The ClHCl− anion: Its chemical bond, vibrations, and free energy

Abstract

The geometry of ClHCl− has D∞h symmetry, [Cl–H–Cl]−, as opposed to C∞v symmetry [Cl–H...Cl−], which is predicted to be favored at the Hartree–Fock (HF) level of calculation with a basis set of [11s9p2d1f(Cl)/5s2p1d(H)]. The chemical bond of the D∞h structure is a nonclassical four-electron three-center bond, while the C∞v geometry corresponds to an ion–dipole complex. The energy difference between the two is small. Calculations with inclusion of configuration revealed that both have predominantly a HF configuration. D∞h is realized by excitations from σ molecular orbitals (MO) to their antibonding σ* MO, whereas excitations to π MO are ineffective. Normal vibrational frequencies were calculated as follows (units are cm−1): G82(MP2): (ν1,σg): 346; (ν3,σu ): 547; (ν2,πu): 854; 1D Analysis(MP4): (ν1,σg ): 338; (ν3,σu): 830; (ν2, πu ):...; 2D Analysis(MP4): (ν1,σg): 310; (ν3,σu) : 709; (ν2,πu) : 877. The potential for the asymmetric vibration is shown to be quartic. The standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes (i.e., at 1 atm and 298.16 K) were evaluated from the reaction, HCl+Cl−→ClHCl−, to be as follows: ΔG°=−16.5 kcal/mol, ΔH°=−23.5 kcal/mol, and ΔS°=−23.5 cal/K/mol.