3.4.2. The IR2 ··· H2O and IR2 ··· MeOH Dimers

The significant non-linearity of the hydrogen bonds in the dimers I··· H2O and I··· MeOH already indicates the presence of additional intermolecular interactions, and indeed, as shown in Figure 4 and discussed previously, there is an additional hydrogen bond of the N-H··· O type in both of these dimers. The structures of these dimers also prove that lone electron pairs on oxygen are readily available, and therefore, both H2O and MeOH will be willing to engage in the formation of accompanying hydrogen bonds. The resulting IR2 ··· H2O and IR2 ··· MeOH dimer structures are shown in Figure 7.

**Figure 7.** The IR2 ··· H2O and IR2 ··· MeOH (R = Me, *<sup>i</sup>* Pr, *<sup>t</sup>* Bu, Ph, Mes, Dipp, Ad) dimers. The values of the dissociation energy are given in bold, while the three values refer to the length of the hydrogen bond, the C-H-D angle and the value of the electron density at the bond critical point of the C··· H-D hydrogen bond.

Among the dimers with water, IDipp2 ··· H2O and IMes2 ··· H2O are characterized by the highest dissociation energy value (11.2 and 10.7 kcal/mol, respectively), and the lowest is for IMe2 ··· H2O (9.0 kcal/mol). It is similar in the case of dimers with MeOH: IDipp2 ··· MeOH (14.4 kcal/mol), IMes2 ··· MeOH (13.1 kcal/mol), IMe2 ··· MeOH (10.4 kcal/mol). As was the case with dimers involving HF or HCN, the presence of any R substituent clearly increases the value of the dissociation energy. Clearly (Table 1, Figure 7), for a given carbene IR2, MeOH forms a stronger complex than H2O. This is also reflected in the shorter distances C··· H. For example, for IPh2 the distances are 1.927 and 1.954 Å, respectively, and for IAd2, they are 1.931 and 1.991 Å, respectively. The dissociation energies for H2O are rather similar and generally slightly lower than those for HCN. It is worth noting that in the case of dimers with H2O and MeOH, the highest (i.e., most negative) values of *E*b, i.e., the binding energy obtained by Equation (1), have been obtained for

the I*<sup>t</sup>* Bu2 carbene, characterized by the formation of an almost linear (ca. 174◦) C··· H-O bond. On the contrary, the smallest values of *E*<sup>b</sup> have been obtained for IMe2 (−6.8 and −7.2 kcal/mol for H2O and MeOH, respectively). However, they are much larger than in the unsubstituted imidazol-2-ylidene (−5.3 and −5.5 kcal/mol, respectively).

Taking into account that the molecular graphs obtained for the IR2 ··· HF and IR2 ··· HCN dimers (Figure 6) in many cases indicated the presence of two, four or even more additional coexisting interactions, it is now worth analyzing the molecular graphs for IR2 ··· H2O and IR2 ··· MeOH dimers. The most interesting examples are shown in Figure 8.

**Figure 8.** Molecular graphs of selected IR2 ··· H2O and IR2 ··· MeOH dimers. Arrows show the presence of bond paths for some accompanying interatomic interactions (hydrogen bonds—red, C-H··· H-C contacts—yellow, and C-H··· *π* hydrogen bonds—light green). Large balls represent atoms (hydrogen—white, carbon—gray, nitrogen—blue, and oxygen—red) and small balls represent critical points (bond critical points—light green, ring critical points—red, and cage critical points—blue).

In the dimers shown in Figure 8, generally one (e.g., I*<sup>i</sup>* Pr2 ··· H2O) or two (e.g., I *t* Bu2 ··· H2O) bond paths are present for the accompanying C-H··· O hydrogen bonds (indicated by red arrows). It is clear, however, that for some carbenes, the MeOH dimers contain additional bond paths for C-H··· H-C contacts (indicated by yellow arrows). Moreover, the presence of a methyl group in MeOH allows in some cases (see the dimers with IMes2 and IDipp2) an additional (i.e., compared with H2O) C-H··· *π*-type hydrogen bond (green arrow). As was the case with the HF and HCN dimers, IDipp2 produces the most intricate molecular graph, suggesting the existence of many interactions accompanying the main C··· H-O hydrogen bond involving the carbene carbon atom. In addition to three C-H··· H-C interactions, numerous accompanying hydrogen bonds are present. Taking this into account, this fact may explain the exceptionally high value of dissociation energy in dimers with IDipp2, and especially in IDipp2 ··· MeOH (Table 1 and Figure 7). Nevertheless, all these accompanying interactions should be much weaker than the leading C··· H-O hydrogen bond involving the carbene carbon atom. For example, C-H··· O hydrogen bonds are generally in the range of −0.6 to −1.5 kcal/mol (e.g., −0.8 kcal/mol in I*<sup>t</sup>* Bu2 ··· H2O, −1.0 kcal/mol in I*<sup>t</sup>* Bu2 ··· MeOH, −1.1 kcal/mol in I*<sup>i</sup>* Pr2 ··· H2O and I*<sup>i</sup>* Pr2 ··· MeOH, and −1.4 kcal/mol in IMes2 ··· H2O), however they are slightly stronger in IAd2 ··· H2O (−1.6 kcal/mol) and especially in IAd2 ··· MeOH (−1.9 kcal/mol). The C-H··· *π* bonds in IMes2 ··· MeOH and IDipp2 ··· MeOH are much weaker (−0.6 kcal/mol). On the other hand, the energy of C-H··· H-C interactions is negligible, below −0.5 kcal/mol (e.g., −0.4 kcal/mol in IAd2 ··· MeOH). On the other hand, the *intra*molecular C-H··· N hydrogen bonds in IDipp2 ··· H2O and IDipp2 ··· MeOH are significantly strong (−2.8 kcal/mol). All these energy values should be compared with the energies of the dominant C··· H-O hydrogen bond, which in the case of IR2 ··· H2O and IR2 ··· MeOH dimers range from −6.6 to −8.4 kcal/mol (Table 1).
