Proximity Effects of Substituents on Halogen Bond Strength

Abstract

It is well known that the presence of an electron-withdrawing substituent (EWS) placed near the halogen (X) atom on a Lewis acid molecule amplifies the ability of this unit to engage in a halogen bond with a base. Quantum calculations are applied to examine how quickly these effects fade as the EWS is moved further and further from the X atom. Conjugated alkene and alkyne chains of varying lengths with a terminal C−I first facilitate analysis as to how the number of these multiple bonds affects the strength of CI••N XB to NH 3 . Then, electron-withdrawing F and CN substituents are placed on the opposite end of the chain, and their effects on the XB properties are monitored as a function of their distance from I. These same EWSs are added to the ortho, meta, and para positions of aromatic iodobenzene. It is found that the XB grows in strength as more triple bonds are added to the alkyne, but there is little change caused by elongating an alkene. The cyano group has a much stronger effect than does F. While F strengthens the XB, its effects are quickly attenuated as it is moved further from I. The consequences of CN substitution are stronger and extend over a longer distance. Placement of an EWS on the phenyl ring diminishes with distance: o > m > p, and the effects of disubstitution are nearly additive. These trends apply not only to energetics but also to geometries, properties of the wave function, σ-hole depth, and NMR shielding.

Characterization of Type I and II Interactions between Halogen Atoms

“…Unlike most of the other properties, this coupling constant is considerably larger for T1 than for T2. It should be pointed out finally that these various spectral changes are not unique to the FX dimers examined here but are general features of halogen bonds in general and even occur in complexes containing closely related interactions such as chalcogen and pnicogen and even hydrogen bonds. ,− …”

Influence of Substituents in the Benzene Ring on the Halogen Bond of Iodobenzene with Ammonia

“…This expectation has indeed been confirmed, as in the ChemPhysChem multiple F-substitution [82] on iodobenzene, or in another work where two F or CN substituents were added to iodobenzene. [30] The latter set of computations found that this disubstitution was additive in the sense that the effects of two groups were nearly precisely double that of a single such substituent.…”

“…The deshielding of the N atom is consistent with earlier studies of related systems, as well as its linear relationship to bond strength. [10,30,72,73] It is intriguing to note that in contrast to the drop of the shielding around the electron donor N atom here, the opposite effect of an increase in [a] Values for R=H: N À 12.0, I À 115.9, C À 6.7 ppm. shielding accompanies the formation of a noncovalent bond to the O atom of N-methylacetamide.…”

“…It stands to reason that the effect of this EWS would be mitigated as it is moved further away from the X, and this expectation has in fact been substantiated. For example, a prior study from this lab [30] documented the reduction of the depth of the σ-hole, along with the ensuing weakening of the XB, as the EWS is moved further from the halogen along an alkyl, alkenyl, or akynyl chain.…”

“…There have been some initial indications that the NMR spectra of halogen bonded systems are connected in some way to the strength of each such bond. [30,35,[41][42][43][44][45] However, the application of spectroscopy to halogen bonding remains in its infancy. Another important goal of the present work is to advance this idea, using substituted halobenzenes as an ideal testbed.…”