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

Abstract

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 Cs 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 S…S 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 N…C and S…S bonds in these two systems.

Keywords

• noncovalent interactions
• structures
• binding energies
• charge-transfer energies
• EOM-CCSD spin-spin coupling constants