Download A Study Proposal for the Creation of Amino Acids in Enantiomeric

Theoretical Astrochemistry at Virginia Tech
A Study Proposal for the
Creation of Amino Acids in
Enantiomeric Excess from
Exposure to UV MagnetoChiral Radiation on
Interstellar Ice Analogues
Ryan Fortenberry
Theoretical Astrochemistry at Virginia Tech
Introduction
•
The chemistry of life is
dominated by the exclusive
utilization of L-amino acids
and D-sugars.
•
What caused this and where
did it first begin?
•
Early Earth from the UreyMiller Experiment.
•
Synthesis in the interstellar
medium (ISM).
http://bill.srnr.arizona.edu/
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Introduction
•
Murchison Meteorite had ees for chiral molecules
(including amino acids) at the 7-9% range.
•
Interstellar explanations include:
•
•
Circularly Polarized Light (CPL)
•
Magneto-Chiral Dichroism
(MChD)
Polarized light and a spinning
meteor
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http://commons.wikipedia.com
Theoretical Astrochemistry at Virginia Tech
Introduction
•
Amino acids have been created from exposure of
“ices” (which include H2O, CH3OH, NH3, CO2, and
similar) to UV light in simulated interstellar conditions.
•
CPL has been used in aqueous environments to create
chiral amino acids from similar starting materials.
•
MChD has been used to create a small ee in the Cr(III)
tris-oxalato complex.
Munoz Caro, G. M.; Meierhenrich, U. J.; Schutte, W. A.; Barbier, B.; Segovia, A. A.; Rosenbauer, H.; Thiemann,
W. H.-P.; Brack, A.; Greenburg, J. M. Nature. 2002, 416, 403-406.
Kawaski, T.; Sato, M.; Ishiguro, S.; Saito, T.; Morishita,Y.; Sato, I.; Nishino, H.; Inoue,Y.; Soai, K. J. Am. Chem. Soc.
Comm. 2005, 126, 3274-3273.
Rikken, G. L. J. A.; Raupach, E. Nature. 2000, 405, 932-935.
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What I Am Proposing
Interstellar UV
Creation of
Synthesis of +
ees by MChD
Amino Acids
Interstellar UV
Synthesis of Amino
Acids with ees
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Circular Dichroism
π �L
�R
ψ ≈ (n − n )
λ
Circular dichroism: natural circular dichroism
(NCD) or magnetic circular dichroism (MCD).
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Magneto-Chiral Dichroism
•
MCD of the absorption factors creates a circular
component in the light beam, and the magnetic field
splits the energy levels for the left- and right-absorbing
enantiomers.
•
•
NCD may then take place.
The light and magnetic field must be either parallel or
anti-parallel with one another for MChD to occur.
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MChD and Asymmetry
B
k
S-Serine
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R-Serine
Theoretical Astrochemistry at Virginia Tech
Magneto-Chiral Dichroism
•
Magneto-chiral dichroism (MChD) is governed by the
equation:
π �↑↑
ψ ≈ (n − n�↑↓ )
λ
•
The ee of a system is predicted to be related to the
absorption indices in the following way:
n�↑↑ − n�↑↓
ee = 2 · �↑↑
n + n�↑↓
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Experimental Setup
• H O, CO , CO, NH ,
CH NH , and
2
Cr
Movable
Bellows
p
m
Pu
t
sta
yo
3
Chamber
Fill Gas
MgF2 Plate
N
oz
zle
Polarimeter
•
•
B
Fill Line
k
Polarized
Light Source
UV
2
•
2
3
CH3OH will be
dispensed from the
fill line and nozzle.
Chamber T & P will
be 10−10 bar and 10K.
118-170 nm UV light
will be shone for 24
hours.
7.5 T magnetic field
introduced collinear
with the light beam. 10
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Experimental Details
•
•
•
•
•
Controls will be tested:
Check polarization of deposited film.
No magnetic field used in order to recreate Munoz Caro
and coworkers’ results.
ees will be measured with a chiral column GC-MS for gross
measurements, but fine measurements will be done by
examination of the polarization over a period of time
immediately after exposure.
The presence of an ee will show a success. Further analysis
could be done with a change in magnetic field strength,
multiprocessing, or studying simpler systems.
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Questions on ORP?
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And Now. . . a Segue
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•
•
•
C2H and C4H
Coupled cluster theory is the “gold standard” of quantum
chemistry.
EOM-CCSD cannot adequately treat double excitations.
Triples (even if approximate) are essential.
Our group has implemented the first triples
including method for excited states
of open-shell molecules: CC3.
•
•
Test CC3 for these radicals of astrochemical importance and
novel yet difficult quantum chemical features.
The remaining difficulty is the question of spin-contamination.
H. Koch, O. Christiansen, P. Jørgensen, A. M. S. de Meràs, and T. Helgaker, J. Chem. Phys. 106, 1808 (1997).
C. E. Smith, R. A. King, and T. D. Crawford, J. Chem. Phys. 122, 054110 (2005).
T. J. Mach, R. A. King, T. D. Crawford, J. Phys. Chem. A. (submitted).
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Excited States
•
Computed UHF- and ROHF-CCSD excited states
using the EOM approach with aug-cc-pVDZ/pVTZ
basis sets for Σ+(A1), Σ-(A2), ∆(A1/A2), and Π(B1/B2)
states.
Purpose of the work:
andfor
ROHF-CC3/aug-cc-pVDZ
To •
testComputed
coupledUHFcluster
vertical excitation energies.
excited states of the same symmetry as CCSD.
•
Summary of states:
C2H: 16 states up to 10.0 eV.
C4H: 13 states up to 8.0 eV.
•
•
R. C. Fortenberry, R. A. King, J. F. Stanton, and T. D. Crawford, J. Chem. Phys. (accepted).
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C2H aug-cc-pVDZ Results
State
CCSD
UHF ROHF
CC3
CCSD
MRCI*
UHF ROHF AEL
1 2Π
1 4Σ+
1 4Δ
2 2Σ+
2 2Π
1 4Π
3 2Σ+
0.997
5.476
6.570
8.354
8.410
8.852
9.334
0.826
5.306
6.382
7.555
8.299
8.625
8.660
Energies in eV
0.798
5.381
6.498
8.067
8.444
8.810
9.137
0.768
5.235
6.318
7.334
8.315
8.519
8.525
1.08
1.14
1.13
1.54
1.07
1.12
1.45
0.60
4.84
5.98
6.73
7.29
6.59
8.11
* A. G. Koures and L. B. Harding, J. Phys. Chem. 95, 1035 (1991).
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C2H
2
Ŝ
Expectation Values
State
CCSD
UHF
ROHF
1 2Π
0.786
0.754
1 4Σ+
3.478
3.509
1 4Δ
3.403
3.484
2 2Σ+
1.064
0.936
2 2Π
2.095
2.213
1 4Π
2.214
2.434
3 2Σ+
1.028
0.844
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Radical Chains Conclusions
• For our test molecules, CCSD is not adequate for
describing the excited states since the role of double
excitations can be significant for even a qualitatively
correct description of the transition.
•
Purpose of the
work:
Spin contamination
is substantial
in many of
states, precluding
assignments
in
To testthe
coupled
cluster fordefinitive
verticalstate
excitation
energies.
some cases.
•
CC3 rectifies some of the problems for excited states
radicals. Triples are essential for such
computations.
R. C. Fortenberry, R. A. King, J. F. Stanton, and T. D. Crawford, J. Chem. Phys. (accepted).
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442.9 nm Allyl and Friends
• Could the H CC
(n=0,1, 2, . . .)
family be the carriers of the DIBs?
2
(n-3)CHCH2
• Suggested as carriers due to:
• The pi conjugation of the chain as n goes up.
• The chemical equivalence of the end H C
2
groups and pseudo-linearity necessary for
the seen experimental results.
C. D. Ball, M. C. McCarthy, and P. Thaddeus, Astrophys. J. 529, L61 (2000).
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Computational Details
•
•
Optimized structures and calculated subsequent
harmonic vibrational frequencies:
•
•
UHF-CCSD/cc-pVTZ - PSI3.
B3LYP/6-31G* - Gaussian03.
Vertical excitation energies:
•
EOM-CCSD/aug-cc-pVDZ - PSI3.
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H2CC(n-3)CHCH2 Cations
n
State
Energy (nm)
Oscillator
Strength
3
1 B2
223.5
0.3841
3
1 A2
216.7
0.0000
4
1 A”
396.9
0.0000
4
1 A’
232.7
0.5068
5
1 A”
468.8
0.0000
5
1 A’
288.8
0.6550
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EOM-CCSD/aug-cc-pVDZ
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EOM-CCSD Comparison
B3LYP Geom. CCSD Geom.
Energy (nm) Energy (nm)
n (cations)
State
3
1 B2
228.6
223.5
3
1 A2
221.3
216.7
4
1 A”
364.4
396.9
4
1 A’
227.3
232.7
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Potential 442.9 nm Carrier?
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Theoretical Astrochemistry at Virginia Tech
Future Directions
•
Explore spin-restricted methods of C2H and C4H in
conjunction with Peter Szalay.
•
Finish the computations necessary for the silicon
project and compare with the results from McCarthy
and coworkers at the CfA.
•
Explore the promising predictions for the cation
where n=9 with adiabatic computations and more
accurate methods.
•
Continue development of the methods for better
comparison to experimentation and observation.
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Acknowledgements
•
•
•
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Dr. Crawford, the Group, and my Committee
•
Dr. David Magers (MC), Dr. Cliff Fortenberry (MC),
and Mrs. Lauren Fortenberry
Virginia Tech Chemistry Department
VSGC, NASA, NSF, and DOE
Dr. Rollin King (Bethel U. MN), Dr. John Stanton
(Texas), and Drs. McCarthy and Thaddeus (CfA)
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Other Questions?
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Temp.
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