*3.2. Investigated Systems*

The subject of the research in this article is the hydrogen bonds between imidazol-2-ylidene (I) and its seven derivatives having a pair of R substituents (methyl = Me, *iso*-propyl = *<sup>i</sup>* Pr, *tert*-butyl = *<sup>t</sup>* Bu, phenyl = Ph, mesityl = Mes, 2,6-diisopropylphenyl = Dipp, 1-adamantyl = Ad) attached to both nitrogens and five popular and important H-D proton donors (HF, HCN, H2O, MeOH, NH3). Dimers (complexes) having this bond will be denoted as IR2 ··· HD for simplicity. Apart from the description of C··· H-D hydrogen bonds in these systems, an important topic will be the possible presence of other accompanying bonds as they can sometimes significantly influence the structure of a complex [80]. Values of some selected parameters characterizing the C··· H-D hydrogen bond or the proton-donor molecule in the IR2 ··· HD dimers are listed in Table 1. Because imidazol-2-ylidene is somewhat different in terms of its binding capacity, the imidazol-2 ylidene complexes will be discussed first.

**Table 1.** Values of selected parameters characterizing the C··· H-D hydrogen bond or the protondonor molecule in the IR2 ··· HD dimers (I = imidazol-2-ylidene; R = Me, *<sup>i</sup>* Pr, *<sup>t</sup>* Bu, Ph, Mes, Dipp, Ad; HD = HF, HCN, H2O, MeOH, NH3): *D*0–dissociation energy (in kcal/mol), *ρ*C···H–the electron density at the bond critical point of C··· H-D (in a.u.), *E*b–binding energy of the C··· H-D hydrogen bond obtained by Equation (1) (in kcal/mol), *d*C···H–length of the C··· H distance (in Å), *α*CHD–the C-H-D angle (in degrees), Δ*d*HD–elongation of the H-D proton-donor bond (in Å) relative to isolated HD (HF–0.917 Å, HCN–1.068 Å, H2O–0.958 Å, MeOH–0.957 Å, and NH3–1.013 Å).



**Table 1.** *Cont.*
