**3. Results and Discussion**

### *3.1. Hydrogen Bonds to Carbenes Observed in the Solid State*

It should be noted that despite the high reactivity of carbene compounds, hydrogen bonds with the participation of the carbene carbon atom as a proton acceptor quite often occur in the crystals. Of course, due to the abundance of C-H bonds, the most common hydrogen bond of this type is C··· H-C, but C··· H-N or C··· H-O can also be found in the Cambridge Structural Database [117]. Some more interesting examples are shown in Figure 2.

**Figure 2.** Examples of the occurrence of hydrogen bonds (marked with a dashed green line) involving a carbene carbon atom in crystallographic structures. The atoms are labeled as follows: carbon—gray, hydrogen—white, nitrogen—blue, oxygen—red, and boron—pink.

There are two C··· H-C type hydrogen bonds in the crystal form of 1,3-diisopropyl-4,5 dimethylimidazol-2-ylidene (GUXJAK02) [118]. Both the dimer between 1,3-dimesityl-imidazol-2 ylidene and diphenylamine (MODVEG) [119] and between 1,3-bis[2,6 diisopropylphenyl]imidazol-2-ylidene and N1,N4-diphenylbenzene-1,4-diamine (LAGYEB) [120] have one C··· H-N hydrogen bond, while the dimer of N-methyl-2-(3-phenyl-2,3-dihydro-1H-imidazol-1-yl)ethanamine (AWIBOY) [121] has two. 1,3-dimesitylimidazol-2-ylidene in methanol (JAPDEK) [122], on the other hand, is the simplest example of the occurrence of a C··· H-O type hydrogen bond and was reported as the first X-ray structure of a carbene– alkohol hydrogen-bonded complex. The -OH group donors are somewhat more complex in QIGPEE (i.e., the dimer of 1-(2-hydroxy-2-methylpropyl)-3-(2,4,6-trimethylphenyl) imidazol-2-ylidene) [123], USINAM (i.e., the dimer between 1,3-dimesitylimidazol-2 ylidene and 2,2,6,6-tetramethylpiperidin-1-ol) [51], and XORMUP (the dimer between 1,3 bis[2,6-bis(propan-2-yl)phenyl]-imidazol-2-ylidene and (2,4,6,2,4,6-hexamethyl[1,1 :3 ,1 terphenyl]-2 -yl)boronic acid) [124]. Interestingly, in the last system, there is the -B(OH)2

group, of which, however, only one -O-H bond is involved in the formation of the C··· H-O bridge with the carbene molecule.

The examples presented show clearly that the carbene carbon can actually engage in hydrogen bonding quite easily by acting as a proton acceptor. It can also be seen that these carbenes have large substituents (most often in the form of mesityl groups) in the 1 and 3 positions of the imidazol-2-ylidene parent molecule. Therefore, the study of these types of systems using theoretical methods is also very important [80]. However, the presence of bulky substituents significantly increases the size of the systems and thus significantly increases the computational cost of theoretical research. Moreover, their presence increases the probability of the occurrence of interactions other than C··· H-D, which additionally complicates the research. Probably for these reasons the amount of theoretical studies on this type of systems is small. As stated in the Introduction, the purpose of this article is to fill this gap to some extent. The investigated systems will be described in the next subsection.
