XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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CCSD(T), MP2 and DFT investigations of Electron Affinities of Uracil: Microsolvation and the Polarized Continuum Model.
M. Melichercik1, L. F. Pasteka1, P. Neogrady1 and M. Urban1,2
1Comenius University, Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences,
Mlynska dolina, 842 15 Bratislava, Slovakia.

2Slovak University of Technology in Bratislava, Institute of Materials Science, Faculty of Materials Science and Technology in Trnava, Bottova 25, 917 24 Trnava, Slovakia.
Electron affinities of Nucleic Acid Bases (NABs) are crucial for understanding the mechanism underlying the radiation damage to living tissues by low-energy electrons, which can cause strand breaks in the DNA duplex. There is a clear evidence that the environment contributes to the stability of the valence bound uracil anion significantly [1-4]. Recently, we presented [5] reference CCSD(T) calculations of the adiabatic electron affinities (AEA) and the vertical detachment energies (VDE) of the free and the microhydrated uracil•(H2O)n (n=1-3). The OVOS technique was used to alleviate large computer demands of CCSD(T) calculations [4,5]. Next, we have extended our AEA and VDE studies to DFT-B3LYP and UMP2 calculations of microhydrated uracil•(H2O)n (n=1-5) complexes treated at the same time by a polarized continuum model (PCM) [6]. Due to the microsolvation AEAs and VDEs increase by 600-660 meV, and by more than 1100 meV, respectively, for U(H2O)5 complexes. With increasing dielectric constant in the PCM model AEAs attained the value of 2000 – 2200 meV and VDEs the value of 2700 – 3000 meV. Electron affinities depend on specific structural features of the microsolvated U(H2O)n complexes.
DFT data for AEAs of microsolvated uracil copy trends of benchmark CCSD(T) results being systematically by 200 – 230 meV higher than CCSD(T) values depending on the particular structure of the complex. MP2 values are underestimated. Although DFT-B3LYP electron affinities differ from benchmark CCSD(T) data significantly, all trends are reproduced reasonably accurately. This gives a support for using DFT in investigations of trends in larger molecules and their complexes. More accurate values can be obtained by tuning DFT by benchmark CCSD(T) results.

This work was supported by the Slovak Research and Development Agency APVV (Contract No. LPP-0155-09) and VEGA-1/0520/10.

References:
[1] Kim S., Schaefer H. F., J. Chem. Phys 125, 144305 (2006).
[2] Eustis S., Wang D., Lyapustina S., Bowen K. H. J. Chem. Phys. 127, 224309 (2007).
[3] Brancato G., Rega N., Barone V. Chem. Phys. Lett. 500, 104 (2010).
[4] Dedíková P., Demovič L., Pitoňák M., Neogrády P., Urban, M. Chem. Phys. Lett. 481, 107 (2009).
[5] Dedíková P., Neogrády P., Urban M. J. Phys. Chem. A 115, 2350 (2011).
[6] Cossi M., Barone V., Mennucci B., Tomasi J. Chem. Phys. Lett. 286, 253 (1998); Tomasi J., Mennucci B., Cammi R. Chem. Rev. 105, 2999 (2005)


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