Halogen bonding (XB) has emerged as an important bonding motif in supramolecules and biological systems. Although regarded as astrong noncovalent interaction, benchmark measurements of the halogen bond energy are scarce. Here,acombined anion photoelectron spectroscopya nd density functional theory (DFT) study of XB in solvated Br À anions is reported. The XB strength between the positivelycharged s-hole on the Br atom of the bromotrichloromethane (CCl 3 Br) molecule and the Br À anion was found to be 0.63 eV (14.5 kcal mol À1 ). In the neutral complexes,B r(CCl 3 Br) 1,2 ,t he attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl 3 Br,w hich is as econd type of halogen bonding,w as estimated to have interaction strengths of 0.15 eV (3.5 kcal mol À1 )a nd 0.12 eV (2.8 kcal mol À1 ).Even though they are generally regarded as electron withdrawing groups,a lready covalently-bonded halogen atoms can in addition interact attractively and directionally by an on-covalent bond to neighbor nucleophiles such as lone pairs and anions. [1][2][3][4] This noncovalent interaction was first referred to as "halogen bonding" (XB) by Dumas and coworkers in 1976. [5,6] Halogen atoms have positive electrostatic potential regions on the opposite end of their s bond due to polarizability;m oreover,t he equatorial sides of these atoms exhibit negative electrostatic potential belts. [7,8] Thep ositive facial site was termed as "s-hole" by Politzer et al. in 2007, [9] although, clearly it is only apositive partial charge present at this site and no full electron is missing in any orbital. Thesize of the s-hole depends on the polarizability of the halogen atom, that is,I> Br > Cl > F, but it can also be tuned by other highly electron-withdrawing functional groups in the molecule. [2] While the positively-charged s-hole interacts with nucleophiles,the negatively-charged equatorial belt interacts with electrophiles,r esulting in two categories of XB inter-actions. [1] XB has rapidly expanded into applications such as crystal engineering. [10][11][12][13][14] It also has provoked the survey of biological structures,w here XB has been found to stabilize inter-a nd intramolecular interactions that can further affect ligand binding,p rotein folding,a nd enzymatic reactions. [15] While presence of XB interactions in the condensed-phase materials and biological chemistry is well known, experimental investigations of XB in the gas phase have been scarce. Nevertheless,t echniques such as molecular beam scattering, [16] rotational spectroscopy, [17][18][19] blackbody infrared radiative dissociation [20] and ion-mobility mass spectrometry have provided significant insight. [21] More recently,o ur group reported the stabilization of otherwise unstable anions by XB using gas-phase anion photoelectron spectroscopy. [22] Theoretical calculations have also been widely used to investigate the nature and applications of XB. [1][2][3][4][23][24][25][26][27][28] Even though many levels of theory have estimated th...
