Methyl Isocyanide: The Low-Frequency Bands ν8 and ν7, and a Determination of the Rotational Constant A0

Journal of Molecular Spectroscopy(1995)

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The weak perpendicular bands of the low-frequency fundamental bands ν8 and ν7 of CH3NC were measured in a long path cell with a high-resolution Fourier transform spectrometer. Analysis of the ν8 band shows its origin to be at ν0 = 267.3173 cm−1, some 4.3 cm−1 higher than previous estimates that were based on an unresolved peak of the bunched-up Q branches seen in a low-resolution scan. The ν7 band, at ν0 = 1130.6588 cm−1, is slightly affected by Fermi resonance with the combination ν4 + ν8, whose band is observed just above ν7. On combining the observed ΔK = +1 transitions of ν−7 + ν−8 we reported recently, with the ΔK = −1 transitions of ν−8 and of the hot band ν−7 + ν−8 − ν−8 accompanying ν7, we have been able to obtain accurate values for the ground state rotational constant A0 = 5.247234(77) cm−1 and for the centrifugal distortion constant D0K = 8.5626(24) × 10−5 cm−1, which had never been experimentally determined. Accurate spectroscopic constants are also reported for the ν8, ν7, and ν4 + ν8 states.
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