FCl:PCX Complexes: Old and New Types of Halogen Bonds

Abstract

MP2/aug'-cc-pVTZ calculations have been performed to investigate the halogen-bonded complexes FCl:PCX, for X = NC, CN, F, H, CCH, CCF, CH(3), Li, and Na. Although stable complexes with a F-Cl···P halogen bond exist that form through the lone pair at P (configuration I), except for FCl:PCCN, the more stable complexes are those in which FCl interacts with the C≡P triple bond through a perturbed π system (configuration II). In complexes I, the nature of the halogen bond changes from traditional to chlorine-shared and the interaction energies increase, as the electron-donating ability of X increases. The anionic complex FCl:PC(-) has a chlorine-transferred halogen bond. SAPT analyses indicate that configuration I complexes with traditional halogen bonds are stabilized primarily by the dispersion interaction. The electrostatic interaction is the most important for configuration I complexes with chlorine-shared halogen bonds and for configuration II complexes except for FCl:PCNa for which the induction term is most important. The F-Cl stretching frequency is red-shifted upon complexation. EOM-CCSD/(qzp,qz2p) spin-spin coupling constants have been obtained for all FCl:PCX complexes with configuration I. (1)J(F-Cl) decreases upon complexation. (2X)J(F-P) values are quadratically dependent upon the F-P distance and are very sensitive to halogen-bond type. (1X)J(Cl-P) tends to increase as the Cl-P distance decreases but then decreases dramatically in the chlorine-transferred complex FCl:PC(-) as the Cl-P interaction approaches that of a covalent Cl-P bond. Values of (1)J(F-Cl) for configuration II are reduced relative to configuration I, reflecting the longer F-Cl distances in II compared to those of the neutral complexes of I. Although the F-P and Cl-P distances in configuration II complexes are shorter than these distances in the corresponding configuration I complexes, (2X)J(F-P) and (1X)J(Cl-P) values are significantly reduced, indicating that coupling through the perturbed C-P π bond is less efficient. The nature of F-P coupling for configuration II is also significantly different, as evidenced by the relative importance of PSO, FC, and SD components.

Using beryllium bonds to change halogen bonds from traditional to chlorine-shared to ion-pair bonds

“…For these calculations, the Ahlrichs 44 qzp basis set was placed on 13 C, 15 N, and 19 F, and the qz2p basis set on 35 Cl. A basis set for 9 Be was developed using the same approach as used previously for 7 Li and 11 B. 45 The Dunning cc-pVDZ basis set was placed on all 1 H atoms.…”

Halogen Bonding with Phosphine: Evidence for Mulliken Inner Complexes and the Importance of Relaxation Energy

“…Although the changes in the underlying mechanism of halogen bonding may be subtle in many cases, there is evidence for unusually strong interactions with significantly different compositions of the forces responsible. This includes the "chlorine-shared" bonds investigated by Del Bene and co-workers, [9][10][11] and "Mulliken inner complexes" that feature a halogen bond.…”

The shape of the halogen atom—anisotropy of electron distribution and its dependence on basis set and method used

“…This partial positive charge on the valence sphere of the halogen atom is often defined within the framework of NBO theory [21] as a sigma hole-a local deficit of an electron charge (a hole) being placed opposite the sigma bond [22,23]. The sigma hole may interact with the local electron charge surplus such as lone electron pairs [17,24,25], pi-type electrons [17,24,26], or even sigma-type electrons [27]; in this way, the mechanism of formation of X-bond can be well defined. However, there are some discrepancies regarding the nature of X-bonding.…”