Proximity Effects of Substituents on Halogen Bond Strength

Document Type

Article

Author ORCID Identifier

Steve Scheiner https://orcid.org/0000-0003-0793-0369

Journal/Book Title

The Journal of Physical Chemistry A

Publication Date

6-3-2021

Publisher

American Chemical Society

Award Number

NSF, Division of Chemistry (CHE) 1954310

Funder

NSF, Division of Chemistry (CHE)

Volume

125

Issue

23

First Page

5069

Last Page

5077

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 NH3. 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.

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