Halogen bonding involving CO and CS with carbon as the electron donor

Abstract

MP2/aug'-cc-pVTZ calculations have been carried out to investigate the halogen-bonded complexes formed when CO and CS act as electron-pair donors through C to ClF, ClNC, ClCl, ClOH, ClCN, ClCCH, and ClNH2. CO forms only complexes stabilized by traditional halogen bonds, and all ClY molecules form traditional halogen-bonded complexes with SC, except ClF which forms only an ion-pair complex. Ion-pair complexes are also found on the SC: ClNC and SC: ClCl surfaces. SC: ClY complexes stabilized by traditional halogen bonds have greater binding energies than the corresponding OC: ClY complexes. The largest binding energies are found for the ion-pair SC-Cl+:-Y complexes. The transition structures which connect the complex and the ion pair on SC: ClNC and SC: ClCl potential surfaces provide the barriers for inter-converting these structures. Charge-transfer from the lone pair on C to the σ-hole on Cl is the primary charge-transfer interaction stabilizing OC: ClY and SC: ClY complexes with traditional halogen bonds. A secondary charge-transfer occurs from the lone pairs on Cl to the in-plane and out-of-plane π antibonding orbitals of ClY This secondary interaction assumes increased importance in the SC: ClNH2 complex, and is a factor leading to its unusual structure. C-O and C-S stretching frequencies and 13C chemical shieldings increase upon complex formation with ClY molecules. These two spectroscopic properties clearly differentiate between SC: ClY complexes and SC-Cl+:-Y ion pairs. Spin-spin coupling constants 1xJ(C-Cl) for OC: ClY complexes increase with decreasing distance. As a function of the C-Cl distance, 1xJ(C-Cl) and 1J(C-Cl) provide a fingerprint of the evolution of the halogen bond from a traditional halogen bond in the complexes, to a chlorine-shared halogen bond in the transition structures, to a covalent bond in the ion pairs.