Isolation of a Halogen-Bonded Complex Formed between Methane and Chlorine Monofluoride and Characterisation by Rotational Spectroscopy and Ab Initio Calculations.

Anthony C Legon,John H Holloway,Elangannan Arunan,David G Lister,Devendra Mani

doi:10.3390/molecules24234257

Abstract

A halogen-bonded complex formed between methane and chlorine monofluoride has been isolated in the gas phase before the reaction between the components and has been characterised through its rotational spectrum, which is of the symmetric-top type but only exhibits K = 0 type transitions at the low effective temperature of the pulsed-jet experiment. Spectroscopic constants for two low-lying states that result from internal rotation of the CH4 subunit were detected for each of the two isotopic varieties H4C···35ClF and H4C···37ClF and were analysed to show that ClF lies on the symmetry axis with Cl located closer than F to the C atom, at the distance r0(C···Cl) Å and with an intermolecular stretching force constant kσ 4 N m−1. Ab initio calculations at the explicitly correlated level CCSD(T)(F12c)/cc-pVTZ-F12 show that in the equilibrium geometry, the ClF molecule lies along a C3 axis of CH4 and Cl is involved in a halogen bond. The Cl atom points at the nucleophilic region identified on the C3 axis, opposite the unique C–H bond and somewhere near the C atom and the tetrahedron face centre, with re(C···Cl) = 3.191 Å. Atoms-in-molecules (AIM) theory shows a bond critical point between Cl and C, confirming the presence of a halogen bond. The energy that is required to dissociate the complex from the equilibrium conformation into its CH4 and ClF components is only De 5 kJ mol−1. A likely path for the internal rotation of the CH4 subunit is identified by calculations at the same level of theory, which also provide the variation of the energy of the system as a function of the motion along that path. The barrier to the motion along the path is only 20 cm−1.

Highlights

It was established, by a comparison of several series of complexes of the type B···HCl andB···ClF, where B is a Lewis base, that a close parallelism exists among the angular geometries of the hydrogen-bonded and halogen-bonded species for a given Lewis base B [1,2]
We report such an investigation conducted by rotational spectroscopy and ab initio calculations
The two groups of hyperfine transitions of a given J were assigned to states with the symmetry labels A and F, where the nomenclature in use is that introduced by Ohshima and Endo [16] in their analysis of the similar spectrum of H4 C···HCl. These authors treated the coupling of the angular momentum arising from the internal rotation of the CH4 subunit with that of the rotation of the whole molecular framework

Summary

Introduction

By a comparison of several series of complexes of the type B···HCl and. Cl [15,16] or Br [17], that is with the δ+ H atom of HX forming a hydrogen bond to a face centre of the methane molecule with a very low potential energy barrier to the internal rotation of the CH4 subunit. The MESP calculated at the MP2(Full)/aug-cc-pVTZ level is given in the Supplementary Material and the results are very similar It was shown some years ago [21] that HCN forms a hydrogen bond to the centre of the face of ethane to give a complex of C3v symmetry. B···ClF angular geometries already referred to, it is of interest to investigate whether methane forms a halogen-bonded complex with ClF that is isomorphous with H4 C···HCl. In this article, we report such an investigation conducted by rotational spectroscopy and ab initio calculations. The blue areas indicate electrophilic regions known as sigma holes (positive +40 kJ mol−1 )

Experimental and Theoretical Methods

Determination of Spectroscopic Constants

ClF D φ

Conclusions