Electron affinities of the lanthanides

The electron affinities (EA's) of the lanthanides (La through Tm) have been determined from the expression EA = IP₁ ? C)r^?1)_n1, where IP₁ is the first ionization potential and (r^?1)_n1 is the relativistic radial integral of an electron in the unfilled shell, the constant C includes the quantum numbers n, 1 of the partly filled shells and the atomic number Z of each element. The EA's vary from +0.5 eV (La) to –0.2eV (Tm) which is consistent with other semi-empirical estimates for certain lanthanide elements.     

Electron affinities of the lanthanides

The electron affinities of the lanthanides (La through Lu) are estimated by considering the energy variations associated with changes in the 4f orbital population. The ground-state electron affinities are all predicted to be within the range +0.5 eV to ?0.3 eV.     

Intrinsic lifetimes of the excited state of DNA and RNA bases

The lifetimes of the excited state of free nucleobases were measured in the gas phase for the first time. They are, respectively, 1.0 and 0.8 ps for the purine bases adenine (shown above) and guanine and 3.2, 2.4, and 6.4 ps for the pyrimidine bases cytosine, uracil, and thymine at 267 nm. The longer lifetimes of the pyrimidine bases may be associated with their higher propensity toward photodegradation, especially in the case of thymine. The ultrashort lifetime of nucleobases conventionally known in solution was found to be an intrinsic molecular property due to extremely facile internal conversion, and therefore the lifetime should be largely independent of the medium at this energy, that is, whether in vacuo, in solution, or in vivo. The evolutionary selection of nucleobases as the durable carriers of genetic information is suggested to be due to their inherent immunity from photochemical reactions.     

On the aromatic character of the heterocyclic bases of DNA and RNA

Studies based on ab initio optimized geometries (at B3LYP/6-311+G** and MP2/6-311+G** levels) and on experimental structures retrieved from the Cambridge Structural Database (CSD) reveal that the nucleobases constituting DNA and RNA differ significantly in their aromatic character, as shown by the geometry-based index of aromaticity HOMA that ranges from 0.466 for thymine to 0.917 for adenine, based on B3LYP/6-311+G** calculations, and 0.495-0.926, respectively, if based on the MP2/6-311+G** level. Aromaticity of the bases decreases markedly with an increase of the number of double-bond C=X (X = N, O) substituents at the rings. H-bonds involving C=O groups in Watson-Crick pairs cause an increase of the aromatic character of the rings.     

Proton affinities of N-heterocyclic carbene super bases

The gas-phase proton affinity of the N-heterocyclic carbene, 1-ethyl-3-methylimidazol-2-ylidene, was determined to be 251.3 +/- 4 kcal/mol using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far. Density functional theory calculations have been carried out at the B3LYP/6-31+G(d) level to compare the high experimental value with that estimated theoretically. Experimental results also show that two other N-heterocyclic carbenes with larger substituents have even higher proton affinities. [structure: see text]     

Theoretical determination of electron affinity and ionization potential of DNA and RNA bases

The ionization potentials and electron affinities of thymine, cytosine, adenine, guanine, and uracil were determined at density functional level using different exchange‐correlation functionals and basis sets. Results showed that the computed ionization potentials are very close to the experimental counterparts. The sign of adiabatic electron affinities of adenine, thymine, and uracil is unaffected by the used level of theory while that for guanine and cytosine depends on both the used potential and basis set. Vertical electron affinities are always negative in agreement with the experimental indications. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1243–1250, 2000     

Effects of alkylation upon the proton affinities of nitrogen and oxygen bases

The protonation energies of alkylated derivatives of NH₃ and OH₂ are calculated at the Hartree–Fock level with the split-valence 4-31G basis set. The methyl, dimethyl, and ethyl amines are studied; oxygen bases include methanol, dimethylether, and ethanol. The geometries of each molecule and its protonated analog are fully optimized. It is found that protonation leads to significant changes in the molecular structures. In particular, the bonds to the N and O atoms are substantially elongated, especially when the other atom involved is C rather than H. The calculated absolute proton affinities are somewhat larger than the experimental values. However, the differences in protonation energies of the various molecules relative to one another agree quantitatively with experiment. Replacement of one H atom of the base by a methyl group induces an increase in proton affinity of some 10 kcal/mol. If a second methyl group is added to the N or O atom, a further increment of about 70% this amount is noted. On the other hand, placement of the second C atom on the first methyl group (to form an ethyl substituent) leads to a smaller increase (～30%). The magnitudes of these alkyl substituent effects are somewhat larger for the oxygen bases than for the amines.     

Electron propagator calculations of molecular electron affinities

An approximate electron propagator method for predictive calculations of molecular electron affinities is proposed. The self-energy accounts for relaxation effects to all orders Additional correlation effects are treated using a diagonal approximation with shifted denominators. Applications to CN, NH₂, and PH₂ are reported.     

Electron attachment to DNA and RNA nucleobases: An EOMCC investigation

We report a benchmark theoretical investigation of both vertical and adiabatic electron affinities of DNA and RNA nucleobases: adenine, guanine, cytosine, thymine, and uracil using equation of motion coupled cluster method. The vertical electron affinity (VEA) values of the first five states of the DNA and RNA nucleobases are computed. It is observed that the first electron attached state is energetically accessible in gas phase. Furthermore, an analysis of the natural orbitals exhibits that the first electron attached states of uracil and thymine are valence‐bound in nature and undergo significant structural changes on attachment of an extra electron, which reflects in the deviation of the adiabatic electron affinity (AEA) than that of the vertical ones. Conversely, the first electron attached states of cytosine, adenine, and guanine are in the category of dipole‐bound anions. Their structure, by and large, remain unaffected on attachment of an extra electron, which is evident from the observed small difference between the AEA and VEA values. VEA and AEA values of all the DNA and RNA nucleobases are found to be negative, which implies that the first electron attached states are not stable rather quasi bound. The results of all previous theoretical calculations are out of track and shows large deviation with respect to the experimentally measured values, whereas, our results are found to be in good agreement. Therefore, our computed values can be used as a reliable standard to calibrate new theoretical methods. © 2015 Wiley Periodicals, Inc.     

Gas phase interaction of zinc ion with purine and pyrimidine DNA and RNA bases

The interaction of uracil, thymine, cytosine, adenine, and guanine with zinc ion was studied at the density functional B3LYP/6‐311+G(2df,2p) level. Different binding sites allowing both mono‐metal and bi‐metal coordination were considered for the different low‐lying tautomers of nucleic acid bases. Zinc ion forms stable compounds with all nucleobases. Except for cytosine, mono‐coordination appears to be less favored than bi‐coordination in the other pyrimidines. Instead, the preferred sites in the case of adenine and guanine were found to be the N7 and O6 and N7 and N6 pairs of atoms, respectively. Zinc ion affinity was evaluated for all the complexes and compared with values previously obtained for other transition metal ions. In the present case, the following order of metal ion affinity (MIA) was found: G>A>C>T>U. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Low-energy electron scattering with the purine bases of DNA/RNA using the R-matrix method

R-matrix calculations on electron collisions with the purine bases found in DNA and RNA (i.e., adenine and guanine) are presented. Resonant anion states of these systems are identified by employing different approximation levels of ab initio theoretical methods, such as the static exchange, the static exchange plus polarization, and the close-coupling methods. The results are compared with other available calculations and experiments. All of these ab initio approximations, which we refer to as a scattering "model," give four shape resonances of (2)A' (π) symmetry within the energy range of 10 eV for both molecules. For adenine, the most sophisticated method, the close-coupling model, gives two very narrow (2)A' (σ) symmetry Feshbach-type resonances at energies above 5 eV. Quantitative results for the total elastic and electronic excitation cross sections are also presented.     

Low-energy electron scattering from DNA and RNA bases: shape resonances and radiation damage

Calculations are carried out to determine elastic-scattering cross sections and resonance energies for low-energy electron impact on uracil and on each of the DNA bases (thymine, cytosine, adenine, and guanine), for isolated molecules in their equilibrium geometry. Our calculations are compared with the available theory and experiment. We also attempt to correlate this information with experimental dissociation patterns through an analysis of the temporary anion structures that are formed by electron capture in shape resonances.     

Open-shell coupled-cluster method: Electron affinities of Li and Na

The open-shell coupled-cluster method and the diagrams needed for its implementation are described. The method is applied to the electron affinities of Li and Na, which are calculated in two ways: as the ionization potential of the anions or as the energy of adding the second electron to the cations. The two schemes give essentially the same results, in very good agreement (<0.02 eV) with experiment. Three-body effects are negligible.     

On the applicability of resonance forms in pyrimidinic bases. II. QTAIM interpretation of the sequence of protonation affinities

The atomic properties of neutral and protonated forms of uracil and some model compounds, computed from B3LYP/6-31++G//B3LYP/6-31G charge densities with the QTAIM theory, indicate that sigma electron reorganization plays a significant role in the protonation processes. This reorganization is substantially different for O=C-C=C and O=C-C-X (X = N, O) units, involving transfers of electron population between all atoms in the first case but not across the C-X bond in the second unit. O-Protonation is basically favored over the N-protonation because of the lower electron population transferred to the proton. The stability sequence of N-protonated forms can be rationalized in terms of the closer position of the proton, when attached to N3, to regions of larger electron population (carbonyl groups).     

Electron attachment to the DNA bases adenine and guanine and dehydrogenation of their anionic derivatives: Density functional study

Density functional theory (DFT) calculations have been used to explore electron attachment to the purines adenine and guanine and their hydrogen atom loss. Calculations show that the dehydrogenation at the N9 site in the adenine and guanine transient anions is the lowest‐cost channel of hydrogen loss, and the N9? H bond scission has Gibbs free energies of dissociation ΔG° of 8.8 kcal mol^?1 for the anionic adenine and 13.9 kcal mol^?1 for the anionic guanine. The relatively high feasibility of low‐energy electron (LEE)‐induced N9? H bond cleavage in the purine nucleobases arises from high electron affinities of their H‐deleted counterparts. Unlike adenine, other N? H bond dissociations are competitive with the N9? H bond fission in the anionic guanine. The replacement of hydrogen in the ring of purine has a significant effect on the N9? H bond fragmentation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Electron attachment and detachment: electron affinities of isomers of trifluoromethylbenzonitrile

Rate constants for electron attachment to the three isomers of trifluoromethylbenzonitrile [(CF(3))(CN)C(6)H(4), or TFMBN] were measured over the temperature range of 303-463 K in a 133-Pa He buffer gas, using a flowing-afterglow Langmuir-probe apparatus. At 303 K, the measured attachment rate constants are 9.0 x 10(-8) (o-TFMBN), 5.5 x 10(-8) (m-TFMBN), and 8.9 x 10(-8) cm(3) s(-1) (p-TFMBN), estimated accurate to +/-25%. The attachment process formed only the parent anion in all three cases. Thermal electron detachment was observed for all three anion isomers, and rate constants for this reverse process were also measured. From the attachment and detachment results, the electron affinities of the three isomers of TFMBN were determined to be 0.70(o-TFMBN), 0.67(m-TFMBN), and 0.83 eV (p-TFMBN), all +/-0.05 eV. G3(MP2) [Gaussian-3 calculations with reduced M?ller-Plesset orders (MP2)] calculations were carried out for the neutrals and anions. Electron affinities derived from these calculations are in good agreement with the experimental values.     

Electron affinities of uracil: microsolvation effects and polarizable continuum model

We present adiabatic electron affinities (AEAs) and the vertical detachment energies (VDEs) of the uracil molecule interacting with one to five water molecules. Credibility of MP2 and DFT/B3LYP calculations is supported by comparison with available benchmark CCSD(T) data. AEAs and VDEs obtained by MP2 and DFT/B3LYP methods copy trends of benchmark CCSD(T) results for the free uracil and uracil-water complexes in the gas phase being by 0.20 - 0.28 eV higher than CCSD(T) values depending on the particular structure of the complex. AEAs and VDEs from MP2 are underestimated by 0.09-0.15 eV. For the free uracil and uracil-(H(2)O)(n) (n = 1,2,3,5) complexes, we also consider the polarizable continuum model (PCM) and discuss the importance of the microsolvation when combined with PCM. AEAs and VDEs of uracil and uracil-water complexes enhance rapidly with increasing relative dielectric constant (ε) of the solvent. Highest AEAs and VDEs of the U(H(2)O)(5) complexes from B3LYP with ε = 78.4 are 2.03 and 2.81 eV, respectively, utilizing the correction from CCSD(T). Specific structural features of the microsolvated uracil-(H(2)O)(n) complexes and their anions are preserved also upon considering PCM in calculations of AEAs and VDEs.     

Gas-phase theoretical prediction of the metal affinity of copper(I) ion for DNA and RNA bases

The most stable tautomeric forms of free DNA and RNA bases were considered as substrates for the interaction of Cu(+) ion. Several suitable attachment sites were selected that involved mono- and bi-coordination of the cation. B3LYP/6-311 + G(2df,2p) bond energies showed that copper ion has the major affinity for guanine and cytosine bases. The proposed values of Cu(+) ion affinity are 59.9, 60.0, 80.2, 88.0 and 69.0 kcal mol(-1) for uracil, thymine, cytosine, guanine and adenine, respectively. The preference for the mono- or bi-coordination depends on the particular tautomer for each base.