In this work we investigate the ability of the uracil‚water complex to form stable anionic systems. As the experimental evidence and theoretical calculations have indicated, the isolated uracil molecule can only attach an excess electron into a diffuse dipole-bound state, while some recent experiments suggest that the uracil‚ water complex can form a more stable valence-type anion. In this work we demonstrate that it is possible to converge ab initio calculations of uracil‚(H 2 O) 3 -to an equilibrium structure that is significantly different from the structure of the neutral cluster and that has a positive and remarkably significant vertical ionization potential. Apart from the valence anion, the uracil‚(H 2 O) 3 complex can form a stable dipolesbound anion, but as the present calculations indicate the electron affinity, which corresponds to this attachment, is very small (13 meV). The structure of the dipole-bound anion is virtually identical with the structure of the neutral complex.
We present results of ab initio calculations of our study of (H2O)3−. The main conclusions of this work are as follows: the most stable cyclic structure of (H2O)3 has a dipole moment too small to form a dipole-bound state with an excess electron; the dipole-bound anion of the water trimer observed experimentally appears to be a hydrated complex of the water dimer anion, (H2O)2−, by a single water molecule. The water trimer anion, (H2O)3−, has an open shape. The calculated vertical electron detachment energy of this anion is predicted to be equal to 141 meV, which is in good agreement with the experimental value of Bowen and co-workers, equal to 142±7 meV. Although the open optimal geometry of the (H2O)3− anion obtained in the present calculations is an equilibrium structure, its energy is higher than the energy of the cyclic equilibrium structure of the neutral complex, indicating that the anion is a metastable system. Based on calculations, we predict significant differences in the IR vibrational spectra of (H2O)3 and (H2O)3−, which may be used for identification of the two species.
Following the experimental characterization of the N,N-dimethylated uracil anion by Bowen and co-workers, we have undertaken an investigation of the influence of the methylation on the electron affinity of the uracil molecule. Both experiment and theory agree that, as it is in the case of the isolated uracil molecule, the methylated uracils can only attach excess electrons into diffuse dipole-bound states. The corresponding electron affinities are very small (several MeV). The bonding effect in the dipole-bound state depends on the magnitude of the molecular dipole and on the size of the molecule. Selective methylation of the uracil molecule can be used to reduce or increase the dipole value and to change the electron affinity of the molecule. The present calculated results are consistent with the experimental determination that N,N-dimethylation of uracil results in reduction of the electron affinity.
The electronic structure perturbations caused by cyclopentadienyl substitutions in the series of complexes (η5-C5H5)2Ru, (η5-C5Me5)(η5-C5H5)Ru, (η5-C5Me5)2Ru, (η5-C5Me5)(η5-C5Cl5)Ru, and (η5-C5Me5)(η5-C5F5)Ru are measured by gas-phase photoelectron spectroscopy. The shifts of the valence metal- and cyclopentadienyl-based ionizations give an indication of the overall electronic effects of methyl and halogen substitutions on the cyclopentadienyl rings. The halogen substituent interaction is an admixture of inductive σ-electron-withdrawing and filled−filled π-electron-overlap effects, which act in opposite directions. The π-overlap interaction is relatively weak in the case of chlorine substitution for hydrogen, and the combined σ and π interactions give rise to an overall withdrawal of electron density from the metal center and increase in the metal d-based ionization energies. Fluorine substituents on cyclopentadienyl make the ring only slightly more electron withdrawing than η5-C5Cl5, despite the much greater electronegativity of fluorine compared to chlorine. The electron withdrawing ability of η5-C5F5 is tempered by the greater filled−filled interaction of the fluorine pπ orbitals with the cyclopentadienyl pπ orbitals, which lessens the stabilization of these orbitals and the withdrawal of electron density from the metal. It is interesting that in each case the metal d-based ionizations are stabilized more than the cyclopentadienyl π-based ionizations with halogen substitution for hydrogen, such that these ionizations begin to merge in the lowest ionization energy band.
We present results of ab initio calculations of the (H2O)4/(H2O)4− system. The main conclusions of this work are as follows: The calculated results predict that water tetramer anions are metastable systems in agreement with weak spectral manifestation of these systems in gas-phase experiments of Bowen and co-workers; the excess electrons in all four structural isomers of water tetramer anions found in the calculations are attached to the clusters by the virtue of dipole-electron interaction; all four (H2O)4− anions found in the calculations are almost isoenergetic but have different vertical electron detachment energies (VDEs) ranging from 22 to 279 meV; the most stable cyclic structure of (H2O)4 has a null dipole moment and does not form a dipole–bound state with an excess electron; the water tetramer anions observed experimentally probably are formed as a result of hydration of the water dimer anion, (H2O)2−, by a neutral water dimer or by hydration of the water trimer anion, (H2O)3−, by a single water molecule; based on calculations, we predict some specific IR vibrational features for the anions which can be used for identification of these species; vibrational analysis of all four anions found in the calculations indicate that they correspond to minima on the potential-energy surface.
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