X-H- n hydrogen bond

The Cl-H- jt hydrogen bond in the crystal structure of toluene-2HCl has been characterized. In this case the it bonding electrons of the aromatic ring of toluene serve as the proton acceptor. The distance of the H atom to the ring center is 232 pm, as shown in Fig. 11.3.2. 

---

The situation is more complicated for X-H- - - n hydrogen bonds where two-centered proton acceptors are possible as for example acetylene or ethylene molecules. However also more than two-center Lewis bases play the role of proton acceptors benzene and the other aromatic, or more general, n-electron systems. The deeper analysis of the latter complexes, like for example, CeHe- - - HF, CsHs"- - - HF and CeHe- HCCH shows that F-H- - - n, F-H- - - C and C-H- - - n hydrogen bonds are observed for them, respectively. It seems that for CeHe- - - HF complex there is two-center proton acceptor while for CeHe- - - HCCH this is six-center acceptor. 

Domagala M, Grabowski SJ (2009) X-H- n and X-H- N hydrogen bonds - acetylene tmd hydrogen cyanide as proton acceptors. Chem Phys 363 42-48... 

Note than in most X-ray structures of aromatic NH-heterocycles, the N-H -N hydrogen bonds are of fundamental importance (Sect. 3.7). 

Under N2, a 1 1 ratio mixture of para carborane 3 and base 5 in toluene at -20°C gave crystals identified by multinuclear NMR spectroscopy as a 1 1 adduct IS.12 Likewise, a 1 2 ratio mixture of 3 and 5 gave a 1 2 adduct 16. An X-ray structural determination of the latter adduct showed two base molecules to be linked solely to one carborane molecule via C-H—N hydrogen bonds. 

The trisilylmethane derivatives described in the section above display two conformational extremes already indicated in Scheme 2. While the alkyl-substituted derivatives 9a and 10a have an adamantoidal structure which is probably determined by weak intramolecular N-H N hydrogen bonding, the aryl-substituted species 11a, 12a, and 13a adopt an "inverted" conformation both in solution (established by NOESY experiments at variable temperature) and in the solid state (X-ray structure analysis) (Fig. 1) [7],... 

---