A new relativistic four-component density functional approach for calculations of NMR shielding tensors has been developed and implemented. It is founded on the matrix formulation of the Dirac-Kohn-Sham (DKS) method. Initially, unperturbed equations are solved with the use of a restricted kinetically balanced basis set for the small component. The second-order coupled perturbed DKS method is then based on the use of restricted magnetically balanced basis sets for the small component. Benchmark relativistic calculations have been carried out for the (1)H and heavy-atom nuclear shielding tensors of the HX series (X=F,Cl,Br,I), where spin-orbit effects are known to be very pronounced. The restricted magnetically balanced basis set allows us to avoid additional approximations and/or strong basis set dependence which arises in some related approaches. The method provides an attractive alternative to existing approximate two-component methods with transformed Hamiltonians for relativistic calculations of chemical shifts and spin-spin coupling constants of heavy-atom systems. In particular, no picture-change effects arise in property calculations.

The accurate first-principles calculation of relative energies of transition metal complexes and clusters is still one of the great challenges for quantum chemistry. Dense lying electronic states and near degeneracies make accurate predictions difficult, and multireference methods with large active spaces are required. Often density functional theory calculations are employed for feasibility reasons, but their actual accuracy for a given system is usually difficult to assess (also because accurate ab initio reference data are lacking). In this work we study the performance of the density matrix renormalization group algorithm for the prediction of relative energies of transition metal complexes and clusters of different spin and molecular structure. In particular, the focus is on the relative energetical order of electronic states of different spin for mononuclear complexes and on the relative energy of different isomers of dinuclear oxo-bridged copper clusters.

Most of the tools for quantifying the extent of chemical bonding between two atoms are quantum-chemical in nature. None of them are unambiguous, however, and different analyses can lead to conflicting interpretations, even concerning the most fundamental question of whether or not atoms are linked by a chemical bond. [1,2] One of the indicators that can be probed experimentally is the indirect spin-spin coupling constant (SSCC). For instance, observation of spinspin coupling across hydrogen bonds [3] has been taken as evidence for covalent contributions to this kind of bonding. For atoms within the same molecule that are close in space, but not linked through a direct formal bond, the question arises, how much of the observed coupling is transmitted through a succession of bonds that eventually links them ("through-bond coupling"), and how much is due to interaction through the overlap of lone pairs ("through-space coupling"). Again, a variety of quantum-chemical tools have been developed to address this question.A rigid scaffold that is used to achieve such spatial proximity is the naphthalene framework, in which substituents in the peri (1,8) positions have a typical separation of around 3 . [4] Through-space J( 19 F, 19 F) SSCCs, long known for their distance dependence, [5a] have been studied in some detail in peri-difluoronaphthalenes. [5b,c] Similarly, J( 31 P, 31 P) values in peri-bis(phosphino)naphthalenes have been attributed to through-space coupling, [6] and J( 77 Se, 77 Se) values in peri-bis(seleno) derivatives have been analyzed in detail through quantum-chemical computations. [7] In systematic studies of naphthalene (N) and acenaphthene (A) derivatives with pnictogen and chalcogen atoms in the peri positions, it became apparent that for the heavier congeners, steric repulsion is partly counterbalanced by attractive interactions. In particular with Te substituents, formally nonbonded, "across-the-bay" distances are significantly shorter than the sum of the van-der-Waals radii, which has been traced back to weak donor-acceptor interactions and the onset of 3-center-4-electron (3c4e) bonding. [8] For instance in N1 and A1 (Scheme 1), Te···Te distances of around 3.3 are observed in the solid state (ca. 0.7 below the sum of the vdW radii), and Wiberg bond indices [9] (WBIs, a measure for the covalent character of a bond, approaching WBI = 1 for a true single bond) of around 0.15 have been computed. Slightly larger WBIs of approximately 0.18 have been obtained for cationic methylated species N2 and A2, despite a slightly longer Te-Te separation (ca. 3.4 ). [10] These unsymmetrical systems show remarkably large J( 125 Te, 125 Te) SSCCs, formally 4 J values, of 1093.0 Hz and 945.8 Hz in N2 and A2, respectively. [10] We now report even larger couplings in N1 and A1, along with computational conformational analysis underlining the potential of this property as a structural and interpretative method.The computed (ZORA-SO/BP//B3LYP level) [11] J( 125 Te, 125 Te) SSCCs in N2 and A2 (1490 and 1377 Hz, respec...

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Across the bay: J(125Te,125Te) spin–spin coupling is a highly sensitive probe into the electronic and geometric structure of 1,8‐peri‐substituted naphthalene tellurium derivatives. The coupling is related to the onset of multicenter bonding in these systems.