Metallo-boranes: A class of unconventional superhalogens defying electron counting rules

Superhalogens are a class of highly electronegative atomic clusters whose electron affinities exceed those of halogens. Due to their potential for promoting unusual reactions and role as weakly coordinating anions...

Oligomeric approach to 2D materials modeling

Oligomeric approach has been originally developed to study electronic properties of conjugated polymers. This approach allows to access electronic properties of 1D systems otherwise difficult to calculate. We successfully extended this method to study electronic properties of 2D materials. In this review we summarize our recent work in this area. It has been established that large graphene nanoflake possess multiconfigurational singlet or even high spin ground state. Doping of 2D systems has also been explored and it has been demonstrated that doping allows to tune their electronic properties, including ionization potentials, electron affinities, reorganization energies and the very nature of the ground state. The electronic properties of novel 2D allotropies of carbon, phosphorus, germanium and silicon have been studied as well as their complexes with Li. Heterostructures, of different 2D allotropies are readily formed. This is an alternative method for tuning of their electronic properties.

Quantum alchemy beyond singlets: Bonding in diatomic molecules with hydrogen

Bonding energies are key for the relative stability of molecules in chemical space. Therefore methods employed to search for relevant molecules in chemical space need to capture the bonding behavior for a wide range of molecules, including radicals. In this work, we investigate the ability of quantum alchemy to do so for exploring hypothetical chemical compounds, here diatomic molecules involving hydrogen with various electronic structures. We evaluate equilibrium bond lengths, ionization ener- gies, and electron affinities of these fundamental systems. We compare and contrast how well manual quantum alchemy calculations, i.e. quantum mechanical calculations in which the nuclear charge is altered, and quantum alchemy approximations using a Taylor series expansion can predict these molecular properties. We also investigate the extent of error cancellation of these approaches in terms of ionization energies and electron affinities when using thermodynamic cycles. Our results suggest that the accuracy of Taylor series expansions are greatly improved by error cancellation in thermodynamic cycles, and errors also appear to be generally system-dependent. Taken together, this work provides insights into how quantum alchemy predictions us- ing a Taylor series expansion may be applied to future studies of non-singlet systems as well as which challenges remain open for these cases.

Three-Body Excitations in Fock-Space Coupled-Cluster: Fourth Order Perturbation Correction to Electron Affinity and Its Relation to Bondonic Formalism

In this paper, we present a formulation of highly correlated Fock-space multi-reference coupled-cluster (FSMRCC) methods, including approximate triples on top of the FSMRCC with singles and doubles, which correct the electron affinities by at least at third and up to the fourth order in perturbation. We discuss various partial fourth-order schemes, which are reliable and yet computationally more efficient than the full fourth-order triples scheme. The third-order scheme is called MRCCSD+. We present two approximate fourth-order schemes, MRCCSD+ and MRCCSD+. The results that are presented allow one to choose an appropriate fourth-order scheme, which is less expensive and right for the problem. All these schemes are based on the effective Hamiltonian scheme, and provide a direct calculation of the vertical electron affinities. We apply these schemes to a prototype molecule, using four different basis sets, as well as BeO and CH+. We have calculated the vertical electron affinities of at the geometry of the neutral molecule. We also present the vertical ionization potentials of the anion at the geometry of the anion ground state. We have also shown how to calculate adiabatic electron affinity, though in that case we lose the advantages of direct calculation. BeO has been examined in two basis sets. For CH+, four different basis sets have been used. We have presented the partial fourth-order schemes to the EA in all the basis sets. The results are analyzed to illustrate the importance of triples, as well as highlight computationally efficient partial fourth-order schemes. The choice of the basis set on the electron affinity calculation is also emphasized. Comparisons with available experimental and theoretical results are presented. The general fourth-order schemes, which are conceptually equivalent with the Fock-space multi-reference coupled-cluster singles, doubles, and triplets (MRCCSD+T) methods, based on bondonic formalism, are also presented here in a composed way, for quantum electronic affinity.

Electronic structure and reactivity of tirapazamine as a radiosensitizer

Tirapazamine (TP) has been shown to enhance the cytotoxic effects of ionizing radiation in hypoxic cells, thus making it a candidate for a radiosensitizer. This selective behavior is often directly linked to the abundance of O2. In this paper, we study the electronic properties of TP in vacuum, micro-hydrated from one up to three molecules of water and embedded in a continuum of water. We discuss electron affinities, charge distribution, and bond dissociation energies of TP, and find that these properties do not change significantly upon hydration. In agreement with its large electron affinity, and bond breaking triggered by electron attachment requires energies higher than 2.5 eV, ruling out the direct formation of bioactive TP radicals. Our results suggest, therefore, that the selective behavior of TP cannot be explained by a one-electron reduction from a neighboring O2 molecule. Alternatively, we propose that TP’s hypoxic selectivity could be a consequence of O2 scavenging hydrogen radicals.

DFT study of low-energy electron interaction with pyridine, pyrazine and their halo derivatives

In this work, the density functional theory with B3LYP hybrid functional was employed to calculate quantities useful for estimating the behavior of pyridine, pyrazine and their derivatives monosubstituted with Cl or Br atom, when exposed to low-energy electron impact. Vertical electron affinities obtained in several Pople basis sets and in aug-cc-pVTZ basis set are reported. Although some of the investigated molecules do not form stable anions, the results are in a satisfactory agreement with the available, albeit sparse experimental data, if the diffuse functions are included in calculations. It was found that the 6-31+G* basis is sufficient and its further enlargement does not significantly change the results. At this level of theory, potential energy curves, supported by enthalpies of dissociation to the neutral and anion fragment, were also determined for the description of the dissociative electron attachment. According to B3LYP, the potential energy curves of the halogen bond are almost repulsive in halopyridines, whereas halopyrazine anions require small activation energy for dissociation. Vertical electron affinities, enthalpies and equilibrium C-X distances (X=H, Cl, Br) were also calculated using Møller-Plesset second-order perturbation theory. Graphic Abstract