No abstract

DNA base-gold interactions are studied theoretically at the DFT level using Au(3) and Au(4) clusters as simple catalytic models for Au particles. The bonding between DNA bases and gold clusters occurs via the anchoring of a Au atom to the N or O atoms of the bases. In the most stable planar base-Au(3) complexes, the Au-N or Au-O anchor bonds are reinforced by N-H...Au bonds. The mechanism of formation of these nonconventional H-bonds is discussed.

In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.

The interaction of 2-hydroxy, 4-hydroxy, and 2,4-dihydroxy tautomers of uracil with a water molecule is studied at the B3LYP/6-31+G(d,p) computational level. Depending on the nature of the tautomers, cyclic or open structures are formed. In most of the cyclic structures, water accepts the OH or NH protons and donates its proton to the O or N atoms of the uracil tautomers. Two anticooperative structures where water acts as a biacceptor are also formed. The intermolecular distances, binding energies, and frequency shifts of the stretching vibrations are rationalized in terms of the deprotonation energies and the proton affinities of the sites involved in complex formation. The relative order of stability of the uracil tautomers, free and complexed with water, is compared. Deprotonation of the O 10 H bond of the 3,4 2 -U tautomer greatly influences the geometry and the binding energies of the water complexes. Complex formation with water also results in a decrease of the deprotonation energy. The concerted double-proton transfer of the tautomerization process U S 1,4 1 -U mediated by a water molecule is also examined. The lowering of the proton-transfer barrier because of excess entropy is discussed.