A double modulation microwave spectrometer was used to determine the linewidth parameter for some rotational components in the nν8 vibrations of methyl cyanide under 13C and 15N substitution. The linewidth parameters for self-broadening of the ΔJ=2←1 rotational components for the ground, ν8, and 2ν8 vibrations were determined over a pressure range of 1–13 mTorr and at a temperature of 300 K. An experimental method is presented to correct for modulation broadening when high derivatives are used to extract the absorption profile information from the signals. The eighth derivative profile was explored to determine if the spectral line shape remains Lorentzian over a range of modulation levels from 10% of Δν to more than 100% of Δν. These tests showed that the ratio of inner to next inner peak separations, designated in this paper as Δ, of the eighth derivative was the same as that for an assumed Lorentzian line shape. Thus, line shapes were assumed to be Lorentzian for theoretical analysis of the derivative profiles and comparisons made between experiment and theory on that basis. Dipole moments for vibrationally excited energy levels for the ν8 and 2ν8 vibrations were calculated from the linewidth parameter data after all corrections had been made for modulation, Doppler, and other nonpressure broadening effects.

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The microwave spectra of the two isotopes ('3CH3'2C15N and 13CH313C1SN) of the methyl cyanide molecule in the 3 u s , U, and v4 vibrational energy levels for the rotational components 1 s J s 5 (for a range of frequencies 17-95 GHz) were experimentally and theoretically examined. Rotational components in each vibration were measured to determine the mutual interactions in each vibration investigated.The method of isotopic substitution was employed for internal tuning of each vibrational level by single and double substitution of 13C in the two sites of the molecule. It was found that relative frequencies within each vibration with respect to another vibration were shifted in a systematic way. The results given in this work were interpreted on the basis of these energy shifts. Large departures between experimentally measured and theoretically predicted frequencies for the quantum sets ( J , K = *l, I = i l ) , KI = 1 in the 3us and U, vibrational states in 13CH3'2C'SN and 13CH3'3C'SN showed anomalous behaviour which was explained as being due to a Fermi resonance between these vibrational modes.

A technique which was employed earlier to calculate the rotational constants of CHaCCH has been extended to the ground and two vibrational levels in the ~' 8 vibration of CH3CN for several isotopic species. The moments of inertia and a computer iteration technique over experimental data for each isotopic species were employed to evaluate the constant A,, in an excited vibrational state for a symmetric top molecule. This method gave good estimates for A,, for each isotope. The angle of bending and the orientation of each molecular system in reference frames, one fixed on the carbon atom at the --C----N site and the other at the center of mass, were explored. These results are discussed in this paper. The method, which was applied by Tam and Roberts to the mqo, n --1, 2, 3, 4, vibrations of CH3CCH earlier and which was extended to the ~t0 = 1 vibration of CH3CCH with 13C isotopic species, has been applied to 13C isotopic species of CH3CN and seems to be a useful tool to extract the value of A o. Each of these molecules shows reasonable dependency of A,. over vibrational levels, values of A,, calculated from the geometrical model are in good agreement with those obtained by fitting the terms in the frequency equations, which contained A,,, to the experimental data through an iteration technique in which the value of A was allowed to vary.