Mercedes Rubio

Ab initio determination of the ionization potentials of DNA and RNA nucleobases.

Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron spectra.

A theoretical study of the electronic spectrum of naphthalene

Abstract Ab initio calculations have been carried out for the singlet and triplet excited states of naphthalene. Excitation energies have been calculated using multiconfigurational second order perturbation theory (CASPT2). The study comprises a total of 32 states, ten singlet and ten triplet excited states, in addition to the 1au→3s, 3p, dipole-allowed 3d, and 2b1u→3s, 3p Rydberg states. Computed excitation energies and oscillator strengths make possible confident assignments of the main features reported in the singlet-singlet and triplet-triplet experimental spectra.

Ab initio determination of the electron affinities of DNA and RNA nucleobases.

High-level quantum-chemical ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute the vertical and adiabatic electron affinities of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. The present results aim for the accurate determination of the intrinsic electron acceptor properties of the isolated nucleic acid bases as described by their electron affinities, establishing an overall set of theoretical reference values at a level not reported before and helping to rule out less reliable theoretical and experimental data and to calibrate theoretical strategies.

Excited states of the water molecule : Analysis of the valence and Rydberg character

The excited states of the water molecule have been analyzed by using the extended quantum-chemical multistate CASPT2 method, namely, MS-CASPT2, in conjunction with large one-electron basis sets of atomic natural orbital type. The study includes 13 singlet and triplet excited states, both valence and 3s-, 3p-, and 3d-members of the Rydberg series converging to the lowest ionization potential and the 3s- and 3p-Rydberg members converging to the second low-lying state of the cation, 1 (2)A(1). The research has been focused on the analysis of the valence or Rydberg character of the low-lying states. The computation of the 1 (1)B(1) state of water at different geometries indicates that it has a predominant 3s-Rydberg character at the equilibrium geometry of the molecule but it becomes progressively a valence state described mainly by the one-electron 1b(1)-->4a(1) promotion, as expected from a textbook of general chemistry, upon elongation of the O-H bonds. The described valence-Rydberg mixing is established to be originated by a molecular orbital (MO) Rydbergization process, as suggested earlier by R. S. Mulliken [Acc. Chem. Res. 9, 7 (1976)]. The same phenomenon occurs also for the 1 (1)A(2) state whereas a more complex behavior has been determined for the 2 (1)A(1) state, where both MO Rydbergization and configurational mixing take place. Similar conclusions have been obtained for the triplet states of the molecule.

Electronic spectra of tetrathiafulvalene and its radical cation: analysis of the performance of the time-dependent DFT approach

Abstract The electronic spectra of tetrathiafulvalene and its radical cation have been studied within the framework of the time-dependent density functional theory by using a conventional hybrid functional. The behaviour of the method has been analyzed through the computed vertical excitation energies for the low-lying electronic excited states. Although the procedure provides a correct description of many of the features of the spectra, deviations in the range 0.4–0.7 eV have been obtained for several transitions, from which one can conclude misleading assignments.

The internal rotational barrier of biphenyl studied with multiconfigurational second-order perturbation theory (CASPT2)

SummaryA detailedab initio study of the molecular structure and rotational barriers of biphenyl has been performed. First, non-dynamical correlation effects involving the π system are taken into account at the CASSCF level. These wave functions are subsequently employed as reference functions in a multiconfigurational second-order perturbation treatment (CASPT2). The performance single-reference approaches is in addition analysed. The molecular geometries of biphenyl in twisted, coplanar, and perpendicular conformations have been optimized at the CASSCF level. A rotational angle of 44.3° is predicted for the minimum energy conformer in agreement with gas-phase electron diffraction data (44.4±1.2°). The highest level of theory employed yields the values 12.93 (6.0±2.1) and 6.40 (6.5±2.5) kJ/mol for the barrier heights at 0° and 90°, respectively (electron diffraction data within parentheses). In the light of the present findings, the reliability of the available experimental data is discussed.

A theoretical study of the electronic spectrum of bithiophene

The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is possible there is agreement between theory and experiment.

A theoretical study of the electronic spectrum of terthiophene

Abstract The electronic spectrum of planar 2,2′:5′,2″-terthiophene has been studied using multiconfigurational second-order perturbation theory. Four valence states are located below the first dipole-allowed Rydberg state. The computed excitation energies (1 1 B 2 : 2.86 eV; 2 1 A 1 ; 3.71 eV; 2 1 B 2 : 4.44 eV; 3 1 A 1 : 4.96 eV) are in agreement with experiment. The first transition results mainly from the HOMO → LUMO single electron promotion and represents the most prominent feature of the spectrum. Comparisons are made with other theoretical calculations and between the electronic spectra of terthiophene and bithiophene.

A theoretical study of the electronic spectrum of biphenyl

Abstract The electronic spectrum of biphenyl in the energy range up to 6.0 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2) and a basis set of ANO type, including polarization functions on all carbon atoms. The calculated spectrum gives conclusive assignments to all valence excited singlet states and the low-lying triplet states. The change of the torsional angle between the two benzene rings in the different excited states is shown to be of considerable importance and explains the different excitation energies observed in the gas phase as compared to solution or crystalline biphenyl at low temperatures. The intense transition to the 1 1B1u state is mostly affected. The first Rydberg transition (3s) is found at 5.6 eV. The appearance of a state of 1Ag symmetry at energies around 5.0 eV is ruled out. The first state of this symmetry appears at 5.9 eV.

Are most of the stationary points in a molecular association minima? Application of Fraga's potential to benzene-benzene

The importance of characterizing the stationary points of the intermolecular potential by means of Hessian eigenvalues is illustrated for the calculation of the benzene–benzene interaction using an atom‐to‐atom pair potential proposed by Fraga (FAAP). Two models, the standard one‐center‐per atom and another using three‐centers‐per atom due to Hunter and Sanders, are used to evaluate the electrostatic contributions and the results are compared. It is found in both cases that although using low‐gradient thresholds allows optimization procedures to avoid many stationary points that are not true minima computing time considerations makes the usual procedure of using high‐gradient thresholds [say, 10−2 kj/(mol Å)] as the most efficient. Moreover, this later procedure can be recommended because the actual minima can be characterized by means of Hessian eigenvalues even if these high‐gradient thresholds are used, and further decreasing of the convergence criterion does not imply significant modifications in the geometric parameters of the minima. The possible advantages of using the three‐centers‐per‐atom model for the calculation of molecular associations between delocalized systems are also discussed on the basis of the agreement of the benzene–benzene results with experimental and theoretical data taken from the literature.