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TAUTOMERISM: a special form of isomerism, inter molecular proton transfer

TAUTOMERISM: a special form of isomerism, inter molecular proton transfer. Prototype case:. Challenge for theory: completely different electronic structures!. Motivation: an embarrassing DISCREPANCY between theory and experiment about CYTOSINE.

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TAUTOMERISM: a special form of isomerism, inter molecular proton transfer

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  1. TAUTOMERISM: a special form of isomerism, intermolecular proton transfer Prototype case: Challenge for theory: completely different electronic structures!

  2. Motivation: an embarrassing DISCREPANCY between theory and experiment about CYTOSINE

  3. Cytosine: one of the four bases in DNA …..

  4. 1: the “canonical” oxo form; 2b: enol; 3a: imino Theoretical results, e.g.: CCSD(T)[f.c.]/cc-pVTZ// rfg Watch out, black-box users!: DFT gives a qualitatively different picture! 1 2b 3a 1.51 0. 1.49 B3LYP/6-311++G(2d,2p): B3PW91/6-311++G(2d,2p): -0.54 0. 1.27 -0.28 0. 1.74

  5. Compare with experiment: From molecular beam MW and noble-gas matrix IR, no reliable quantitative results, but the picture is: Abundancies (for isolated specii): 2b > 1 >> 3a While theory from above:2b > 1  3a Serious discrepancy about the ‘rare’IMINO form between theory and experiment!!

  6. Section 1. Test calculations Check the reliability of several methods on small molecules. Test calculations on three systems:

  7. Vinylalcohol Acetaldehyde

  8. Acetaldimine Vinylamine

  9. Formamide Formamidic acid

  10. Methods: Electronic theory: RHF, B3LYP, MP2, CCSD(T) Basis sets: from 6-31G(d,p) to 6-311++G(3df, 3pd) and cc-PVTZ to aug-cc-PV5Z

  11. Test 1. Acetaldehyde – Vinylalcohol Tautomer Pair a In calculations with Pople-type basis sets (6-31…) geometries and energies calculated at the same level. b E in atomic units.

  12. Acetaldehyde – Vinylalcohol Tautomer Pair Contnd. C Geometry selected as standard: MP2/aug-PVTZ

  13. Test 2. Acetaldimine - Vinylamine pair, incl. Trans. State _________________________________________ a Energy relative to vinylamine

  14. Acetaldimine - Vinylamine contnd. ______________________________________________________ aEnergy relative to vinylamine. bAt MP2/aug-PVTZ geometry.

  15. Test 3. Formamide – Formamidic acid

  16. Formamide – Formamidic acid, contnd.

  17. CONCLUSIONS ON THE TESTS: _________________________________________________________________ Even the best results are uncertain to ~ 1 kcal/mol

  18. Section 2. The effect of water Model: supermolecule, a water molecule added explicitly

  19. Formamide plus water: water may mediate proton transfer

  20. (Formamide – Formamidic acid) Monohydrate

  21. The special case of formaldehyde-hydrate: b3lyp/631G(d,p) works reasonably, as compared to “best”: 10.1 vs. 9.2 and 19.4 vs. 22.8 1: b3lyp/31Gdp, 2: b3lyp/311G++2d2p, 3: MP2/31Gdp, 4: MP2/aug-PVTZ, 5: MP2/PVQZ, 6: CCSD/aug-PVTZ, 7: CCSD(T)/aug-PVTZ, 8: CCSD/PVQZ, 9: CCSD(T)/PVQZ

  22. Section 3: Dynamics: Try to ‘see’ the process of water-mediated tautomerization Specifically: is there an intermediate state ? Method: ab initio dynamics* Contrary to Car-Parrinello, the wave function is truly recalculated at each point …. * Pulay, P., Fogarasi, G.: Fock matrix dynamics, CPL 386, 272-278 (2004)

  23. About ab initio dynamics The notion of reaction mechanisms is based on the Born-Oppenheimer (B-O) approximation: atoms move on a potential energy surface (PES) defined by the electronic energy as a function of nuclear positions. In the simplest models reactions follow the minimum energy pathway (MEP), going through a transition state (TS). The MEP expressed in mass-weighted Cartesians is referred to as the internal reaction coordinate, IRC. Recent computations have shown that reactions may follow a route totally different from the IRC. (W.L. Hase, Science 2002; M. Dupuis, Science 2003).

  24. True dynamics calculations require knowledge of the complete PES, and recent methods generate it "on the fly". The well-known Car-Parrinello method is most efficient computationally because the electronic wave function is "propagated", and not optimized, at the trajectory points. As a consequence, the system is moving close to, but not exactly on the B-O surface. In B-O dynamics, the wave function of a QC method is fully optimized in each step along the trajectory. Energy and first derivatives are determined from ab initio wf, and atomic movements calculated from them classically. This is the approach adopted here.

  25. Dynamics details: 1 fs time steps, just 200 steps per trajectory. But several hundred trajectories. The QC method: DFT(B3LYP)/6-31G** Call PQS to show dynamics Two different cases: 1391c and 1397c

  26. The End

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