3.4.3. The IR2 ··· NH3 Dimers

The values of dissociation (*D*0) and binding (*E*b) energies listed in Table 1 show that the dimers with NH3 as well as the C··· H-N hydrogen bonds in them should be the weakest. Namely, the *D*<sup>0</sup> values are from 4.5 kcal/mol in I*<sup>t</sup>* Bu2 ··· NH3 to 7.3 kcal/mol in IDipp2 ··· NH3. The *E*<sup>b</sup> values form a fairly narrow range from −3.2 kcal/mol (not including I··· NH3 with a much lower value of −1.6 kcal/mol) to −3.7 for IDipp2 ··· NH3. Such small values of *D*<sup>0</sup> and *E*<sup>b</sup> may result from the fact that the N-H bond in ammonia is a worse proton donor than the O-H bond in water or methanol. The C-H-D angle (*α*CHD) values show that in none of the dimers, the C··· H-N hydrogen bond is linear (although in I*<sup>t</sup>* Bu2 ··· NH3 and IAd2 ··· NH3 this angle is ca. 174◦); quite the opposite, the deviation from linearity is significant (157◦–166◦), although much smaller than in I··· NH3 (123◦). Such large deviations from linearity may indicate the presence of additional coexisting interactions. The structures of the IR2 ··· NH3 dimers are shown in Figure 9, while their molecular graphs can be found in Figure 10.

**Figure 9.** The IR2 ··· NH3 (R = Me, *<sup>i</sup>* Pr, *<sup>t</sup>* Bu, Ph, Mes, Dipp, and 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.

**Figure 10.** Molecular graphs of the IR2 ··· NH3 dimers. Arrows show the presence of bond paths for some accompanying interatomic interactions (hydrogen bonds—red, C-H··· H-C or C-H··· H-N contacts—yellow, and N··· *π* contact—dark green). Large balls represent atoms (hydrogen—white, carbon—gray, and nitrogen—blue) and small balls represent critical points (bond critical points—light green, ring critical points—red, and cage critical points—blue).

A careful comparison of the molecular graphs obtained for the IR2 ··· H2O and IR2 ··· MeOH dimers (Figure 8) with the molecular graphs obtained for the IR2 ··· NH3 dimers (Figure 10) shows some impoverishment in the second case. In addition, there is a visible change in the nature of some accompanying interactions in some systems with the same IR2 carbene. For example, the molecular graph of the I*<sup>i</sup>* Pr2 ··· NH3 dimer contains

only one bond path for the interaction (C-H··· N) accompanying the dominant C··· H-N hydrogen bond, while I*<sup>i</sup>* Pr2 ··· MeOH in addition to the additional C-H··· O also contains a bond path for C-H··· H-C. The I*<sup>t</sup>* Bu2 ··· H2O dimer contains two bond paths for C-H··· O hydrogen bonds, whereas I*<sup>t</sup>* Bu2 ··· NH3 also has two additional bond paths, but for C-H··· H-C interactions. The situation is similar for the molecular graph involving Ad; in the case of dimer with H2O, there are two bond paths for C-H··· O hydrogen bonds, whereas in the case of IAd2 ··· NH3, there are two bond paths for C-H··· H-N. Keeping in mind that C-H··· H-C interactions are weaker than C-H··· O/N hydrogen bonds, together with a smaller number of accompanying interactions, this finding may explain the weaker strength of complexes with NH3. Similarly, it can be seen that IDipp2 creates many bond paths, of which as many as three (except three for N-H bonds, of course) lead to the nitrogen atom of the ammonia molecule. Two of them represent very weak C-H··· N hydrogen bonds (−0.1 and −1.3 kcal/mol), while the other, interestingly, determines the N··· *π* contact (−0.5 kcal/mol). Of course, all these interactions are much weaker than the dominant C··· H-N hydrogen bond (−3.2 kcal/mol). Nevertheless, their multitude makes the dissociation energy for IDipp2 ··· NH3 clearly the highest (7.3 kcal/mol) among all the IR2 ··· NH3 dimers (see Table 1 or Figure 9). Of course, it should be noted that a similar dissociation energy value was obtained for a simple I··· NH3 dimer (7.2 kcal/mol); however, as already shown (Figure 4), such a large value in this case results from the presence of a strong N-H··· N hydrogen bond (−4.6 kcal/mol), which overwhelms the weaker C··· H-N bond (−1.6 kcal/mol).
