STUDIES ON TAUTOMERISM Energies and dynamics

1. STUDIES ON TAUTOMERISM Energies and dynamics Central European Symposium on Theoretical Chemistry (CEST-2009), Dobogókő, Hungary, September 25-28, 2009. Géza Fogarasi Eötvös University, Institute of Chemistry, Laboratory of Theoretical Chemistry,Pf. 32, Budapest, Hungary, H-1518 Motivation: an embarrassing DISCREPANCY between theory and experiment about CYTOSINE (logo above). Before continuing with that project, test calculations on small prototype molecules are inevitable. Results 2. Explicite DYNAMICScalculations Watermay mediate the proton transfer in formamide Method: ab initio Born-Oppenheimer 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). b a d c Scheme 1. Systems studied: a) acetaldehyde – vinylalcohol; b) acetaldimine – vinylamine; c) formamide – formamidic acid;d) formamidine – formamidine. 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. • Methods • Standard quantum chemistry, from Hartree-Fock to Coupled Cluster, CCSD(T), basis sets from 3-21G to aug-cc-pV5Z. • Ab initio Born-Oppenheimer dynamics, B3LYP and MP2, 6-31G(d,p). • Computer programs used: PQS [1], ACES II [2]. Procedure [3] 1. Start geometry not too far from equilibrium 2. The atomic nuclei get a random kicksuch that the average kinetic energy corresponds to a selected temperature. 3. Energy and FORCES calculated from ab initio Quantum Chemistry ‘on the fly’ 4. Movement of atoms calculated classically 5. GOTO 3 Results 1. Energies of tautomer pairs and transition states. Watch for the role of water(monohydrate). TAUTOMERIZATION CAPTURED! Tautomerization of formamide to formamidic acid by water mediatiion. Six snapshots at time intervals of 5 fs. Method: MP2/6-31G(d,p). CONCLUSION Tautomerization is strongly facilitated by water, as studied in form of monohydrates. The TS barrier is lowered by more than a factor of 2. Explicit dynamics calculations capture the mechanism: tautomerization proceeds through a synchronous, concerted process. References [1]PQS version 2.4, Parallel Quantum Solutions, 2013 Green Acres Road, Fayetteville, AR 72703. Private version from Peter Pulay. [2] ACES II, Mainz-Austin-Budapest version, J.F. Stanton, J. Gauss, J. D. Watts, P. G. Szalay, R. J. Bartlett; with contributions from A.A. Auer, D.B. Bernholdt, O. Christiansen, M.E. Harding, M. Heckert, O. Heun, C. Huber, D. Jonsson, J. Jusélius, W.J. Lauderdale, T. Metzroth, C. Michauk, D.P. O'Neill, D.R. Price, K. Ruud, F. Schiffmann, A. Tajti, M.E. Varner, J. Vázquez and the integral packages MOLECULE (J. Almlöf and P.R. Taylor), PROPS (P.R. Taylor), and ABACUS (T. Helgaker, H.J. Aa. Jensen, P. Jorgensen, J. Olsen), [3] Pulay, P., Fogarasi, G.: Fock matrix dynamics, CPL 386, 272-278 (2004) Acknowledgement.Financial support has been provided by a Hungarian science grant NKTH-OTKA-A07, no. K 68427.