Thiago Messias Cardozo

Energy partitioning for generalized product functions: The interference contribution to the energy of generalized valence bond and spin coupled wave functions

The main driving force for the formation of the covalent bond is the quantum-mechanical interference effect among one-electron states, as has been suggested in several works by the use of partition schemes to calculate the interference contributions to the energy. However, due to some difficulties associated with the original approaches, calculations were only carried out for a few, mostly diatomic molecules. In this work, we propose a general approach of partitioning based on generalized product functions with generalized valence bond at the perfect pairing approximation and spin-coupled groups, which should allow the investigation of a broader array of molecules, and hopefully, shed light on the nature of the chemical bond in molecules with unusual chemical features. Among other things, this approach lends itself naturally to the investigation of interference in individual bonds or groups of bonds in a molecule.

Interference Effect and the Nature of the π-Bonding in 1,3-Butadiene

The nature of the chemical bond in 1,3-butadiene is analyzed by applying the recently developed generalized product function energy partitioning (GPF-EP) scheme, which allows the calculation of the quantum mechanical interference contribution to the energy in a meaningful and intuitive fashion. The method is applied to investigate the breakage of the middle C-C bond, and the rotation along the torsional angle defined by the carbon atoms. A comparison between bonding in ethylene and butadiene is also performed. It is shown that bond delocalization plays no role in the properties of a conjugated molecule and that existing alternative explanations should be revisited.

Chemical Bonding in the N2 Molecule and the Role of the Quantum Mechanical Interference Effect

The chemical bond in the N(2) molecule is analyzed from the perspective of the quantum mechanical interference effect by means of the recently developed generalized product function energy partitioning (GPF-EP) scheme. The analysis is carried out at the GVB-PP and SC levels, which constitute interpretable independent particle models, while ensuring the correct dissociation behavior for the molecule. The results suggest that some current ideas concerning the bond in the N(2) molecule should be revised. It is shown that, in the absence of the interference effect, there is no chemical bond in the N(2) molecule. The influence of the basis set on the energy partitioning is also evaluated. The interference contributions to the energy are substantially less sensitive to the choice of the basis set than the reference energy, making the investigation of the relative importance of inteference effects in larger systems feasible.

Nature of the chemical bond and origin of the inverted dipole moment in boron fluoride: a generalized valence bond approach.

The generalized product function energy partitioning (GPF-EP) method has been applied to investigate the nature of the chemical bond and the origin of the inverted dipole moment of the BF molecule. The calculations were carried out with GPF wave functions treating all of the core electrons as a single Hartree-Fock group and the valence electrons at the generalized valence bond perfect-pairing (GVB-PP) or full GVB levels, with the cc-pVTZ basis set. The results show that the chemical structure of both X (1)Σ(+) and a (3)Π states is composed of a single bond. The lower dissociation energy of the excited state is attributed to a stabilizing intraatomic singlet coupling involving the B 2sp-like lobe orbitals after bond dissociation. An increase of electron density on the B atom caused by the reorientation of the boron 2sp-like lobe orbitals is identified as the main responsible effect for the electric dipole inversion in the ground state of BF. Finally, it is shown that π back-bonding from fluorine to boron plays a minor role in the electron density displacement to the bonding region in both states. Moreover, this effect is associated with changes in the quasi-classical component of the electron density only and does not contribute to covalency in either of the states. Therefore, at least for the case of the BF molecule, the term back-bonding is misleading, since it does not contribute to the bond formation.

Interference Energy in C–H and C–C Bonds of Saturated Hydrocarbons: Dependence on the Type of Chain and Relationship to Bond Dissociation Energy

Interference energy for C-H and C-C bonds of a set of saturated hydrocarbons is calculated by the generalized product function energy partitioning (GPF-EP) method in order to investigate its sensitivity to the type of chain and also its contribution to the bond dissociation energy. All GPF groups corresponding to chemical bonds are calculated by use of GVB-PP wave functions to ensure the correct description of bond dissociation. The results show that the interference energies are practically the same for all the C-H bonds, presenting only small variations (0.5 kcal.mol(-1)) due to the structural changes in going from linear to branched and cyclic chains. A similar trend is verified for the C-C bonds, the sole exception being the cyclopropane molecule, for which only the C-C bond exhibits a more significant variation. On the other hand, although the interference energy is quantitatively the most important contribution to the bond dissociation energy (DE), one cannot predict DE only from the bond interference energy. Differences in the dissociation energies of C-C and C-H bonds due to structural changes in the saturated hydrocarbons can be mainly attributed to quasi-classical effects.

Absorption and Fluorescence Spectra of Poly(p-phenylenevinylene) (PPV) Oligomers: An ab Initio Simulation

The absorption and fluorescence spectra of poly(p-phenylenevinylene) (PPV) oligomers with up to seven repeat units were theoretically investigated using the algebraic diagrammatic construction method to second order, ADC(2), combined with the resolution-of-the-identity (RI) approach. The ground and first excited state geometries of the oligomers were fully optimized. Vertical excitation energies and oscillator strengths of the first four transitions were computed. The vibrational broadening of the absorption and fluorescence spectra was studied using a semiclassical nuclear ensemble method. After correcting for basis set and solvent effects, we achieved a balanced description of the absorption and fluorescence spectra by means of the ADC(2) approach. This fact is documented by the computed Stokes shift along the PPV series, which is in good agreement with the experimental values. The experimentally observed band width of the UV absorption and fluorescence spectra is well reproduced by the present simulations showing that the nuclear ensemble generated should be well suitable for consecutive surface hopping dynamics simulations.

The Nature of the Singlet and Triplet States of Cyclobutadiene as Revealed by Quantum Interference.

The generalized product function energy partitioning (GPF-EP) method is applied to the description of the cyclobutadiene molecule. The GPF wave function was built to reproduce generalized valence bond (GVB) and spin-coupled (SC) wave functions. The influence of quasiclassical and quantum interference contributions to each chemical bond of the system are analyzed along the automerization reaction coordinate for the lowest singlet and triplet states. The results show that the interference effect on the π space reduces the electronic energy of the singlet cyclobutadiene relative to the second-order Jahn-Teller distortion, which takes the molecule from a D4h to a D2h structure. Our results also suggest that the π space of the (1) B1g state of the square cyclobutadiene is composed of a weak four center-four electron bond, whereas the (3) A2g state has a four center-two electron π bond. Finally, we also show that, although strain effects are nonnegligible, the thermodynamics of the main decomposition pathway of cyclobutadiene in the gas phase is dominated by the π space interference.