INTRAMOLECULAR HYDROGEN BONDING AND THE ANHARMONICITY OF THE NH2 STRETCHING VIBRATIONS IN SUBSTITUTED ANILINES

Abstract

The fundamental and first overtone NH2 stretching bands in 55 ortho-, meta-, and para-substituted anilines, measured in dilute carbon tetrachloride solution, have been compared. For all ortho-substituted compounds (except o-alkyl, o-COCH3, and o-COOCH2CH3) splitting of the symmetric band is observed in the first overtone region. On the basis of unusually low mechanical anharmonicity the high-frequency component is assigned to a vibration arising in an intramolecularly bonded NH2 group. The mechanical anharmonicity of the "free" NH2 group is found to be more dependent on the electronic nature of meta and para substituents for the symmetric mode than for the asymmetric mode, with minor changes on ortho substitution. The relative electrical anharmonicities for both modes have been estimated from changes in the ratio of the integrated intensity of the fundamental band to that of the first overtone band. For the asymmetric mode the electrical anharmonicity is independent of the electronic nature of the substituents, but depends on their ring position. For the symmetric mode the electrical anharmonicity decreases as charge separation increases in the molecule, with no significant difference for ortho-substituted compounds. These results are in qualitative agreement with a mechanism previously proposed for these vibrations.