What Types of Noncovalent Bonds Stabilize Dimers (XCP)2, for X = CN, Cl, F, and H?

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out in search of equilibrium dimers on (XCP)2 potential energy surfaces, for X = CN, Cl, F, and H. Five equilibrium dimers with D∞h, C∞v, Cs, C2h, and C2 symmetries exist on the (ClCP)2 potential energy surface, four on the (FCP)2 and (HCP)2 surfaces, and three on the (NCCP)2 surface. These dimers are stabilized by traditional halogen, pnicogen, and tetrel bonds, and one of them by a hydrogen bond. The binding energies of the dimers (XCP)2 vary from 3.0 to 22.0 kJ·mol-1, with the strongest and weakest bonds found for complexes on the (NCCP)2 surface. The binding energies of the linear D∞h and C∞v dimers on each surface differ by no more than 1.0 kJ·mol-1, except for (NCCP)2, which has D∞h and C∞v complexes with binding energies of 3.0 and 11.0 kJ·mol-1, respectively. The highly symmetric complexes with D∞h and C∞v symmetry are found on all surfaces and are the most weakly bound complexes on each surface. The structures of these dimers, the nature and strengths of charge-transfer interactions, the molecular graphs, and the molecular electrostatic potentials are useful for determining the type of intermolecular bond that stabilizes the dimers. EOM-CCSD spin-spin coupling constants 1pJ(P-P) for complexes with P···P pnicogen bonds and D∞h symmetry are the largest coupling constants, ranging from 119 to 170 Hz. These increase with decreasing distance and follow a second-order trendline. The nature of the spin-spin coupling constants of these complexes is consistent with the type of noncovalent bond that stabilizes the dimers.