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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/ 2 Theoretical Astrochemistry at Virginia Tech 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 3 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. 4 Theoretical Astrochemistry at Virginia Tech What I Am Proposing Interstellar UV Creation of Synthesis of + ees by MChD Amino Acids Interstellar UV Synthesis of Amino Acids with ees 5 Theoretical Astrochemistry at Virginia Tech Circular Dichroism π �L �R ψ ≈ (n − n ) λ Circular dichroism: natural circular dichroism (NCD) or magnetic circular dichroism (MCD). 6 Theoretical Astrochemistry at Virginia Tech 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. 7 Theoretical Astrochemistry at Virginia Tech MChD and Asymmetry B k S-Serine 8 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�↑↓ 9 Theoretical Astrochemistry at Virginia Tech 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 Theoretical Astrochemistry at Virginia Tech 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. 11 Theoretical Astrochemistry at Virginia Tech Questions on ORP? 12 Theoretical Astrochemistry at Virginia Tech And Now. . . a Segue 13 Theoretical Astrochemistry at Virginia Tech • • • 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). 14 Theoretical Astrochemistry at Virginia Tech 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). 15 Theoretical Astrochemistry at Virginia Tech 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). 16 Theoretical Astrochemistry at Virginia Tech 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 17 Theoretical Astrochemistry at Virginia Tech 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). 18 Theoretical Astrochemistry at Virginia Tech 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). 19 Theoretical Astrochemistry at Virginia Tech 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. 20 Theoretical Astrochemistry at Virginia Tech 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 21 EOM-CCSD/aug-cc-pVDZ Theoretical Astrochemistry at Virginia Tech 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 22 Theoretical Astrochemistry at Virginia Tech Potential 442.9 nm Carrier? 23 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. 24 Theoretical Astrochemistry at Virginia Tech Acknowledgements • • • • 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) 25 Theoretical Astrochemistry at Virginia Tech Other Questions? 26 Theoretical Astrochemistry at Virginia Tech Temp. 27