Theory and practice of uncommon molecular electronic configurations

Abstract

The electronic configuration of the molecule is the foundation of its structure and reactivity. The spin state is one of the key characteristics arising from the ordering of electrons within the molecule's set of orbitals. Organic molecules that have open-shell ground states and interesting physicochemical properties, particularly those influencing their spin alignment, are of immense interest within the up-and-coming field of molecular electronics. In this advanced review, we scrutinize various qualitative rules of orbital occupation and spin alignment, viz., the aufbau principle, Hund's multiplicity rule, and dynamic spin polarization concept, through the prism of quantum mechanics. While such rules hold in selected simple cases, in general the spin state of a system depends on a combination of electronic factors that include Coulomb and Pauli repulsion, nuclear attraction, kinetic energy, orbital relaxation, and static correlation. A number of fascinating chemical systems with spin states that fluctuate between triplet and open-shell singlet, and are responsive to irradiation, pH, and other external stimuli, are highlighted. In addition, we outline a range of organic molecules with intriguing non-aufbau orbital configurations. In such quasi-closed-shell systems, the singly occupied molecular orbital (SOMO) is energetically lower than one or more doubly occupied orbitals. As a result, the SOMO is not affected by electron attachment to or removal from the molecule, and the products of such redox processes are polyradicals. These peculiar species possess attractive conductive and magnetic properties, and a number of them that have already been developed into molecular electronics applications are highlighted in this review. How to cite this article:WIREs Comput Mol Sci 2015Sci , 5:440-459. doi: 10.1002Sci /wcms.1233 INTRODUCTIONT he electronic configuration and state of a molecule characterize the distribution of electrons across the corresponding one-electron wavefunctions, i.e., orbitals. 1 This model description, albeit approximate, is nonetheless indispensable in explaining and predicting molecular geometry and various chemical and physical properties. As orbitals comprise not only the spatial component, but also the spin, they also give rise to such key features as the nature of configuration shell-open or closed-and the multiplicity of the electronic state. While the majority of stable organic molecules have a closed-shell singlet ground state, species with unpaired electrons display unique chemical reactivity and are capable of carrying magnetism and conductivity functionalities. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Although historically the latter have been the domain of metals and, in particular, transition metal complexes, in the recent years the focus has ...

Exchange Rules for Diradical π-Conjugated Hydrocarbons

“…Naively, one could expect that diradicals always had a ground state with S = 1, due to the similitude with the case of open-shell atoms. Whereas this is often the case in a variety of systems 3,[11][12][13][14][15] , in others there is a violation of the Hund's rule 12,[16][17][18][19][20][21] . Therefore, the sign of the exchange interaction in this class of diradicals is not always the same 5 .…”

Role of exact exchange in thermally-assisted-occupation density functional theory: A proposal of new hybrid schemes

“…Nonetheless, owing to the inappropriate treatment of nonlocal XC effects [12,13], KS-DFAs can perform very poorly in situations where the self-interaction error (SIE) [12][13][14][15], noncovalent interaction error (NCIE) [16][17][18], or static correlation error (SCE) [12,[19][20][21][22] is pronounced. Over the years, considerable efforts have been made to resolve the qualitative failures of KS-DFAs at a reasonable computational cost.…”

“…In spite of their computational efficiency, KS-DFAs, hybrid functionals, and double-hybrid functionals can perform very poorly for systems with strong static correlation effects (i.e., multi-reference systems) [12,[19][20][21][22]. Within KS-DFT, fully nonlocal XC functionals, such as those based on the random phase approximation (RPA), may be adopted for a reliably accurate description of strong static correlation effects.…”

Effect of Li Termination on the Electronic and Hydrogen Storage Properties of Linear Carbon Chains: A TAO-DFT Study

“…Theoretically, the popular Kohn-Sham density functional theory (KS-DFT) [34] with conventional semilocal [35], hybrid [36][37][38][39], and double-hybrid [40][41][42][43] exchange-correlation (XC) density functionals can provide unreliable results for systems with strong static correlation effects [44]. For accurate prediction of the properties of these systems, high-level ab initio multi-reference methods are typically needed [45]. Nonetheless, accurate multi-reference calculations are prohibitively expensive for large systems (especially for geometry optimization).…”