N^…C and S^…S Interactions in Complexes, Molecules, and Transition Structures HN(CH)SX:SCO, for X = F, Cl, NC, CCH, H,...

Ab initio Møller-Plesset perturbation theory (MP2)/aug'-cc-pVTZ calculations have been carried out in search of complexes, molecules, and transition structures on HN(CH)SX:SCO potential energy surfaces for X = F, Cl, NC, CCH, H, and CN. Equilibrium complexes on these surfaces have C₁ symmetry, but these have binding energies that are no more than 0.5 kJ·mol^-1 greater than the corresponding C_s complexes which are vibrationally averaged equilibrium complexes. The binding energies of these span a narrow range and are independent of the N-C distance across the tetrel bond, but they exhibit a second-order dependence on the S-S distance across the chalcogen bond. Charge-transfer interactions stabilize all of these complexes. Only the potential energy surfaces HN(CH)SF:SCO and HN(CH)SCl:SCO have bound molecules that have short covalent N-C bonds and significantly shorter SS chalcogen bonds compared to the complexes. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants ^1tJ(N-C) for the HN(CH)SX:SCO complexes are small and exhibit no dependence on the N-C distance, while ^1cJ(S-S) exhibit a second-order dependence on the S-S distance, increasing as the S-S distance decreases. Coupling constants ^1tJ(N-C) and ^1cJ(S-S) as a function of the N-C and S-S distances, respectively, in HN(CH)SF:SCO and HN(CH)SCl:SCO increase in the transition structures and then decrease in the molecules. These changes reflect the changing nature of the NC and SS bonds in these two systems.