Temperature Effect on the IR Absorption Spectra of Hydrogen Bonded (N[sbnd]…N)+Homoconjugated Cations

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Abstract
The effect of temperature on the absorption spectra from 50 to 3200 cm−1 of a number of amine semiperchlorates and perchlorates of amines with two basic centres in which the hydrogen bridges (N [sbnd] H…N)+ are formed, was investigated. Three broad absorption regions may be distinguished in the spectrum, viz., between 100 and 900 cm−1, 600 and 1800 cm−1 and 1700 and 2900 cm−1. The latter one may be assigned to the stretching vibration νS (N [sbnd] H…N). A shift of this band peak (which is usually a doublet at room temperature for aromatic amines) towards longer wavelengths on decreasing the temperature may be an evidence that the bridge becomes shorter. Simultaneously with these changes, most salts under investigation exhibit a strong increase in low-frequency absorption intensity at about 400–600 cm−1. It may be suggested that this absorption results from a transition between the split levels of the zero vibrational state.

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