This paper is concerned with the first two-step spin crossover presented by a polynuclear molecular compound, viz., the dinuclear iron(II) complex [Fe(bt)(NCS)2]2bpym, where bt stands for 2,2'-bi-2-thiazoline and bpym for the bridging ligand 2,2'-bipyrimidine. The synthesis of the compound is described. Variable-temperature magnetic susceptibility and 57Fe Mossbauer spectrometry data provide evidence for an overall 5 = 2 (HS) ** S = 0 (LS) spin-crossover behavior. They show that the transition takes place in two steps. The so-called "step 1" and "step 2" are centered around 163 and 197 K, respectively. The former is more abrupt than the latter. The thermal variation of the quadrupole splitting of the HS doublet (A£qHS) confirms that the sample is made of a single structural phase and roughly accounts for the intramolecular processes LS,LS ** LS,HS (step 1) and LS,HS ** HS,HS (step 2). The differential scanning calorimetry diagram exhibits two peaks, a sharp one pointing at 164 K and a broad one with a maximum at 194 K. The overall enthalpy and entropy variations are found to be AH = 13.3 ± 0.5 kj mol™1 11and AS = 82 ± 6 J K™1 mol'1, respectively. The cornerstone of the theoretical approach which has been developed to account for a two-step transition in iron(II) dinuclear species is that the enthalpy HSq of the mixed-spin species SQ (S = spin singlet; Q = spin quintet) is not exactly halfway between the enthalpies Hss and Hqq of the like-spin species SS and QQ, respectively. It turns out that it can be written HSq = (Hss + Hqq)/2 + W. W ^0 is due to both electrostatic and vibronic effects. W > 0 makes the transition more abrupt; on the other hand, W < 0 may lead to a two-step transition.Actually, the two-step character is due to the synergistic effect of intramolecular interactions favoring SQ (W < 0) and intermolecular interactions favoring like-species domains (intermolecular interaction parameter y > 0). Our model leads to quite a satisfying fitting of the magnetic data with AH = 13.16 kJ mol™1, AS = 73.69 J K™1 mol™1, W = -478 J mol™1, and 7 = 2.572 kJ mol™1. Furthermore, using those energy parameters results in a simulation of the heat capacity versus temperature curve showing the main features of the experimental curve, in particular a sharp and intense peak around 163 K and a broader and less intense peak around 197 K.

Starting from the formula proposed by Gerloch and McMeeking in 1975, the electronic g-matrix is expressed as a sum of two matrices called Lambda and Sigma describing the orbital and spin contributions respectively. This approach is applied on benchmark diatomic and triatomic molecules, and on TiF3 and Cu(NH3)4(2+) using either CASPT2 or CCSD(T) methods to calculate the spin-free states and SO-RASSI to calculate spin-orbit coupling. Results compare very well to experimental data and to previous theoretical work; and, for each molecule, the anisotropy of the g-matrix is modeled by the mean of a few parameters.

The electronic structure and magnetic properties of neptunyl(VI), NpO2(2+), and two neptunyl complexes, [NpO2(NO3)3](-) and [NpO2Cl4](2-), were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal-field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin-orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g-factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g-factors were calculated for the ground and excited states. For [NpO2Cl4](2-), a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn-Sham DFT with standard functionals can produce reasonable g-factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward.

Electronic structures and magnetic properties of actinyl ions AnO2(n+) (An = U, Np, and Pu) and the equatorially coordinated carbonate complexes [UO2(CO3)3](5–), [NpO2(CO3)3](4–), and [PuO2(CO3)3](4–) are investigated by ab initio quantum chemical calculations. The complex [PuO2(NO3)3](−) is also included because of experimentally available g-factors and for comparison with a previous study of [NpO2(NO3)3](−) (Chem.—Eur. J. 2014, 20, 7994-8011). The results are rationalized with the help of crystal-field (CF)-type models with parameters extracted from the ab initio calculations, and with the help of natural orbitals and natural spin orbitals contributing to the magnetic properties and the unpaired spin distribution, generated from the spin–orbit wave functions. These orbitals resemble textbooklike representations of the actinide 5f orbitals. Calculated paramagnetic susceptibilities are used to estimate dipolar 13C chemical shifts for the carbonate ligands. Their signs and order of magnitude are compared to paramagnetic effects observed experimentally in NMR spectra. The results indicate that the experimental spectra are also influenced by contact shifts.

Selective memory: Using actinides in designing molecular nanomagnets could provide better performance and higher anisotropy barriers, owing to the peculiar properties of the 5f electron shell. Neptunocene is found to display an open magnetic hysteresis cycle at low temperatures (see picture), and interaction with the hyperfine degrees of freedom determines whether the magnetic relaxation is fast or slow at a given field value.

The bond distances and coordination numbers of the predominant Np(VII) complex in strongly alkaline solution have been determined using EXAFS transmission measurements. The stoichiometry and structure of NpO 4 (OH) 2 3has been deduced by combining these data with different structure models, mostly determined by using DFT based methods. The experimental and theory based distance Np(VII)-O oxo is 1.89 4 and 1.90 Å, respectively, whereas the Np(VII)-OHdistance is 2.32 6 and 2.33 Å, respectively. Theory based geometry and bond distances have been obtained also for other Np(VII) and Np(VI) complexes, NpO 2 (OH) 4 -, NpO 4 -, NpO 4 (OH) 2 4-, and NpO 2 (OH) 4 2-, NpO 4 2-. The "NpO 6 "unit has a square bipyramidal geometry both in NpO 4 (OH) 2 3-/4and in NpO 2 (OH) 4 1-/2-, albeit with some difference in bond distances. The close similarity in structure indicates that no major rearrangements are necessary on electron transfer between Np(VI) and Np(VII), a possible explanation for the stable and reproducible Np(VII)/Np(VI) redox potential observed in alkaline solution. The structure data indicate that new Np(VII) species may be identified by oxidation of Np(VI) solutions at lower hydroxide concentrations.

The first spectroscopic evidence of a double-bridged silicon molecule is reported. The submillimeterwave rotational spectrum of the disilyne S12H2 has been observed in a low-power silane plasma cooled at liquid-nitrogen temperature, with argon as buffer gas. These measurements led to the determination of the ro structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the 29 Si and 30 Si monosubstituted forms.PACS numbers: 33.10.Ev, 33.20.Bx, 35.20.Dp, 35.20.Pa There is presently a growing interest in the study of small unsaturated silicon compounds, related to the vapor deposition of amorphous silicon films. 1 " 3 Detection of some monosilicon intermediate species [SiH" (Ref. 4); SiH" + (Ref. 5), n<3] by high-resolution spectroscopy has already led to the development of nonintrusive diagnostic techniques of silane plasmas. 2,3,6 In contrast, experimental data on unsaturated disilicon hydrides are very scarce, although recent mass-spectrometry investigations 7 have shown that Si2H2 and Si2H4 can be abundant in SiH4 plasmas and therefore can play a key role in deposition processes (Refs. 1-3, and 8, and references therein).The study of these highly reactive species also has its own interest, for a fundamental comparison between carbon and silicon bonding properties. For example, ab initio calculations have shown that the simplest unsaturated hydrides SiiFh and Si2H4 exhibit geometries very different from the analogous C-containing molecules, and that the smaller basis sets are not adequate to properly describe the structures and the relative stabilities of the various isomers. 9 " 15 In particular, it has been shown that addition of Si d functions and of electron correlation makes the nonclassical bridged structure of the disilyne Si(H2)Si in its singlet ground state more stable than the silasilene F^SiSi classical structure, which is the minimum-energy structure at the HF/3-21G level. 14 The energy difference between the two structures is rather small, in the range of 7-13 kcal/mol. 9 " 12,14 In fact, bridged structures are well known in molecules involving electron-deficient atoms such as boron. 16 Nevertheless, very little is experimentally known regarding the existence of such three-center bonds in the group-IV compounds. Protonated acetylene, recently investigated by ir spectroscopy 17 and Coulomb-explosion experiments, 18 is the only example of such a situation. The nonclassical bridged form HC(H)CH + is calculated to be lower in energy than the classical form F^CCH* by about 4-7 kcal/mol, 19,20 which is consistent with the experiments. However, no complete structural information on C2H3 -1 " can be deduced from these experiments at the moment.In this Letter, we present the first spectroscopic evidence of the disilyne Si2H2, and the first determination of a bridged structure for a silicon-containing molecule.In the course of an investigation of a silane plasma by submillimeter-wave spectroscopy, some unidentified lines were observed a...

Two relaxation processes of the magnetization in an antiferromagnetically coupled Dy2 metallacrown-based complex.