Дирадикальні та мультирадикальні системи і квантовохімічні методи розрахунків їхніх властивостей

• Datum: 2025
• Hauptverfasser: Кремень, О. С., Лобанов, В. В.
• Format: Artikel
• Sprache: Englisch
• Veröffentlicht: Chuiko Institute of Surface Chemistry National Academy of Sciences of Ukraine 2025
• Schlagworte: дирадикальний стан; дирадикальний характер; синглетні стани з відкритою оболонкою; нелінійні оптичні властивості; дирадикальні стани фулеренів; числа заповнення натуральних орбіталей; константа магнітної взаємодії
• Online Zugang: https://surfacezbir.com.ua/index.php/surface/article/view/802
• Назва журналу: Surface

Intense interest in diradical (di- and multi-) compounds arises from their unique nonlinear optical (NLO) properties, manifested as responses to strong laser electric fields. Their NLO behavior finds numerous applications in spectroscopy, materials science, engineering, and in photon-based data collection, storage, processing, and transmission. The electronic structure of open-shell diradical systems is classified according to the magnitude of their diradical character (y) into three categories: (i) closed-shell systems (y = 0); (ii) intermediate diradical systems (0 < y < 1); and (iii) pure open-shell systems (y = 1). Due to the interactions between unpaired electrons in diradical species, they cannot simply be regarded as a joint system of two independent radical centers. A complete description of the electronic structure of diradical species requires consideration of both singlet and triplet open-shell states. Results obtained within a simple two-center model lead to the emergence of a new class of open-shell singlet systems, which are expected to surpass traditional closed-shell NLO systems. Based on this principle, practical guidelines for molecular design and optimization of diradical characteristics are proposed, thereby enhancing NLO responses through first-principles calculations performed on realistic singlet open-shell molecular systems. Computational data on fullerenes are valuable not only for the development of efficient NLO materials but also for understanding the origin of multiradical character in certain bonds within such systems. Although the studied fullerenes possess singlet ground states, their intermediate diradical character may result in narrowing of the energy gap between singlet states and higher spin multiplets. In the context of molecular magnetism, it has been shown that the main effect beyond CAS, determined by the DDCI2 method (constructed from second-order perturbation theory terms), arises from external space determinants of the 1h + 1p type and represents a fourth- and higher-order correction. This effect consists of dynamic re-polarization of ionic structures of valence bonds. Beyond the DDCI2 space, which does not provide quantitative agreement with experiment, it is necessary to account for 2h + 1p and 1h + 2p excitations. Their influence is significant and does not correspond to ligand dynamic polarization via charge transfer to the metal atom, but rather occurs through dynamic coupling of ligand-metal transition dipoles with transition dipoles of surrounding electrons, thereby increasing the effective hopping integral of dispersive origin. Within Noodleman’s method, it has been demonstrated that for the generation of wave functions and matrix elements used in the calculation of higher spin multiplet energies, one may employ either the unrestricted Hartree–Fock approach or spin-polarized density functional theory. One of the advantages of broken-symmetry wave functions is that, being single-configuration wave functions, they are easily visualized. Moreover, a broken-symmetry wave function provides a weighted average of pure spin states, which are orthogonal and do not interact with the system’s overall Hamiltonian. This is a powerful result that can be exploited to evaluate the energies and properties of pure spin states.