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A new family of heterometallic pentanuclear complexes of formulas [Mn(IV)Mn(2)(III)Ln(2)(III)O(2)(benz)(4)(mdea)(3)(NO3)(2)(MeOH)] (Ln = Dy (1-Dy), Tb (2-Tb), Gd (3-Gd), Eu (4-Eu), Sm (5-Sm), Nd (6-Nd), Pr (7-Pr); benz(H) = benzoic acid; mdeaH(2)= N-methyldiethanolamine) and [Mn(IV)Mn(2)(III)Ln(2)(III)O(2)(o-tol)(4)(mdea)(3)(NO3)(2)(MeOH)] (Ln = Gd (8-Gd), Eu (9-Eu); o-tol(H) = o-toluic acid) have been isolated and structurally, magnetically, and theoretically characterized. dc magnetic susceptibility measurements reveal dominant antiferromagnetic magnetic interactions for each complex, except for 2-Tb and 3-Gd, which reveal an upturn in the chi T-M product at low temperatures. The magnetic interactions between the spin centers in the Gd derivatives, 3-Gd and 8-Gd, which display markedly different chi T-M vs T profiles, were found to be due to the interactions of the Gd-III-Gd-III ions which change from ferromagnetic (3-Gd) to antiferromagnetic (8-Gd) due to structural differences. ac magnetic susceptibility measurements reveal a nonzero out-of-phase component for 1-Dy and 7-Pr, but no maxima were observed above 2 K (H-dc = 0 Oe), which suggests single-molecule magnet (SMM) behavior. Out-of-phase signals were observed for complexes 2-Tb, 4-Eu, 8-Gd, and 9-Eu, in the presence of a static dc field (H-dc = 2000, 3000 Oe). The anisotropic nature of the lanthanide ions in the benzoate series (1-Dy, 2-Tb, 5-Sm, 6-Nd, and 7-Pr) were thoroughly investigated using ab initio methods. CASSCF calculations predict that the origin of SMM behavior in 1-Dy and 7-Pr and the applied field SMM behavior in 2-Tb does not solely originate from the single-ion anisotropy of the lanthanide ions. To fully understand the relaxation mechanism, we have employed the Lines model to fit the susceptibility data using the POLY_ANISO program, which suggests that the zero-field SMM behavior observed in complexes 1-Dy and 7-Pr is due to weak Mn-III/IV-Ln(III) and Ln(III)-Ln(III) couplings and an unfavorable Ln(III)/Mn-III/Mn-IV anisotropy. In complexes 4-Eu, 8-Gd, and 9-Eu ab initio calculations indicate that the anisotropy of the Mn-III Cions solely gives rise to the possibility of SMM behavior. Complex 7-Pr is a Pr(III)-containing complex that displays zero-field SMM behavior, which is rare, and our study suggests the possibility of coupling weak SOC lanthanide metal ions to anisotropic transition-metal ions to derive SMM characteristics; however, enhancing the exchange coupling in {3d-4f} complexes is still a stubborn hurdle in harnessing new generation {3d-4f} SMMs.

A series of trinuclear Ln(3) complexes (Ln(III) = Yb (1), Er (2), Dy (3) and Gd (4)) were prepared from the trischelate bidentate ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT). 1 and 2, exhibited field-induced single-molecule-magnet (SMM) behavior with estimated U-eff values of 21.30 and 13.86 K, respectively. Complex 3 behaved as a SMM even at zero field, and two different thermally assisted relaxation processes were detected with U-eff values of 29.6 K (fast relaxation process, FR) and 69 K (slow relaxation process, SR) due to the existence of two magnetically different D-III centers in the molecule. Ab initio studies reveal that all the Dy3+ centers have almost an Ising ground state. The local anisotropy axes are not coplanar but form angles with the Dy3 plane in the range 58-78 degrees. The magnetic interaction between the anisotropic Dy3+ ions is antiferromagnetic in nature and very weak in magnitude. However, due to the extreme feebleness of the magnetic interaction with regard to the local excitation energies, the magnetization blockade is most probably of single -ion origin. Calculations support the existence of two relaxation processes, which take place through the first excited state following an Orbach/Raman mechanism. Finally, for complex 4, the magnetocaloric effect was simulated using the magnetic parameters extracted from the fit of the magnetization and susceptibility data and demonstrated that the simulated -Delta S-m values were almost coincident with those extracted from the integration of the field dependence of the magnetization. The simulated MCE value at 2 K and 5 T (20.46 J kg(-1) K-1) makes complex 4 an attractive candidate for cryogenic magnetization.

Extensive ab initio CASSCF/RASSI-SO/SINGLE_ANISO/POLY_ANI-SO calculations have been undertaken on eight structurally similar previously synthesized [Cu-II(L)(C3H6O)Ln(III)(NO3)(3)] (Ln = Dy (1), Tb (3), Ho (5), and Er (7)) and {(VO)-O-IV(L)(C3H6O)Ln(III)(NO3)(3)} (Ln = Dy (2), Tb (4), Ho (6), and Er (8)) (here H2L = N,M-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dirnethylpropane) complexes (crystal structures reported earlier). Our estimated exchange interactions (J) using the Lines model for complexes 1-8 (1.55 cm(-1), 0.15 cm(-1), 5.30 cm(-1), 0.06 cm(-1), 1.05 cm(-1), -0.18 cm(-1), 0.24 cm(-1), and 0.02 cm(-1) for complexes 1-8 respectively) match well with the experimental values (HE-EPR and pulse magnetization technique) reported earlier and offer confidence in the methodology employed. We have established the mechanism of magnetic coupling for this series to rationalize the observation that LnCu complexes are strongly coupled compared to LnV complexes. Besides, the results procured based on the BS-DFT method imply a crucial role of overlap between the 3d and 4f orbitals, the formally empty Sd/6s/6p orbitals of Ln(III) ion in the exchange coupling mechanism. To probe the origin/absence of magnetization relaxation observed in these complexes 1-8, both the single ion and the exchange anisotropy are analyzed. Our calculations reveal that stronger exchange interaction quenches the quantum tunnelling of magnetization behavior in these complexes; however, for LnV complexes the exchange interaction was too small to offer a large blockade barrier. In the quest to obtain a stronger exchange interaction, we have assessed several models and have developed a magneto-structural correlation. An antagonizing behavior between the J(CuDy) and U-cal values are noted for the Dy-O-O-Cu dihedral angle correlation developed on complex 1. This highlights the subtle nature of the magnetic anisotropy in this class of complexes and postulates that both the single-ion anisotropy and the exchange interaction are needed to be targeted simultaneously to achieve a new generation {3d-4f} single molecule magnets (SMM).

A coordination complex family comprising of six new dinuclear symmetric lanthanide complexes, namely, [Ln(2)(L-x)(2)(L')(2)(CH3OH)(2)]center dot yG (H2Lx: three related yet distinct Schiff-base linkers; x = 1-3, according to the nomenclature of the Schiff-base linker employed herein. HL': 2,6-dimethoxyphenol. yG refers to crystallographically assigned guest solvent species in the respective complexes; y = number of solvent molecules; Ln(III) = Dy/Gd) were isolated employing a mixed-ligand strategy stemming out of a strategic variation of the functionalities introduced among the constituent Schiff-base linkers. The purposeful introduction of three diverse auxiliary groups with delicate differences in their electrostatic natures affects the local anisotropy and magnetic coupling of Ln(III) ion environment in the ensuing Ln(2) dinuclear complexes, consequentially resulting into distinctly dynamical magnetic behaviors among the investigated new-fangled family of isotypic Ln(2) complexes. Among the entire family, subtle alterations in the chemical moieties render two of the Dy-2 analogues to behave as single molecule magnets, while the other Dy-2 congener merely exhibits slow relaxation of the magnetization. The current observation marks one of the rare paradigms, wherein magnetic behavior modulation was achieved by virtue of the omnipresent influence of subtly tuned linker functionalities among the constituent motifs of the lanthanide nanomagnets. To rationalize the observed difference in the magnetic coupling, density functional theory and ab initio calculations (CASSCF/RASSI-SO/POLY_ANISO) were performed on all six complexes. Subtle difference in the bond angles leads to difference in the J values observed for Gd-2 complexes, while difference in the tunnel splitting associated with the structural alterations lead to variation in the magnetization blockade in the Dy-2 complexes.

Low-spin ground states and low-lying excited states of higher spin were investigated for four pyrene oxoverdazyl monoradicals 14 and eight pyrene dioxoverdazyl diradicals 512. The ground states for quartet and quintet spin symmetries that are in reality excited states were found in the region of 565-775 nm above the respective electronic ground states. We calculated the adiabatic magnetic exchange coupling constant in the electronic ground state of each isolated biradical (5-12) by unrestricted density functional theory. A number of hybrid functionals such as B3LYP, PBE0, M06, and M06-2X were used. We also used range-separated functionals such as LC-omega PBE and omega B97XD to compare their effects on the coupling constant and the relative energy of the high-spin state. Molecular geometries were optimized for the doublet and quartet spin states of every monoradical (14), and the broken symmetry and triplet solutions were optimized for every biradical (512), by systematically using 6-311G, 6-311G(d,p), and 6-311++G(d,p) basis sets with each functional. The geometry of each quintet diradical (512) was optimized using 6-311G basis set. B3LYP produced the best spin values. The excited state (quartet or quintet)ground state energy difference (Delta E) increases in the presence of para-phenylene connectors. These energy differences were predicted here. The nature of spin coupling and consequently the ground state spin agree with spin alternation rule and the calculated atomic spin population. The adiabatic coupling constants were predicted for the biradicals (5-12) in their electronic ground states. Electron paramagnetic resonance parameters were determined at 6-311++G** level for the ground state and the quartet state of 1 and compared with the available experimental data. Low-lying excited states were found for the radical center (oxoverdazyl), pyrene, molecule 1, and diradical 5 by time-dependent density functional theory (TDDFT) method using B3LYP hybrid, 6-311++G(d,p) basis set, and the molecular geometry in the electronic ground state. Data from these calculations were used to discuss possible mechanisms for the achievement of the high-spin (excited) states in 1 and 5 and to predict a similar outcome for radicals 24 and 612 upon excitation. A comprehensive mechanism for the first excitation is proposed here. In particular, we show that the initial excitation of 1 involves large contributions from mixed transitions between pyrene and oxoverdazyl moieties, whereas the initial excitation of 5 is basically that of only the pyrene fragment. Subsequent internal conversion and intersystem crossing are likely to lead to the high-spin states of lower energy. Sample spin-flip TDDFT calculations were also done to confirm the energetic location and composition of the quartet state of 1 and the quintet state of 5.

Electronic and magnetic properties of silicon-substituted meta-xylylene polyradicals containing meta-phenylene groups as couplers and silicon atoms in >Si-H groups as radical sites are quantum chemically investigated. The polyradicals are found to be stable with alternately periodic structure. They have high-spin ground states and show ferromagnetic interaction among all pairs of radical sites. The silicon sites are somewhat creased. A periodic calculation on the dimeric unit predicts the long-chain polyradical to be a one-dimensional ferromagnet. Band energy calculation gives a large gap between virtual and valence bands. The large energy gap owes its origin to the radical sites being out of plane of the phenylene rings, a twist between successive monomer units, and the difference between the p(z) orbital energy in carbon and silicon. The polyradical chains are electron insulators unlike substituted polysilene chains, though they are predicted to be good ferromagnets.

We have investigated unsubstituted and methyl substituted polyradical chains of meta-xylylene by using density functional theory-broken symmetry methodology (DFT-BS). Optimization of geometry in the high-spin and low-spin states have been done at B3LYP/6-31G(d,p) and M06-2X/6-31G(d,p) levels in unrestricted methodology. Single-point calculations on the high-spin optimized geometries have been done by using the 6-311G(d,p) basis set. Each polyradical has been found to be nonplanar with a high-spin ground state. Each has a coupling constant larger than thermal energy. For each group of polyradicals, the coupling constant has been found to exponentially decrease with increase in the number of phenylene groups. The B3LYP infinite chain limit has been estimated for both the unsubstituted and substituted polyradicals. The individual inter-radical-site coupling constants have been estimated for the triradicals and tetraradicals using HDVV Hamiltonian in ORCA 3.0.1 code. These are also generally large and positive, revealing a strong intersite ferromagnetic interaction. The intersite coupling constant too decreases with increasing distance between the radical centers. Finally, we have used CRYSTAL09 package for calculations on the infinitely long one-dimensional and periodic polyradical chains. The coupling constants estimated from the periodic calculations are quite large at about 500 cm(-1) and somewhat greater than the limiting values calculated for the polyradicals with an increasing number of phenylene groups. This happens as the individual polyradicals of increasing size progressively deviate from periodicity, and thereby lessens the strength of through-bond spin-spin coupling. The calculated band gap of similar to 4.5 eV indicates that the infinitely long one-dimensional chains must be ferromagnetic and electron insulators.

The redox systems [Ru(L)(bpy)(2)](k), [Ru(L)(2)(bpy)](m), and [Ru(L)(3)](n) containing the potentially redox-active ligand 9-oxidophenalenone = L- were investigated by spectroelectrochemistry (UV-vis-near-IR and electron paramagnetic resonance) in conjunction with density functional theory (DFT) calculations. Compounds [Ru(L-)(bpy)(2)]ClO4 ([1]ClO4) and [Ru(L-)(2)(bpy)]ClO4 ([2]ClO4) were structurally characterized. In addition to establishing electron-transfer processes involving the Ru-II/Ru-III/Ru-IV. and bpy(0)/bpy(center dot-) couples, evidence for the noninnocent behavior of L- was obtained from [Ru-IV(L-center dot)(L-)(bpy)](3+), which exhibits strong near-IR absorption due to ligand-to-ligand charge transfer. In contrast, the lability of the electrogenerated anion [Ru(L)(2)(bpy)](-) attributed to a resonance situation [Ru-II(L center dot 2-)(L-)(bpy)](-)/[Ru-II(L-)(2) (bpy(center dot-))](-), as suggested by DFT calculations.

The Schlenk diradical has been known since 1915. After a detailed experimental work by Rajca, its magnetic nature has remained more or less unexplored. We have investigated by quantum chemical calculations the nature of magnetic coupling in 11 substituted Schlenk diradicals. Substitution has been considered at the fifth carbon atom of the meta-phenylene moiety. The UB3LYP method has been used to study 12 diradicals including the original one. The 6-311G(d,p) basis set has been employed for optimization of molecular geometry in both singlet and triplet states for each species. The singlet optimization has led to the optimization of the broken-symmetry structure for 10 species including the unsubstituted one. This development makes it possible to carry out further broken symmetry calculations in two ways. The triplet calculation has been done using 6-311++G(d,p) basis set and the optimized triplet geometry in both procedures. The broken symmetry calculations have used the optimized geometries of either the triplet states or the broken symmetry solutions. The first method leads to the prediction of electron paramagnetic resonance (EPR) compatible magnetic exchange coupling constant (J) in the range 517-617 cm(-1). A direct optimization of the broken symmetry geometry gives rise to a lower estimate of J, in the range of 411-525 cm(-1) and compatible with macroscopic Curie studies. The calculated J for the unsubstituted Schlenk diradical is 512 cm(-1) that can be compared with 455 cm(-1) estimated by Rajca. In both cases, introduction of groups with +M and +I effects (Ingold's notation) decreases the J value from that for the unsubstituted Schlenk diradical while -I and -M groups at the same position increases J. These trends have been explained in terms of Hammett constants, atomic spin densities, and dihedral angles.

We have investigated the magnetic properties of four recently synthesized stable TMM-type nitroxide diradicals. Four new diradicals are proposed by tailoring one of the species in such a way that both conjugation and planarity increase. As a remarkable consequence, the intramolecular ferromagnetic exchange interaction was found to be quite high in the proposed radicals. The calculated coupling constants were in the range of +102 to +140 cm(-1). The MO and spin density analysis are provided to interpret the exchange interactions. We observed the existence of intramolecular pi-pi-interactions for the species 2. This slightly increased the J value by shortening the length of the spacer between the two spin sources.

The ground-state spins of seven diradicals belonging to the fused ring system have been investigated by ab initio restricted and unrestricted formalisms. The systems under study are (1) 4-oxy-2-naphthalenyl methyl, (2) 1,8-naphthalenediylbis(methyl), (3) 8-imino-1-naphthalenyl methyl, (4) 1,8-naphthalenediylbis(amidogen), (5) 8-methyl-1-naphthyl carbene, (6) 8-methyl-1-naphthalenyl imidogen, and (7) 8-methyl-1-naphthyl diazomethane. Out of the seven molecules, only I was theoretically investigated earlier. To our knowledge, for 2-7, this work represents the first ab initio investigation. A variety of basis sets have been employed in these calculations. For each spin state, the molecular geometry has been fully optimized at the unrestricted Hartree-Fock (UHF) level using the STO-3G, 4-31G, 6-311G(d), and 6-311G(d,p) basis sets. The UHF optimized geometries have been used for Moller-Plesset (MP) and coupled cluster (CC) calculations as well as the density functional (UB3LYP) treatment. Results in the unrestricted formalism have been given only at UHF and UB3LYP levels for the 6-311G(d) basis. The UHF calculations yield an unrealistically large singlet-triplet (S-T) splitting. Splittings calculated with different bases disagree seriously. The S-T gap is smaller in the split-valence bases. The basis set truncation error can be considerably overcome by calculations involving electron correlation. For these diradicals, any meaningful result would require larger bases with polarization functions. Apart form this difficulty, the optimized geometry turned out to be highly spin-contaminated. The spin-contamination can be significantly reduced by the density functional UB3LYP treatment. Nevertheless, for most of the diradicals, the UB3LYP method did not yield a systematic trend. To avoid spin contamination completely, we have repeated computations in the restricted (open-shell) Hartree-Fock framework. Geometry optimizations were carried out using STO-3G, 6-311G(d), and 6-311G(d,p) bases at the R(O)HF level and 6-311G(d,p) basis at the R(O)B3LYP level for each spin state. The R(O)B3LYP/6-311G(d,p) optimized geometry yields the best total energy for each spin state and hence the most reliable S-T energy difference. Molecules 1-6 are found as ground-state triplets. The calculated results are in agreement with the available experimental findings. Molecules 3 and 7 have widely different geometries in the singlet and triplet states. The calculations using 6-311G(d) and 6-311G(d,p) basis sets show that in molecule 3 the substituents of naphthalene are -NH2 and -CH in singlet but -NH and -CH2 in triplet. The two optimized geometries are tautomeric forms. Molecule 7 is expected to be either a ground-state triplet with a very little S-T gap or a ground-state singlet. This prediction is borne out by the computed results. The R(O)B3LYP/6-311G(d,p) calculation yields a S-T splitting of -21.9 kcal mol(-1). The singlet state becomes stabilized by forming an additional condensed ring. The UHF spin density plots obtained from the 4-3 1 G optimized geometries manifest the phenomenon of spin alternation in the ground state.