Large-amplitude vibrations and microwave band spectra. Part 2.—1-Adamantarnine

Abstract

The microwave spectra of 1-adamantamine, [²H₁]1-adamantamine and [²H₂]1-adamantamine have been observed at room temperature in the frequency range 8–40 GHz. The spectra consist of both µ_a R-branch and µ_c Q-branch bands. The µ_a R-branch bands of the three isotopic species appear as doublets and this is attributed to the inversion of the amine group. A simple treatment of the internal rotation and inversion in which the molecule is regarded as a symmetric rotor shows that six series of ΔK=+1 µ_c Q-branch bands should be associated with the ground vibrational state. The band frequencies are shown to follow approximately the formulae v=a(2K+1) or v=a(2K+1)+b, where the constants a are the sum of the rotational constant A–B and contributions from the internal motions. The formulae are verified experimentally for [²H₂]1-adamantamine. The barrier to internal rotation in [²H₂]1-adamantamine is determined to be V₃= 9.0 (9) kJ mol^–1[750 (70) cm^–1] and the inversion splitting to be Δ^°_i= 1250 (30) MHz.