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...

Adenine, an essential building block of nucleic acids present in all living systems, can occur in several tautomeric forms. The phenomenon of tautomerism can be investigated by several experimental methods, including nuclear magnetic resonance. In this study, long-range (1)H-(13)C and (1)H-(15)N coupling constants for N-alkyl derivatives related to four tautomers of adenine are investigated in DMSO and DMF solutions. To investigate the structural dependence of the coupling constants and to understand how polarization propagates in the system, Fermi contact (FC) terms were calculated for the individual isomers and analyzed by using density functional theory (DFT), and the coupling pathways were visualized using real-space functions. The coupling electron deformation densities (CDD) of several (1)H-X (X = (13)C, (15)N) pairs are evaluated and compared. In order to analyze the CDD in more detail, a new approach to break down the CDD into contributions from Boys or Pipek-Mezey localized molecular orbitals (LMOs) has been developed. A similar approach has been applied to split the value of the FC contribution to the J coupling into the LMO contributions. On the basis of chemical concepts, the contributions of sigma-bonds, pi-electrons, and lone pairs of electrons are discussed. The lone pair of electrons at the nitrogen atom contributes significantly to the (1)H-C horizontal line(15)N coupling, whereas the (1)H-C=N-(13)C coupling is affected in a somewhat different way. Surprisingly, the contribution of the intervening C horizontal lineN bond to the FC term for (1)H-C=(15)N coupling originates exclusively in sigma-electrons, with a vanishingly small contribution calculated for the pi-electrons of this fragment. This behavior is rationalized by introducing the concept of "hard and soft J elements" derived from the polarizability of the individual components.