Cyanoacetylene, hydrogen bonding

Figure 9. Infinite molecular chains linked by C-H - N hydrogen bonds crystalline hydrogen cyanide (N=C-H, top, drawn based on published atomic coordinates [43]) and cyanoacetylene (N=C-C=C-H, bottom, drawn using published atomic coordinates [29]).

However, in the case of CH donors and pyrrole, a weak NH donor, the correlation is limited by base type. In Figure 4.8, the hydrogen-bond enthalpies of HC=CC=N are plotted versus the 4-fluorophenol affinity scale in CCI4. The results may be described by three roughly parallel lines the upper line for n bases, the middle line for sulfur, oxygen and Nsp bases and the lower line for Nsp and Nsp bases. This line separation amounts to about 5 kJ mol between the middle and lower lines. Thus, whereas the cyanoacetylene affinities... 

Figure 4.8 Limited linear enthalpy relationship for formation of hydrogen-bonded complexes of cyanoacetylene and 4-fluorophenol. The upper line represents n bases (benzene, mesity-lene, hexamethylbenzene), the middle line sulfur (tetrahydrothiophene), Nsp (acetonitrile) and O (ethers, CO, SO, PO) bases, and the lower line pyridine and tertiary amine bases.

Figure 6.1 shows the stockholder decomposition of the theoretical deformation density of the cyanoacetylene molecule, H—Cs=C—C=N (Hirshfeld 1977b). The overlap density in the bonds is distributed between the bonded atoms. The assignment of part of the density near the hydrogen nucleus to the adjacent carbon atom manifests the difference between fuzzy and discrete boundary partitioning methods. 

In conclusion, the reaction mechanism for the CN + C2H2 reaction — the prototype of the class of reactions CN + H-(C=C)n-H — is relatively simple amongst the possible approaches, the addition of the CN radical to the triple bond on the C side is the favored one the dissociation of the cis trans forms of the addition intermediates leads to the cyanoacetylene product and atomic hydrogen. 

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