Hydrogen Bonding versus Halogen Bonding: Spectroscopic Investigation of Gas‐Phase Complexes Involving Bromide and Chloromethanes

Abstract

Hydrogen bonding and halogen bonding are important noncovalent interactions that are known to occur in large molecular systems, such as in proteins and crystal structures. Although these interactions are important on a large scale, studying hydrogen and halogen bonding in small, gas-phase chemical species allows for the binding strengths to be determined and compared at a fundamental level. In this study, anion photoelectron spectra are presented for the gas-phase complexes involving bromide and the four chloromethanes, CH 3 Cl, CH 2 Cl 2 , CHCl 3 , and CCl 4 . The stabilisation energy and electron binding energy associated with each complex are determined experimentally, and the spectra are rationalised by high-level CCSD(T) calculations to determine the non-covalent interactions binding the complexes. These calculations involve nucleophilic bromide and electrophilic bromine interactions with chloromethanes, where the binding motifs, dissociation energies and vertical detachment energies are compared in terms of hydrogen bonding and halogen bonding.

Definition of the Halogen Bond (IUPAC Recommendations 2013): A Revisit

“…Weak noncovalent interactions are weak forces that typically occur between neutral molecular entities, while stronger interactions appear when one of the participating sites (electron density donor) has a negative charge (a polarity opposite to that of the electron density acceptor). The former interactions in the gas-phase have been investigated using a variety of spectroscopic techniques (e.g., rotational, 16,17 vibrational, 18,19 and anion photoelectron spectroscopy 20 and mass spectrometry 21 ), whereas strongly bonded interactions are ion−molecule or ion-pair adducts that have been reported primarily in the crystalline phase. Although chemical reactions are widely understood as a result of the making and breaking of chemical bonds, 22 noncovalent interactions, while generally weaker than covalent bonds, also develop during the course of a chemical reaction.…”

“…The former interactions in the gas-phase have been investigated using a variety of spectroscopic techniques ( e.g. , rotational, , vibrational, , and anion photoelectron spectroscopy and mass spectrometry), whereas strongly bonded interactions are ion–molecule or ion-pair adducts that have been reported primarily in the crystalline phase. Although chemical reactions are widely understood as a result of the making and breaking of chemical bonds, noncovalent interactions, while generally weaker than covalent bonds, also develop during the course of a chemical reaction.…”