Phenol bonding with two acetonitrile molecules

FIGURE 54. The acetonitrile dimer. Bond lengths are in A. Adapted from Reference 761 with permission 

The cyclic ACN dimer is formed thanks to two weak C—H- -N hydrogen bonds characterized by N- -H bond lengths of 2.633 A and a bond angle of 137.6°. They are manifested spectroscopically by the appearance of two far-IR bands N) and 

H- N) at 87 and 89 cm , respectively. Two CN stretching vibrations are also organized into the symmetric and asymmetric bands placed very close to each other, at 2358.1 and 2358.9 cm. Consequently, we conclude that the formation of the cyclic ACN dimer leads to a red shift of y(C=N) of the free acetonitrile molecule by 5-6 cm.  

---