dipole – bound state.

It enters an antibonding * orbital to form a metastable anion that can weaken a N-H (or C-H) bond.

N1N1C6C6

Suggested mechanism

An electron is captured in a dipole – bound state.

It enters an antibonding * orbital to form a metastable anion that can weaken a N-H (or C-H) bond.

H can dissociate and [M-H]- remains.

Example: Adenine

What we worked on …

2. Neutral and anionic energy surface –where do they cross ?

3. Why do similar molecules show different spectra ?

1. Which H dissociates most easily ?

(calculations of BDE)

(calculation of stable and metastable potential energy curves)

(analysis of molecular orbitals)

1. Which H dissociates most easily ?

BDE System

of H from neutral anion

C2 4.74 3.63

N6 4.69 1.72

C8 5.06 2.53

N9 4.38 0.94

QC calculations (G2(MP2) on adenine):

BDE System

of H from neutral anion

C2 4.74 3.63

N6 4.69 1.72

C8 5.06 2.53

N9 4.38 0.94

Extrapolation methods for BDE:

Low qual. methodLarge basis set

Low qual. methodSmall basis set

High qual. methodLarge basis set

High qual. methodSmall basis set

Basis setenergy

correction

Correlation energy correction

(LL-LS)+ (HL-LS)

Extrapolation methods - G2:

  6-11G(d,p) 6-11+G(d,p) 6-11G(2df,p) 6-11+G(3df,2p)

MP2 I K L D

MP4 J B C  

QCISD(T) A     X

We want to arrive at X:E[+] = E[B] - E[J]; E[2df] = E[C] - E[J]

12= (E[D] - E[I]) - (E[K] - E[I]) - (E[L] - E[I]) ==E[D] + E[I] - E[K] - E[L]

E[X] = E[A] + E[+] + E[2df] + 12

average error in ∆Hf = ±1.59 kcal/mol = 0.06 eV

DBS

(A-H)-+H

*(N(9)-H)

01

EA(A-H)-

r(N(9)-H) (Å)

Ep

ot (

eV

)

EA(P-H)-

Mechanism of dissociation:Potential energy curves:

• Neutral curve:

YES

• Stable part of the anionic curve: (excited state but below neutral curve)

• Metastable part of the anionic curve (above neutral):

• Avoided crossing: (In principle easy, accuracy is difficult …)

Can we calculate these curves ?

YES

YES

NOWith extrapolation methods for the ‘metastable energy’

YES

X

Mechanism of dissociation:Potential energy curves:

Mechanism of dissociation:Potential energy curves:

In principle, the probability of

M + e- [M-H]- + H

can be calculated from these curves !

(via tunneling rates; the accuracy is a problem)

Mechanism of dissociation:Potential energy curves:

[A-H]- … H potential energy:

Eanion = Eneutral + Eelectron

Eneutral are calculated from UB3LYP/aug-cc-pVTZ

Eelectron are calculated from UOVGF/aug-cc-pVTZ

[A-H] … H:

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

ESOMO = f(rN9-H)

UOVGF/aug-cc-pVTZ

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

ESOMO = f(rN9-H)

Metastable part

UOVGF/aug-cc-pVTZ

-8

-6

-4

-2

0

2

4

0 1 2 3 4 5

En

erg

y in

eV

%q

r[N9-H] = 1.3 Å

• Stabilise the anion by slightly increasing the nuclear charge.• extrapolate.

ESOMO = f(rN9-H)

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV -8

-6

-4

-2

0

2

4

0 1 2 3 4 5

En

erg

y in

eV

%q

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

-6

-4

-2

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

ESOMO = f(rN9-H)

+

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

0

2

4

6

0 1 2 3 4

r[N9-H]

En

erg

y in

eV

[A-H] … H neutral and anionic curves:

Why do similar purine derivatives show different spectra ?

LUMO & Electrostatic potential:

ESP: negative, positive

Purine Adenine Dimethyladenine

Concluding …

Some (but not all) features of the DEA process can be predicted.

Ssummary of experimental and theoretical work published in Angew. Chemie IE 46, p.5238 (2007)

Thank you …Thank you …