Spin-spin coupling constants transmitted through Ir-H...H-N dihydrogen bonds.

Relativity matters: Calculations of NMR shielding tensors and spin-spin coupling constants transmitted through Ir-H...H-N dihydrogen bonds are presented. The picture shows one of the simplified models employed. It is shown that the spin-orbit relativistic effects influence the NMR shielding constants far more than the spin-spin coupling constants.We present calculations of NMR shielding tensors and spin-spin coupling constants transmitted through Ir-H...H-N dihydrogen bonds. For this purpose, three six-coordinated complexes of iridium have been selected as models of heavy metal complexes and their NMR properties have been calculated within the DFT-ZORA (density functional theory zeroth-order regular approximation) methodology. The influence of intramolecular interactions between hydrogen atoms from IrH(3) and NH(2) groups (including both single dihydrogen bonding and bifurcated hydrogen bonding) on the NMR properties is discussed and the results are compared with the experimental observations. In complexes where dihydrogen bonding occurs, the calculated value of "through-space" (1h)J(H(a)H(b)) is in the range 1.6-7.9 Hz [depending on the model, but with R(H(a)-H(b))<2.0 A], while the experimental values are 2-5 Hz for similar H(a)--H(b) distances. The (2h)J(NH) coupling is also sizeable, ranging from approximately -5.1 Hz for dihydrogen bonds of 2 A up to -7.1 Hz for very short (1.6 A) dihydrogen bonds, and should therefore be experimentally accessible when using (15)N-labelled compounds. The dihydrogen-bond transmitted spin-spin coupling constants (1h)J(HH) and (2h)J(NH) and the shielding tensor of the H(a) atom are the most sensitive probes of the H(a)-H(b) distance, which potentially makes them attractive tools to determine the structure of molecules. The main conclusions are qualitatively similar to those drawn from nonrelativistic calculations in small inorganic complexes, and the spin-orbit relativistic effects influence the NMR shielding constants far more than the spin-spin coupling constants.