A causal relation connecting aromaticity with the current aromaticity descriptors used in the literature and compliant with a quantum mechanics theoretical background is described. © 2015 Wiley Periodicals, Inc.

A concise description of Kekulé's historical origin of aromaticity and the current state of the field is given. Still, space is left for a discussion about the existence of aromaticity's quantum mechanical foundation. Quantum multimolecular polyhedra (QMP), based on density functions sets attached to QMP vertices, collective QMP distances, QSPR fundamental equation, and aromaticity descriptors are proposed as a way to construct an equation able to estimate aromaticity via expectation values of Hermitian operators.

Open Shell organic radicals are principal species involved in many diverse areas such as combustion, photochemistry, and polymer chemistry. Computational studies of such species with an accurate method like coupled-cluster with single and double and perturbative triple (CCSD(T)) may be restricted to systems of modest size due to the steep computational scaling of the method. Herein, we assess the accuracy of extrapolated CCSD(T) energies determined using the connectivity-based hierarchy (CBH) method on medium to large sized radicals. In our method, an MP2 calculation on the target radical is coupled with CCSD(T) energies of fragments determined uniquely by our hierarchy to perform accurate extrapolations. A careful assessment is done with a robust CBH-rad49 test set comprising of 49 diverse cyclic and acyclic radicals with a variety of functional groups. We demonstrate that the extrapolation method with CBH-2 or CBH-3 is sufficient to obtain sub-kcal accuracy. ROMP2 and PMP2 calculations with both Pople-style and Dunning-style basis-sets resulted in mean absolute errors for CCSD(T) extrapolation (full CCSD(T)—extrapolated CCSD(T)) within 0.5 kcal/mol. Further speedup for such CCSD(T) extrapolations are obtained with ROHF-based RI-MP2 calculations. Challenging systems with (a) high ring strain, (b) delocalized character, and (c) spin contamination are identified and analyzed in detail. Finally, we apply our extrapolation method on 10 larger radicals containing 10−15 heavy atoms, where accurate CCSD(T) energies are obtained at a fractional cost of full CCSD(T) calculations. © 2015 Wiley Periodicals, Inc.

Highly accurate extrapolated coupled-cluster with single and double and perturbative triple (CCSD(T)) energies were obtained using the Connectivity-Based Hierarchy method for medium to large sized radicals. A careful assessment was performed with a robust test set comprised of 49 diverse radicals including challenging systems with high ring strain and spin contamination. The most expensive calculation is MP2 on the entire radical, thereby breaking the existing bottleneck for calculating CCSD(T) energies of large open-shell organic molecules.

The Continuum in the variation of the X-Z bond length change from blue-shifting to red-shifting through zero- shifting in the X-Z---Y complex is inevitable. This has been analyzed by ab-initio molecular orbital calculations using Z= Hydrogen, Halogens, Chalcogens, and Pnicogens as prototypical examples. Our analysis revealed that, the competition between negative hyperconjugation within the donor (X-Z) molecule and Charge Transfer (CT) from the acceptor (Y) molecule is the primary reason for the X-Z bond length change. Here, we report that, the proper tuning of X- and Y-group for a particular Z- can change the blue-shifting nature of X-Z bond to zero-shifting and further to red-shifting. This observation led to the proposal of a continuum in the variation of the X-Z bond length during the formation of X-Z---Y complex. The varying number of orbitals and electrons available around the Z-atom differentiates various classes of weak interactions and leads to interactions dramatically different from the H-Bond. Our explanations based on the model of anti-bonding orbitals can be transferred from one class of weak interactions to another. We further take the idea of continuum to the nature of chemical bonding in general. © 2015 Wiley Periodicals, Inc.

Red- and blue- shift in the X-Z bonds during the X-Z---Y complex formation has been analyzed. A continuum in the X-Z bond length is observed for various classes of weak bonds such as H-bonds, halogen-bonds, chalcogen-bonds, and pnicogen-bonds. The balance between negative hyperconjugation within the X-Z molecule and charge transfer from Y-group provides a working model to explain the observations. The definition of the continuum in the weak (X-Z---Y) interactions is profitably extended to include strong (Z-Y) chemical bonds as well.

A series of paracyclophane (PC) bridged mixed-valence (MV) bis-triarylamine radical cations with different ([2.2], [3.3], [4.4]) linkers, with and without additional ethynyl spacers, have been studied by quantum-chemical calculations (BLYP35-D3/TZVP/COSMO) of ground-state structures, thermal electron-transfer barriers, hyperfine couplings, and lowest-lying excited states. Such PC-bridged MV systems are important intra-molecular model systems for inter-molecular electron transfer (ET) via π-stacked aromatics, since they allow enforcement of a more or less well-defined geometrical arrangement. Closely comparable ET barriers and electronic couplings for all [2.2] and [3.3] bridges are found for these class-II MV systems, irrespective of the use of pseudo-para and pseudo-meta connections. While the latter observation contradicts notions of quantum interference for off-resonant conduction through molecular wires, it agrees with the less intricate nodal structures of the highest occupied molecular orbitals. The ET in such MV systems may be more closely connected with hole conduction in the resonant regime. Computations on model cations, in which the [2.2] linkers have been truncated, confirm predominant through-space π-π electronic coupling. Systems with [4.4] PC bridges exhibit far more structural flexibility and concomitantly weaker electronic interactions between the redox centers. © 2015 Wiley Periodicals, Inc.

Electron transfer through the π-stacked faces of paracyclophane bridge units in bis-triarylamine mixed-valence systems has been studied using a previously established quantum-chemical protocol. Pseudo-meta and pseudo-para connected systems exhibit very similar electronic couplings and thermal electron-transfer barriers, explained by resonant hole transfer. Through-space electron transfer through the π-stack dominates over through-bond transfer through the linkers.

We investigated by computational means the kinetics and stationary behavior of stochastic dynamics on an ensemble of rough two-dimensional energy landscapes. There are no obvious separations of temporal scales in these systems, which constitute a simple model for the behavior of glasses and some biomaterials. Even though there are significant computational challenges present in these systems due to the large number of metastable states, the Milestoning method is able to compute their kinetic and thermodynamic properties exactly. We observe two clearly distinguished regimes in the overall kinetics: one in which diffusive behavior dominates and another that follows an Arrhenius law (despite the absence of a dominant barrier). We compare our results with those obtained with an exactly-solvable one-dimensional model, and with the results from the rough one-dimensional energy model introduced by Zwanzig. © 2015 Wiley Periodicals, Inc.

We compute, using the Milestoning method, the stationary flux (shown), the mean first passage time of Brownian trajectories, and the free energy (not shown) on a large ensemble of random energy landscapes with varying degrees of roughness and at a wide range of temperatures. We find two different behaviors: a diffusive regime for high temperatures and an Arrhenius-like regime for low temperatures.