Theoretical design of the biradical character in 1,3-diphosphacyclobutanediyl and homologous structures

Abstract

The electronic nature of 1,3-diphosphacyclobutane-2,4-diyl is explored with wavefunction based and density functional methods. According to MCSCF calculations the singlet state of the title compound is a biradicaloid with closed shell character, the number of unpaired electrons, assigned upon the analysis of the natural orbitals, is close to one. The participation of closed shell contributions in the overall wavefunction arises from a strong mixing of canonical structures, which emphasizes (a) the phosphorane type of bonding as well as (b) π-delocalization within the ring system. The bonding situation changes when σ-attracting substituents, e.g. amino groups, are attached to the phosphorus atoms. They inhibit possible cyclic π-delocalization and enhance the biradical character within the ring system.

The Niecke Biradicals and Their Congeners – The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties

“…amino groups) increase the inversion barrier at trigonal phosphorus centers, thus favoring a triplet over a singlet. This analysis has been predicted already some time ago by quantum chemical investigations , . (c) Radical stabilizing substituents, such as phenyl stabilize preferentially structure a .…”

“…On this basis the static electron correlation is accounted for by CAS(2,2) between HOMO und LUMO and the CASPT2 procedure takes proper care of the change in ring strain. In comparison with the DFT calculations seem to overestimate somewhat the best MCSCF values for singlet (S)–triplet (T) energy separations, for reasons noted earlier , …”

“…Here we will show that these species should reveal unusual bonding features. The four‐membered ring system (I) owe their stability to the interaction of the lone pairs at the phosphorus centers, forming a stable biradicaloid , . The lone pairs are defined by a bonding (n) as well as antibonding (n*) orbital, as depicted in a simplified manner (Scheme ).…”

“…The results, symmetry considerations, etc., are based on previous reports , , . All MCSCF calculations were performed with the MOLPRO program systems, the DFT calculations conducted with the UHF approach (for singlets and triplets) and examination of spin‐contamination.…”

Open-Shell Singlet Nature and σ-/π-Conjugation Effects on the Third-Order Nonlinear Optical Properties of Si Chains: Polysilane and Poly(disilene-1,2-diyl)

“…On the other hand, the above theoretical investigations are mostly performed on the assumption that Si-containing conjugated systems are closed-shell systems. However, recent theoretical analyses of small-size conjugated systems involving Si and Ge atoms suggest that there exist open-shell singlet systems with a variety of open-shell natures due to their weak hybridization of s and p orbitals. ,− Recently, the open-shell nature, which is quantum-chemically well-defined by diradical character ( y ), , is theoretically found to be strongly correlated to the third-order NLO properties (second hyperpolarizabilities, γ): open-shell systems with intermediate y (0 < y < 1) exhibit larger γ than closed-shell ( y = 0) and pure open-shell ( y = 1) systems of similar size, − which is referred to as “the y –γ correlation”. Hence, poly­(disilene-1,2-diyl), the Si analogue of polyacetylene, is an attractive candidate for efficient NLO molecules because of its weak SiSi double bonds with nonzero diradical character as well as the extension of σ- and π-conjugation.…”