The structural dependence of the coupling constant in a series of [L3Cu(μ-C2O4)CuL3]2+complexes is analyzed by means of ab initio difference-dedicated configuration interaction (DDCI2) calculations on the model (μ-oxalato)bis[triamminecopper(II)] cation, [(NH3)6Cu2(μ-C2O4)]2+, in which the nitrogen-coordinated ligands have been substituted by NH3. Two types of geometrical structures have been considered: three different C 2 h geometries and four crystallographic centrosymmetric geometries taken from [(Et5dien)2Cu2(μ-C2O4)](BPh4)2 and [(Et5dien)2Cu2(μ-C2O4)](PF6)2 (Et5dien = 1,1,4,7,7-pentaethyldiethylenetriamine), [(tmen,2-MeIm)2Cu2(μ-C2O4)](PF6)2 (tmen = N,N,N‘,N‘-tetramethylethylenediamine and 2-MeIm = 2-methylimidazole), and [(dien)2Cu2(μ-C2O4)](ClO4)2 (dien = diethylenetriamine). The results show that the antiferromagnetic coupling is strongly underestimated when pure DDCI2 calculations are performed, but when the CI space includes the relaxation of the oxalato-copper charge transfer, quantitative agreement with the experimental results is reached with an error smaller than 5 cm-1. The role of the external ligands in the model is also discussed by means of broken symmetry DFT calculations. At this level of theory, a very different influence of the ligands is predicted by different exchange-correlation functionals. Therefore, the use of DFT to investigate this effect should be considered with caution.
The behavior of a verdazyl-based radical bound to open-shell transition metal ions in the structurally and magnetically characterized [M(hfac)2imvd(o)] (M = Mn, Ni; hfac = (1,1,1,5,5,5)hexafluoroacetylacetonate; imvd(o) = 3-(2'-imidazolyl)-1,5-dimethyl-6-oxoverdazyl) complexes is rationalized using ab initio wave-function-based calculations analysis. The calculated exchange coupling constants J (H = -J(s(M) x s(imvd(o)); J(Mn)(calcd) = -63 cm(-1), J(Ni)(calcd) = 205 cm(-1)) are in excellent agreement with the experimental ones (J(Mn)(exp) = -63 cm(-1), J(Ni)(exp) = 193 cm(-1)). Even though both rings are involved through the binding mode of the imvd(o) radical, the spin density remains essentially localized on the nitrogen-rich ring. The singularity stems from its bidentate coordinating character. The analysis of the correlated wave function suggests that the verdazyl-based radical acts as a pi* donor ligand which allows ligand-to-metal charge transfer and excludes metal-to-ligand charge transfer. This reflects the weak covalent character of the M-imvd(o) pi coordination bond. From a magnetic point of view, the through-space exchange governs the ferromagnetic character in the Ni derivative up to 153 cm(-1) as expected from a description limited to the magnetic orbitals. Nevertheless, the CI expansion displays the participation of excited doublet and quartet states (spin polarization) on the verdazyl moiety which leads to a significant additional ferromagnetic contribution (52 cm(-1)). In the [Mn(hfac)2imvd(o)] analogue, the antiferromagnetic contribution arising from kinetic exchange is only one-third of the observed exchange coupling constant. It is necessary to introduce dynamical correlation effects to quantitatively recover the exchange interaction in this compound. Since the pi* donor and spin-polarized characters of the verdazyl moiety dominate over the negligible polarizability of the imidazole part, it is concluded that the noninnocent nature of the imvd(o) radical is held by the verdazyl ring part.
In dense interstellar environments, Polycyclic Aromatic Hydrocarbons (PAHs) are likely to condense onto or integrate into water ice mantles covering dust grains. Understanding the role of ice in the photo-induced processes involving adsorbed PAHs is therefore a key issue in astrochemistry. This requires (i) the knowledge of PAH-ice interactions, i.e. PAH-ice adsorption energies and local structures at the PAH-ice interface, as well as (ii) the understanding of the fate of electrons in the PAH-ice system upon excitation. Regarding (i), in this work, we determined the lowest energy structures of PAH-ice systems for a variety of PAHs ranging from naphthalene to ovalene on three types of ice - crystalline (Ih and Ic) and amorphous (low density) - using an explicit description of the electrons and a finite-sized system. The electronic structure was determined using the Self Consistent Charge Density Functional based Tight Binding (SCC-DFTB) scheme with modified Mulliken charges in order to ensure a good description of the studied systems. Regarding (ii), the influence of the interaction with ice on the Vertical Ionisation Potentials (VIPs) of the series of PAHs was determined using the constrained SCC-DFTB scheme benchmarked against correlated wavefunction results for PAH-(HO) (n = 1-6, 13) clusters. The results show a deviation equal, at most, to ∼1.4 eV of the VIPs of PAHs adsorbed on ice with respect to the gas phase values. Our results are discussed in the light of experimental data and previous theoretical studies.
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