An ab Initio Study of Linear XH−HX Hydrogen Halide Anions

Abstract

This paper examines the stabilities of singly charged anions with the linear opposed dipole structures: FH-HF, ClH-HCl and BrH-HBr at MP2 and CCSD(T) levels. With 6-311+G(3df,2p) basis sets on Cl and Br and aug-cc-pvdz on H and F, MP2 structures were obtained for all three systems. Since the neutral dimers with opposed dipoles are unstable, the energies for the electron detachment processes were evaluated relative to separated free neutrals, viz.: XH-HX‚f e -+ 2HX. At the CCSD(T) level these energies were 0.26 and 0.84 V for HCl and HBr, respectively, but the value for HF is negative. The addition of diffuse orbitals at the central point between the two H atoms lowered the energy and caused the H-H distance in the FH-HF species to increase. Also the excess electron moved into the diffuse orbitals and the species assumed a solvated electron structure, a finding which is in agreement with previous work. In contrast, for the HCl and HBr species the addition of diffuse orbitals had no significant effect. The optimized geometries, charge and spin distributions, and the adiabatic electron detachment energies all remained essentially unchanged. This result and other molecular parameters indicated that in the ClH-HCl and BrH-HBr anions the extra electrons were located in valence orbitals made up from the σ* orbitals of the two HX molecules. The nature of these orbitals was discussed. Since the dipole moment of the HCl.HCl neutral dimer is considered too small for formation of a dipole-bound anion state (as occurs with HF.HF), it is quite probable that HCl dimer anion observed experimentally is of the XH-HX‚type.

Nonpolar Dihydrogen Bonds—On a Gliding Scale from Weak Dihydrogen Interaction to Covalent HH in Symmetric Radical Cations [HnE‐H‐H‐EHn]+

“…In addition, the authors noted a considerable lengthening of the X À H bond and correspondingly short H À H contacts (0.98 and 0.92 ). [18] These are indeed noticeable features, and we observed the same phenomenon while sidetracking from work with oneelectron reduction processes in water clusters and multiply protonated peptides. From our own quantum chemical calculations of analogous systems it turned out that HCl and HBr are the only neutral hydrides of rows 2, 3, and 4 of the periodic [19] However, these endeavours led us to discover a new class of molecules, radical cations, also dimeric, and isoelectronic with the anions described above.…”

“…In analogy with the anions, we may consider the [H n E-H-H-EH n ] + dimer to be composed of two H n E À H + molecules plus an electron. In that sense we observe, as for the hydrogen halide dimer anions, [18] a significant lengthening of the centrally pointing EÀH bond in going from the monomer…”

“…On the basis of experience [18,19] with the quantum chemical description of the isolectronic anions […”

“…+ (EH n = element hydride, E = element of Groups [15][16][17][18], have been investigated by applying quantum-chemical methods. All structures investigated give rise to symmetric potential energy minimum structures.…”

“…[17] A novel mode of dihydrogen bonding was recently reported by Rauk and Armstrong in a computational study. [18] Despite the fact that neither HCl nor HBr have positive values of their electron affinities, they are both able to bind an electron in their dimeric forms, [X-H-H-X] À (X = Cl, Br). By adapting this symmetric bonding arrangement the dimers become thermodynamically stable.…”