Does Chlorine in CH3Cl Behave as a Genuine Halogen Bond Donor?

Abstract

The CH3Cl molecule has been used in several studies as an example purportedly to demonstrate that while Cl is weakly negative, a positive potential can be induced on its axial surface by the electric field of a reasonably strong Lewis base (such as O=CH2). The induced positive potential then has the ability to attract the negative site of the Lewis base, thus explaining the importance of polarization leading to the formation of the H3C–Cl···O=CH2 complex. By examining the nature of the chlorine’s surface in CH3Cl using the molecular electrostatic surface potential (MESP) approach, with MP2/aug-cc-pVTZ, we show that this view is not correct. The results of our calculations demonstrate that the local potential associated with the axial surface of the Cl atom is inherently positive. Therefore, it should be able to inherently act as a halogen bond donor. This is shown to be the case by examining several halogen-bonded complexes of CH3Cl with a series of negative sites. In addition, it is also shown that the lateral portions of Cl in CH3Cl features a belt of negative electrostatic potential that can participate in forming halogen-, chalcogen-, and hydrogen-bonded interactions. The results of the theoretical models used, viz. the quantum theory of atoms in molecules; the reduced density gradient noncovalent index; the natural bond orbital analysis; and the symmetry adapted perturbation theory show that Cl-centered intermolecular bonding interactions revealed in a series of 18 binary complexes do not involve a polarization-induced potential on the Cl atom.

The Phosphorus Bond, or the Phosphorus-Centered Pnictogen Bond: The Covalently Bound Phosphorus Atom in Molecular Entities and Crystals as a Pnictogen Bond Donor

“…Associated with these depleted charge density regions are σ-holes on the halogen derivatives, whose strength increases in the opposite order: P–Cl (10.0 kcal mol −1 ) < P–Br (16.1 kcal mol −1 ) < P–I (17.0 kcal mol −1 ), concordant with an increase in the polarizability of X. The apparent absence of a σ-hole on F in PF 3 is not unexpected and has been observed in other chloro- and fluorinated compounds [ 60 , 61 , 62 , 99 ].…”

“…(electrons Bohr −3 ) isoelectron density that arbitrarily defines the van der Waals surface of a molecular entity was used to compute the electrostatic potential. While any definition of a molecular surface is necessarily arbitrary [ 52 , 60 ], as we have demonstrated elsewhere [ 61 , 62 ]—and show also in this study (vide infra)—the use of the suggestions of Bader and coworkers [ 63 ], as well as others [ 64 , 65 ], that the 0.001/0.002 a.u. contour of charge density ρ (r) encompasses more than 90–95% of the molecule electronic charge is still found in many reports, especially when it concerns the electrostatic surface potential on molecular entities.…”

“…While the (arbitrary) use of the 0.001 a.u. isodensity envelope is often recommended for computing electrostatic potential [ 100 , 101 ], it may not always be appropriate [ 60 , 61 , 62 ]. Its use in the present case, for instance, suggested that V S,max is neutral on the extensions of the three P–F bonds in PF 3 .…”

Maximal occupation by bases of π‐hole bands surrounding linear molecules

“…This vacant orbital lies along the extension of the AR bond, and is thus coincident with the σ‐hole, so can easily overlap with the lone pair of the approaching nucleophile. There has been a good deal of study of these sorts of noncovalent bonds 13–17 . The quantitative aspects of the relationship between the depth of the σ‐hole and the strength of the bond has been thoroughly explored, as well as the way in which other factors such as charge transfer and dispersion enter into the equation 18–26 .…”

σ- and π-Hole Interactions

“…In reference [23], Varadwaj et al studied theoretically the CH 3 Cl molecule and its complexes with ten Lewis bases to demonstrate that CH 3 Cl is a genuine halogen bond donor. They have evidenced that the electronic charge density distribution around the Cl is anisotropic.…”