Collisions of Low-Energy Electrons with Formamide
Márcio H. F. Bettega
Departamento de Física
Universidade Federal do Paraná
XXXIII ENFMC–05/12/2010
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 1 / 17
Outline
Outline
Motivation.General,Formamide (HCONH2) and formamide dimer (HCONH2...HCONH2).
Schwinger multichannel method.
Results.
Summary.
Acknowledgments.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 2 / 17
Motivation
Motivation
Electron collisions with molecules of biological relevance (biomolecules).
SSBs and DSBs.
DNA damage - single- and double-strand breaking in DNA iscaused by secondary low-energy (up to 20 eV) electronsgenerated by the ionizing radiation(X-rays, β-rays, γ-rays).B. Boudaïffa, P. Cloutier, D. Hunting, M. A. Huels, and L. Sanche, Science 287, 1658
(2000).
Radiation-induced DNA damage can lead to the cell death orcan initiate the development of cancer cells.
The understanding of the fundamental mechanism of DNAdamage can help in cancer treatment.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 3 / 17
Motivation Motivation
DNA molecule
Deoxyribonucleic Acid (DNA)
National Human Genome Research Institute
A
T
A
S
S
S
S
S
S
P
P
P
P
P
PS
S
S
S
S
S
P
P
P
P
P
P
A
T
G C
C G
G C
T
Hydrogenbonds
Base pairsSugar-
phosphatebackbone
Sugar-phosphatebackbone
Base pair
Nucleotide
National Institutes of Health Division of Intramural Research
Transient negative ions (TNI) - shape,core-excited, or Feshbach resonances:dissociative electron attachment (DEA) →electron-molecule collision problem.
Double-bonds and hydrogen bonds.
DNA subunits, organic molecules, model(small) systems - biomolecules.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 4 / 17
Motivation
Motivation
Types of resonances
Shape resonance (elastic scattering, low-energy - one particle): the continuum electron istrapped by an unoccupied molecular orbital (angular momentum barrier) - π∗(indirect-symmetry breaking) and σ∗ (direct).Indirect mechanism: T. N. Rescigno, C. S. Trevisan, and A. E. Orel, Phys. Rev. Lett. 96, 213201 (2006) – HCOOH + e− →HCOO− + H
Core-excited and Feshbach resonances (electronic excitation - two particles-one hole): themolecule is excited and the continuum electron is trapped by an unoccupied molecular orbital.core-excited (shape) resonance: the energy of the TNI is above energy of the parent (neutral) stateFeshbach resonance: the energy of the TNI is below the energy of the parent state.
E(N) (ground state)
E(N+1)
E(N) (excited − parent − state)
E(N+1)
E(N+1)
E(N) (ground state)
shape resonance core−excited (shape) and Feshbach resonances
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 5 / 17
Formamide and formamide dimer Structures
Structures of formamide and formamide dimer.
Formamide: π∗ shape resonance at 2.05 eV (Seydou et al. – experiment) and at 3.77 eV(Goumans et al. – theory) (recall Josue’s talk).
Formamide dimer: we expect 2 π∗ shape resonances.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 6 / 17
SMC method Introduction
Schwinger multichannel method
K. Takatsuka and V. McKoy, PRA 24, 2473 (1981),K. Takatsuka and V. McKoy, PRA 30, 1734 (1984)
Variational approach for the scattering amplitude;
Formulated for applications to low-energy electron-molecule collisions;
Capable of addressing important aspects of these collisions as:molecular targets of general geometry;exchange interactions (ab initio);effects arising from the polarization of the target by the incident electron (ab initio);electronic excitation (multichannel coupling).
In the present implementation of the SMC method, to represent the core electrons we employthe local-density norm-conserving pseudopotentials of Bachelet, Hamann and Schlüter [PRB26, 4199 (1982)].
M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, PRA 47, 1111 (1993).
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 7 / 17
SMC method Computational Details
Computational Details.
Formamide.
Group: Cs
Basis set: C, N, and O: 5s5p3d; H: 4s/3s1p
Geometry: experimental (http://cccbdb.nist.gov)
Orbtials: improved virtual orbitals (< 27.212 eV)
Singlets and triplets;
CSFs: A′: 6402; A′ ′: 5376.
Dipole moment: 4.28 D (calc.) – 3.72 D (expt.)(we neglected the long range dipole interaction -no effect on the resonance’s location, only in thebackground scattering).
Formamide dimer.
Group: Cs(C2h)
Basis set: C, N, and O: 5s4p2d; H: 4s/3s1p
Geometry: optimized (GAMESS) - MP2aug-cc-pVDZ
Orbtials: improved virtual orbitals (< 20.41 eV)
Singlets and triplets;
CSFs: A′ ′(Bg + Au): 8546;
Bg and Au: partial wave analysis.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 8 / 17
Results Results
Results
Previous studies on electron interactions with formamide.
Seydou et al.: electron attachment to isolated formamide and to formamide clusters by measuring thederivative of the electronic current. In the case of the isolated formamide they found a resonance at 2.05eV.M. Seydou, A. Modelli, B. Lucas, K. Konate, C. Desfrançois, and J. P. Schermann, Eur. Phys. J. D 35, 199 (2005).
Goumans et al.: DEA to formamide (possible mechanisms of DEA upon stretching the C–H, N–H and C–Nbonds). π∗ (A′ ′ symmetry) located at 3.77 eV, and a σ∗ (A′ symmetry) located at 14.9 eV.
Dissociation: HCO + NH−2 (breakage of the C–N bond). The π∗ resonance initiate the dissociationmechanism (as in formic acid).
Mechanism: coupling between π∗ and σ∗ anions (symmetry breaking, as in formic-acid).
T. P. M. Goumans, F. A. Gianturco, F. Sebastianelli, I. Baccarelli, and J. L. Rivail, J. Chem. Theory Comp. 5, 217 (2009).
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 9 / 17
Results Results
Results
Indirect mechanism for formamide.
The electron is captured by a π∗ resonance on the C=Odouble bond. The C–N bond is stretched, and at a givenvalue of R the symmetry is broken (pyramidalization of theNH2). The π∗ anion crosses a σ∗ anion, leading to theC–N bond breakage.
σ∗
Req
E
π∗
R
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 10 / 17
Results Results
Cross sections for formamide.
3 6 9 12 150
60
120
180
SESEP
3 6 9 12 15energy (eV)
0
30
60
90
120
150
cro
ss
se
cti
on
(a
0
2)
3 6 9 12 150
30
60
90
120
150
SESEP
3 6 9 12 15energy (eV)
0
30
60
90
cro
ss
se
cti
on
(a
0
2)
A´
A´´
Cross sections.
π∗ shape resonance in the A′ ′ symmetry at 4.5eV (SE) and at 2.5 eV (SEP). σ∗ resonance at∼ 15 eV (SE).
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 11 / 17
Results Results
LUMO, LUMO+1, LUMO+2
(a)
(c)(d)
(b)Minimal basis set calculations.
Minimal basis set (6-31G(d)) calculations at the MP2(geometry optimization) and at the Hartree-Fock (energy)levels with GAMESS.
We computed the Fock operator’s eigenvalues (ε) to theLUMO (π∗
a′ ′ ), LUMO+1 (σ∗a′ ), LUMO+2 (σ∗
a′ ), andLUMO+5 (σ∗
a′ ) orbitals.
Koopmans’ Theorem: electron affinity =EN − EN+1 = −ε.
vertical electron attachment energy (VAE) = − verticalelectron affinity.
Empirical scaling formula: VAE = a + b εcomputedS. W. Staley and J. T. Strnad, J. Phys. Chem. 98, 116(1994).
After the scaling: LUMO = 2.10 eV, LUMO+5 = 8.0 eV.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 12 / 17
Results Results
Cross sections for formamide dimer.
0 3 6 9 12 15energy (eV)
0
30
60
90
cro
ss s
ecti
on
(a
0
2)
0 3 6 9 12 15energy (eV)
0
30
60
90
cro
ss s
ecti
on
(a
0
2)
Au
Bg
Cross sections (preliminary results).
π∗ shape resonance in the Bg symmetry at 3.7 eV (SE) andat 2.0 eV (SEP) and in the Au symmetry at 4.2 eV (SE) andat 2.6 eV (SEP).
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 13 / 17
Results Results
LUMO and LUMO+1
Minimal basis set calculations.
Minimal basis set (6-31G(d)) calculations at the MP2 (geometryoptimization) and at the Hartree-Fock (energy) levels with GAMESS.
We computed the Fock operator’s eigenvalues (ε) to the LUMO (π∗) andLUMO+1 (π∗) orbitals.
After scaling: LUMO (au) = 1.99 eV, LUMO+1 (bg) = 2.12 eV.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 14 / 17
Summary
Summary
π∗ resonance at 2.5 eV, in agreement with the experimental value of 2.05 eV and with thecomputed LUMO of 2.10 eV (after scaling). This resonance is suggested to initiate thedissociation mechanism of formamide along the C–N bond.
The LUMO+1, LUMO+2, and LUMO+5 plots indicate the possibility of breakage of the N–H,C–H, and C–N bonds.
Formamide dimer: two π∗ shape resonances at 2.0 eV (Bg) and at 2.6 eV (Au). LUMO (au)= 1.99 eV, LUMO+1 (bg) = 2.12 eV (after scaling). Polarization?Poster 141 by Freitas, Varella, Sanchez, and Bettega – formic acid dimer.
Future work: look at the π∗ and σ∗ resonances upon stretching the N–H, C–H, and C–Nbonds.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 15 / 17
Acknowledgments
Acknowledgments
Prof. Carlos de Carvalho for computational support at DFis-UFPR and LCPAD-UFPR, and Profs.Sergio d’A. Sanchez and Marcio Varella for fruitful discussions.
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 16 / 17
VIII WFME
M. H. F. Bettega (UFPR) Collisions with Formamide XXXIII ENFMC–05/12/2010 17 / 17
