Knowledge Bank

The infrared spectra of monomeric and dimeric species of $CH3CN$, $CD3CN$, $CH3NC$, and $CD3NC$ in solid argon at $20\circ K$ have been investigated using samples prepared by a pulsed matrix isolation technique. For methyl cyanide, assignments of bands due to monomers and dimers including the mixed $CH3CN-CD3CN$ dimers, were made from concentration studies. The results for the dimer are consistent with a structure of $C2h$ symmetry, in accord with the antiparallel dipole model suggested by second virial coefficient and viscosity data. Similar studies with $CH3NC$, $CD3NC$ and $CH3N13C$ suggest a dimer with the same structure. The analysis clarifies the assignments of bands in the $\nu 4,\nu 1\nu 5,\nu 3$ region of the $CH3CN$ monomer and the $\nu 2,\nu 1,2\nu 6$ region of monomeric $CD2NC$. Force constant calculations for methyl cyanide show a decrease in the $C\equiv N$ stretching and an increase in the $C-C$ stretching force constants upon dimerization.