Table 1 Cl···H and Cl···C distances (d_Cl···H, d_Cl···C in Å), H-binding angles (θ: ∠Cl···H–C in degree), and binding energies (BE in kcal/mol) for complexes of (a) bis-imidazolium and (b) tetraalkylammoinium^a.

^aAll the atom positions excluding H were fixed to the corresponding positions that were determined by X-ray crystallography. As for the H atom positions, both the X-ray and the B97D/cc-pVDZ optimized geometries are reported as X-ray(left)/B97D(right) in the columns of d_Cl···H and θ, because in the original X-ray data one of H positions was missing and some of the X-ray H positions were not reliable enough due to too long/short CH bond lengths and unreliable bond angles due to the uncertainty in resolution. Standard uncertainties (s.u.) of all contact distances and angles for (a) are in parentheses, while those for (b) are not available (ref. 38). The natural bond orbital charges of atoms (q in au) are: (a) q_Cl = −0.904; q_H = 0.18–0.26; q_C = ~0.2(1), −0.3(6, 7), −0.6(2–5); (b) q_Cl = −0.859; q_H = 0.21–0.26; q_C = ~−0.4 (−0.2, −0.7). The BEs were estimated for the Cl⁻···CH₄ interaction, where the C of CH₄ had the X-ray geometry of C_ali for each C_ali–H group and all the H atoms were at the B97D optimized geometry with the typical –CH₃ structure for the remaining three H atoms. The optimally computed bond distance d_Cl···H for the pure Cl⁻···CH₄ interaction is 2.726 Å and the BE is 3.04 kcal/mol. The vdW distance for Cl···H-C and Cl···H interactions are r^vdW_Cl···H-C = 4.01–4.08 Å and r^vdW_Cl···H = 2.9–2.97 Å, respectively.

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