My research interests are in the area of molecular electronic structure theory and involve methodological work and code development, as well as theoretical studies of bonding, reactivity and other electronic properties of molecular systems in organic, inorganic chemistry and materials science. The ability of a modern form of valence-bond VB theory, the spin-coupled SC method, to include a significant amount of electron correlation within a wavefunction, which remains easy-to-visualize and interpret, provides us with a powerful tool for studying not just bonding in isolated molecules, but also the bond-breaking and bond-formation processes that take place during chemical reaction.
In a nutshell, our SC approach for interpreting chemical reaction mechanisms uses existing efficient implementations of high-level molecular orbital methods to obtain transition structures and sequences of geometries along the reaction path, which is followed by SC calculations at these geometries and a detailed analysis of the results.
This methodology is particularly suitable for describing the electronic mechanisms of pericyclic reactions, such as the Diels-Alder reaction between butadiene and ethene and the 1,3-dipolar cycloadditions between fulminic acid and ethyne. For each pericyclic reaction we have studied, it has been possible to obtain a very clear picture of the electronic rearrangements taking place as the system follows the reaction path from reactants, through the transition structure, to products.
We can observe directly the evolution of the bonds being broken or formed during the reaction. During some reactions, for example, the Diels-Alder reaction, bonds break and reform in a homolytic way, the major recoupling of the spins of the active orbitals takes place at the transition structure and involves a resonance pattern very similar to that in benzene.
This strongly suggests that these reactions involve aromatic transition structures. An entirely different rearrangement is observed for reactions involving more polar reactants, for example, the 1,3-dipolar cycloaddition between fulminic acid and ethyne: The bond breaking and formation now involve shifts of whole electron pairs rather that spin-recouplings.
This indicates a heterolytic mechanism and a non-aromatic transition structure. The amount of detail revealed by the SC interpretations of chemical reaction mechanisms has been providing us with insights into the changes in the electronic structure that take place during a wide range of chemical reactions which are not readily available from any other quantum-chemical approaches. SC orbitals describing a C-H bond that will break [P. Replacing the sixty carbon atoms of C 60 by phosphorous atoms produces P We found that no quantum chemical method was able to stabilize P 60 into a C 60 cage-like structure.
We suggest then that this structure is not stable under normal circumstances. The next step was to consider that phosphorus also has a pentavalent bonding capacity. A stable structure was obtained. Figure 8. The spherical core is composed by P atoms. The external ones are O atoms.
Two more molecules were generated in this way: N 70 which is the natural extension of [ 37 ], and a hybrid cage-like structure with formula C 30 N 30 obtained by replacing one of the hemispheres of C 60 by nitrogen atoms. We generated several more similar structures but, as we are interested only in showing the goodness of the EHT method to deal with band structures, we present here the most interesting ones.
All systems are stable.
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We also included N 60 for the sake of completeness. These molecules are interesting candidates for nanoelectronic devices when coupled with other molecular systems. For the sake of comparison, the energy of the highest molecular orbital HOMO of all the molecules has been placed at zero. Figure 9. Density of states for C 30 N Figure Density of states for N 60 , N 70 and P 60 O It is suggested that the place where electrons are almost free to move is the C-N junction which is the natural place to bind this system to another one.
More theoretical work is needed to know the exact nature closed shell vs open shell of this interesting system. N 60 shows an overlap between the VB and the CB. Therefore this molecule should be, at least, a semiconductor. N 70 presents a VB-CB gap of about 1. Moreover, as in the case of N 60 , this molecule also could be a candidate for explosives or rocket propellants. P 60 O 60 is a quite interesting system because it has no gap between the valence and conduction bands.
This indicates that it could have metallic properties.
Nevertheless, its implementation into a electronic device seems difficult because of its reactivity. Footnote 1. In solid state physics the chemical potential, m, is often called the Fermi level. Footnote 2. In our case the bandwidths were calculated as follows. From the numerical set obtained from the Gaussian broadening we inspect visually the energies at which the DOS of a given band tends to zero at both sides. Their difference is the bandwidth.
Calculating curly arrows from ab initio wavefunctions | Nature Communications
In the case of overlapping bands we used the energy of the middle point of the overlap. This is an approximation that could be ameliorated in the future. Footnote 3. The off-diagonal elements are the interaction energies between two atomic orbitals. Interaction energies are the average of the binding energies ie, the VSIE multiplied by the overlap integral over the two atomic orbitals.
The prediction of the band structure of new fullerene-like structures using EHT is a valid approach. We also thank Prof. John A. Weaver for his written permission to reproduce some figures of this work. Kroto, J. Heath, S. O'Brien, R. Curl, and R. Smalley, Nature , Lamb, K. Fostiropoulos, D. Hebard, M.
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