Francisco B. C. Machado

A theoretical investigation of the low‐lying electronic states of the molecule BeH+

A theoretical study at the multireference singles and doubles configuration‐interaction level is reported for the six lowest electronic states of BeH+. Of the four new states described, the 1Π and 3Π are bound states supporting at least 8 and 12 vibrational levels, respectively. Two 3Σ+ states are repulsive. For the two known 1Σ+ states improved results are also presented. For all these states, dipole moments and transition moments have been computed. For the bound states, besides the vibrational levels, transition probabilities, radiative lifetimes, vibrationally averaged dipole moments, and a set of spectroscopic constants are also reported.

On the assignment of transitions involving some 2Π Rydberg states of the BeH molecule

Abstract Very accurate potential energy curves for several low-lying excited 2 Π states of the BeH molecule are reported using the multireference configuration approach. New features in the potential energy curves of the B and G  2 Π states not considered previously, like the existence of a double minimum, invalidate some of the arguments used in the assignment of transitions arising from these states. An alternative interpretation of some of the transitions and improved spectroscopic constants are presented. For the 3d states, for which experimental data are still uncertain about their nature, this study allows us to make an unambiguous correlation with the 3d 2 Π state.

What is so mysterious about the electronic states of SCI

The 15 lowest lying doublet electronic states of the molecule SCI have been investigated theoretically at a high level of correlation treatment (MRCI). For the ground state (X 2II), spectroscopic constants were obtained from a set of eight vibrational intervals. This result extends our knowledge about this state beyond the experimentally known data that presently were derived from only two bands. Spin-orbit constants, transition probabilities and radiative lifetimes complement its spectroscopic characterization. For the excited electronic states, a global view of the doublet states is presented that can help us understand the scarcity of experimental data on electronic transitions for this system and also the difficulty of assigning the only two transitions so far recorded. Most of these states are repulsive, and for the few high lying bound ones, of Rydberg character, avoided crossings restrict the number of accessible vibrational states. Crossing by repulsive states and predissociation is also a factor that can prevent further emissions. Two new bound excited states, 2δ and 2Σ, predicted in this study, are of direct relevance to an interpretation of the limited experimental data available on electronic transitions.

An ab initio study of the equilibrium geometry and vibrational frequencies of hydrazine

Abstract Accurate optimized equilibrium geometries, harmonic frequencies and rotational constants of the gauche conform (C 2 symmetry) of hydrazine (N 2 H 4 ) have been calculated employing a large series of basis sets and several ab initio methods. Our best estimate computed equilibrium geometries are r NN =1.434 A , r NH i =1.013 A , r NH o =1.010 A , θ NNH i =111.3°, θ NNH o =106.8° and θ H i NNH o =89.7°. These predictions were used as reference in the analysis of the experimental and theoretical data, and we expected that the present results could guide future investigations in this direction.

Energetic and structural features of the CH4+O(3P)→CH3+OH abstraction reaction: Does perturbation theory from a multiconfiguration reference state (finally) provide a balanced treatment of transition states?

The stationary points of the CH4+O(3P)→CH3+OH abstraction reaction have been identified at the fully optimized reaction space (FORS) level. For three sets of geometries (FORS plus unrestricted and restricted-open-shell Moller–Plesset second order perturbation theory), single-point calculations by unrestricted Moller–Plesset fourth order perturbation theory (UMP4), by unrestricted coupled cluster theory with single and double excitations and a quasiperturbative treatment of fourth- and fifth-order triple-excitation terms (CCDS(T)), and by multireference Moller–Plesset second order perturbation theory (MRMP2) were also performed for the classical barrier height and energy of the reaction. Calculations carried out at the MRMP2/cc-pVTZ//FORS/cc-pVTZ level predict values for the forward vibrationally adiabatic barrier height and for the energy of the reaction at 0 K equal to 10.3 and 2.0 kcal/mol, respectively. This is in excellent agreement with experiments that show values of the activation energies in the r...

A high level theoretical investigation of the N2O4→2 NO2 dissociation reaction: Is there a transition state?

The N2O4→2 NO2 dissociation reaction was investigated at a high level of theory using the couple cluster with all single and double excitations and connected triples [CCSD(T)] and complete active space self-consistent field approaches, and the cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. Only at the coupled cluster level a first-order saddle point was found connecting reactant and products. Collectively, structural, vibrational, and thermodynamic data for the three stationary points represent the best theoretical description of this reaction system to date, and are in good agreement with available experimental results. Unimolecular transition state theory rate constants (k∞) were also evaluated at 250, 298.15, and 350 K. At the CCSD(T)/cc-pVTZ level of calculation these results are 0.62×101, 1.90×103, and 1.66×105 s−1, respectively. Known experimental results at 298 K vary from 1.7×105 to 1.0×106 s−1. Including an estimate for basis set superposition error, we predict ΔH2980 for the dissociation reaction...

Synthesis, characterization, and computational study of a new dimethoxy-chalcone.

Chalcones are an important class of medicinal compounds and are known for taking part in various biological activities as in anti-inflammatory, anti-leishmania, antimitotic, and antiviral. Chemically, chalcones consist of open-chain flavonoids in which the two aromatic rings are joined by a three-carbon α,β-unsaturated carbonyl system. The wide action spectrum has attracted our attention to synthesize, crystallize, and characterize the dimethoxy-chalcone C18H18O3. Aiming to understand the process of crystal lattice stabilization, a combination of technique has been used including X-ray diffraction, infrared spectroscopy and computational molecular modeling. The theoretical calculations were carried out by the density functional method (DFT) with the M06-2X functional, with the 6-311+G(d,p) basis set. The vibrational wavenumbers were calculated and the scaled values were compared with experimental FT-IR spectrum. The intermolecular interactions were quantified and intercontacts in the crystal structure were analyzed using Hirshfeld surfaces. Bond distances and angles described by the X-ray diffraction and theoretical calculation are very similar. The C-H….O contacts contributing to assemble the supramolecular architecture are also responsible for the molecular structure assembly.

A product branching ratio controlled by vibrational adiabaticity and variational effects: Kinetics of the H + trans-N2H2 reactions

The abstraction and addition reactions of H with trans-N(2)H(2) are studied by high-level ab initio methods and density functional theory. Rate constants were calculated for these two reactions by multistructural variational transition state theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal∕mol lower than that of the abstraction reaction, but the addition rates are lower than the abstraction rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K.

Hydrogen Abstraction from the Hydrazine Molecule by an Oxygen Atom

Thermochemical and kinetics properties of the hydrogen abstraction from the hydrazine molecule (N2H4) by an oxygen atom were computed using high-level ab initio methods and the M06-2X DFT functional with aug-cc-pVXZ (X = T, Q) and maug-cc-pVTZ basis sets, respectively. The properties along the reaction path were obtained using the dual-level methodology to build the minimum energy path with the potential energy surface obtained with the M06-2X method and thermochemical properties corrected with the CCSD(T)/CBS//M06-2X/maug-cc-pVTZ results. The thermal rate constants were calculated in the framework of variational transition-state theory. Wells on both sides of the reaction (reactants and products) were found and considered in the chemical kinetics calculations. Additionally, the product yields were investigated by means of a study of the triplet and singlet surfaces of the N2H4 + O → N2H2 + H2O reaction.

Nuclear motion dependence of the electric field gradient at the 9Be nucleus in BeH

Abstract The electric field gradient (q) at the beryllium nucleus in BeH+ has been calculated as a function of the internuclear distance using the best BeH+ wavefunction available in the literature. Nuclear motion effects on q are computed as an average of q(R) over the vibrational wavefunction. It is shown that Buckinghams expression is inappropriate to compute such a correction and therefore the results reported by Diercksen and Sadlej on BeH+ and other systems do not have the accuracy they imply to have.