Antonio Carlos Borin

New Relativistic Atomic Natural Orbital Basis Sets for Lanthanide Atoms with Applications to the Ce Diatom and LuF3

New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are included as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.

A Three-State Model for the Photophysics of Guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.

Azo−Hydrazone Tautomerism in Protonated Aminoazobenzenes: Resonance Raman Spectroscopy and Quantum-Chemical Calculations

The protonation effect on the vibrational and electronic spectra of 4-aminoazobenzene and 4-(dimethylamino)azobenzene was investigated by resonance Raman spectroscopy, and the results were discussed on the basis of quantum-chemical calculations. Although this class of molecular systems has been investigated in the past concerning the azo-hydrazone tautomerism, the present work is the first to use CASSCF/CASPT2 calculations to unveil the structure of both tautomers as well the nature of the molecular orbitals involved in chromophoric moieties responsible for the resonance Raman enhancement patterns. More specifically both the resonance Raman and theoretical results show clearly that in the neutral species, the charge transfer transition involves mainly the azo moiety, whereas in the protonated forms there is a great difference, depending on the tautomer. In fact, for the azo tautomer the transition is similar to that observed in the corresponding neutral species, whereas in the hydrazone tautomer such a transition is much more delocalized due to the contribution of the quinoid structure. The characterization of protonated species and the understanding of the tautomerization mechanism are crucial for controlling molecular properties depending on the polarity and pH of the medium.

A theoretical study of the electronic structure and spectroscopic properties of a new diatomic molecule, BeC

The 13 lowest‐lying electronic states of a new diatomic molecule, BeC, are described theoretically for the first time using the multireference single and double excitations configuration interaction approach. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing each of them. Dissociation and vertical excitation energies, and dipole moment functions complete the electronic structure description. Vibrational and rotational levels spacings, on the other hand, allowed the calculation of a whole set of spectroscopic constants characterizing each state. In its ground state, a 3Σ−, this molecule is more strongly bound (De=2.39 eV) than the recently described species BeN (De=1.34 eV) but still less strongly bound than BeO (De=4.69 eV) and BeF (De=5.82 eV). The first excited triplet state (3Π) shows an avoided crossing at short internuclear distances which should certainly reflect in perturbative effects in its spectrum. A distinctive feature ...

A theoretical description of the ion BeC+ and the photoelectron spectrum of BeC

Abstract The seven lowest-lying electronic states of the ion BeC + are described theoretically using the multi-reference single and double excitations configuration interaction approach. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing each of them. A whole set of spectroscopic constants completes the characterization of each state. In its ground states (…3δ1π 2 4δ, X 4 Σ − ) this molecule is more strongly bound ( D e =3.18 eV) than the neutral species ( D e = 2.39 eV) and consequently has a shorter equilibrium distance (2.960 a 0 ) compared to 3.150 a 0 of BeC. The first two excited states are a 2 Π ( R e = 3.420 a 0 ) and a 2 Δ ( R e = 2.973 a 0 ) at energies 0.528 and 1.582 eV. The first excited quartet ( 4 Π) lies at 1.971 eV ( R e = 3.616 a 0 ). Of the remaining doublets, the state d 2 Σ + is distinct for exhibiting a double minimum, with the inner equilibrium distance equal to 2.965 a 0 and the outer one equal to 3.681 a 0 ; a barrier of 0.25 eV at 3.263 a 0 separates the two wells. A combination of these results with those previously reported for the neutral species allowed the calculation of vertical and adiabatic ionization potentials and the prediction of photoelectron spectra based on the Franck-Condon distrubutions for transitions from the ground state of BeC to selected bound states of BeC + .

On the deactivation mechanisms of adenine-thymine base pair.

In this contribution, the multiconfigurational second-order perturbation theory method based on a complete active space reference wave function (CASSCF/CASPT2) is applied to study all possible single and double proton/hydrogen transfers between the nucleobases in the adenine-thymine (AT) base pair, analyzing the role of excited states with different nature [localized (LE) and charge transfer (CT)], and considering concerted as well as step-wise mechanisms. According to the findings, once the lowest excited states, localized in adenine, are populated during UV irradiation of the Watson-Crick base pair, the proton transfer in the N-O bridge does not require high energy in order to populate a CT state. The latter state will immediately relax toward a crossing with the ground state, which will funnel the system to either the canonical structure or the imino-enol tautomer. The base pair is also capable of repairing itself easily since the imino-enol species is unstable to thermal conversion.

Proton/Hydrogen Transfer Mechanisms in the Guanine-Cytosine Base Pair: Photostability and Tautomerism.

Proton/hydrogen-transfer processes have been broadly studied in the past 50 years to explain the photostability and the spontaneous tautomerism in the DNA base pairs. In the present study, the CASSCF/CASPT2 methodology is used to map the two-dimensional potential energy surfaces along the stretched NH reaction coordinates of the guanine-cytosine (GC) base pair. Concerted and stepwise pathways are explored initially in vacuo, and three mechanisms are studied: the stepwise double proton transfer, the stepwise double hydrogen transfer, and the concerted double proton transfer. The results are consistent with previous findings related to the photostability of the GC base pair, and a new contribution to tautomerism is provided. The C-based imino-oxo and imino-enol GC tautomers, which can be generated during the UV irradiation of the Watson-Crick base pair, have analogous radiationless energy-decay channels to those of the canonical base pair. In addition, the C-based imino-enol GC tautomer is thermally less stable. A study of the GC base pair is carried out subsequently taking into account the DNA surroundings in the biological environment. The most important stationary points are computed using the quantum mechanics/molecular mechanics (QM/MM) approach, suggesting a similar scenario for the proton/hydrogen-transfer phenomena in vacuo and in DNA. Finally, the static model is complemented by ab initio dynamic simulations, which show that vibrations at the hydrogen bonds can indeed originate hydrogen-transfer processes in the GC base pair. The relevance of the present findings for the rationalization of the preservation of the genetic code and mutagenesis is discussed.

The lowest triplet and singlet electronic states of the molecule SO

Abstract A comprehensive study of the singlet and triplet states of SO, correlating with the first two dissociation channels, is carried out using a state-averaged complete active space self-consistent field/internally contracted multireference configuration-interaction approach, and correlation-consistent polarized-valence quadruple-zeta basis sets. Potential energy curves and dipole moment functions for all states are presented. Besides providing an accurate and global description of the manifold of electronic states known to date, reliable evidence of two 1 Π bound states, for which experimental evidences were reported very recently, and a discussion of the vibrational numbering problem of the transitions c 1 Σ − → a 1 Δ , and A ′ 3 Δ → X 3 Σ − , for which there has been some conflict in the available experimental data, are also presented. These results are of special relevance to elucidate further experimental assignments in the electronic spectroscopy of SO.

Photoinduced Processes in Nucleic Acids

Photoinduced processes in nucleic acids are phenomena of fundamental interest in diverse fields, from prebiotic studies, through medical research on carcinogenesis, to the development of bioorganic photodevices. In this contribution we survey many aspects of the research across the boundaries. Starting from a historical background, where the main milestones are identified, we review the main findings of the physical-chemical research of photoinduced processes on several types of nucleic-acid fragments, from monomers to duplexes. We also discuss a number of different issues which are still under debate.

On the Relaxation Mechanisms of 6-Azauracil

The nonadiabatic photochemistry of 6-azauracil has been studied by means of the CASPT2//CASSCF protocol and double-ζ plus polarization ANO basis sets. Minimum energy states, transition states, minimum energy paths, and surface intersections have been computed in order to obtain an accurate description of several potential energy hypersurfaces. It is concluded that, after absorption of ultraviolet radiation (248 nm), two main relaxation mechanisms may occur, via which the lowest (3)(ππ*) state can be populated. The first one takes place via a conical intersection involving the bright (1)(ππ*) and the lowest (1)(nπ*) states, ((1)ππ*/(1)nπ*)(CI), from which a low-energy singlet-triplet crossing, ((1)nπ*/(3)ππ*)(STC), connecting the (1)(nπ*) state to the lowest (3)(ππ*) triplet state is accessible. The second mechanism arises via a singlet-triplet crossing, ((1)ππ*/(3)nπ*)(STC), leading to a conical intersection in the triplet manifold, ((3)nπ*/(3)ππ*)(CI), evolving to the lowest (3)(ππ*) state. Further radiationless decay to the ground state is possible through a (gs/(3)ππ*)(STC).