The Raman spectra of 10 isotopic species of polycrystalline NMA were recorded at 35 K between 6 and 200 cm −1 . Below 110 cm −1 at least 8 transitions are observed which are mainly due to external modes. Bands located between 110 and 170 cm −1 show unexpected isotopic effects and are interpreted taking into account IR and NIS spectra. A new internal torsional mode around the bond is proposed. The corresponding potential function involves two minima (Δω = ± 10°) separated by a weak barrier (∼600 cal/mole) indicating a nonplanar molecular skeleton.
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Fillaux et al. (1978) studied this question.
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