Knowledge Bank

“The infrared spectra of seventeen amine hydrohalides have been measured in the solid state, in aqueous solutions and in their conjugate amine and their $N+H$ stretching and scissoring bands are discussed. The stretching bands of hydrogen bonded aliphatic and aromatic amine hydrohalides fall into the same part of the spectrum. In the solid state, the hydrogen bonds are of type $N+-H-X$. The aliphatic amine salts usually exhibit many sharp bands while the aromatic amine salts have broad but composite bands. Tertiary amine salts show a hypsochromic shift in the order hydrochloride, hydrobromide, hydriodide; primary and secondary aliphatic amine salts show a slight bathochromic shift in the same order. Primary and secondary aromatic amine salts behave less regularly although in most cases the shift is hypsochromic. These facts can be explained qualitatively by considering the charge distribution in the ionized omino groups and the contribution of electrostatic and delocalization effects to the energy of the hydrogen bonds. The anharmonicity of the potential surface which causes the appearance of strong combination bands may lead to the broadening of bands or to many sharp bands. There is no parallelism in this case between shift and broadening of the bands. The assignment of a strong band which appears in many cases near $2000cm-1$ is discussed. The above mentioned characteristic band shifting is absent in the spectra of aqueous solutions and it is concluded that in this case hydrogen bonding is not of the $N+-H-X$ but of the $N+-H-O$ type.”