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Using a two-site valence configuration interaction model, we reveal the diradical character (y) dependences of the third-harmonic generation (THG) spectra of singlet diradicaloids, which are described by the photon energy-dependent THG second hyperpolarizabilities γ. We particularly examine three- and two-photon resonance peaks associated with the dipole-allowed transition to the first excited state and with the dipole-forbidden transition to the second excited state, respectively. When increasing y, both peaks move to lower-energy region and become close to each other. Although the THG γ spectral intensities show resonance enhancements for any y value, we observe for THG γ the same bell-shape behavior as a function of y, as for the static γ, irrespective of the photon energy. [ABSTRACT FROM AUTHOR]

We predict the magnetic exchange coupling constant ( J) for 27 m-phenylene-based nitronyl nitroxide (NN) diradicals with nine different substituents in three unique (common ortho, ortho- meta and common meta) positions on the coupler unit by using the broken-symmetry density functional methodology. For all investigated diradicals, J values are computed using B3LYP, B3LYP-D3 and M06-2X functionals with 6-311+G(d,p) basis set. The J value is larger than J and closer to the observed value for the unsubstituted species. Substitutions at common ortho position always produce a greater angle of twist between the spin source and the coupler units. When the twist angle is very large, the nature of intramolecular magnetic interaction changes from ferromagnetic to antiferromagnetic. In these cases, the coupler-NN bond order becomes small. Substitution at the common meta position of m-phenylene in the diradical has little steric and hydrogen-bonding effects. Electron-withdrawing groups reveal a specific trend for single-atom substitution. An ortho substitution generally decreases J and a meta substitution always increases J with a decreasing −I effect. Variation of J with planarity as well as Hammett constant is investigated. The nucleus-independent chemical shift value is found to decrease from the corresponding mono-substituted phenyl derivatives. The dependence of J on these factors is explored. [ABSTRACT FROM AUTHOR]

We have developed a method for analyzing the (hyper)polarizabilities of open-shell molecular systems. This method employs the (hyper)polarizability densities based on the natural orbitals and occupation numbers, which enables us to analyze the contributions of odd electrons having various open-shell (diradical) characters. Within broken-symmetry, i.e., spin-unrestricted, single-determinant molecular orbital and density functional theory approaches, we can also remove the spin contamination effects on these quantities through spin projection. To do that, an approximate spin projected method has been elaborated and applied to the analysis of the (hyper)polarizability of multi-radical systems. As examples, typical open-shell singlet systems, 1,3-dipoles and rectangular graphene nanoflakes, are examined. [ABSTRACT FROM AUTHOR]