Microwave spectra of CH314NC and CH315NC in the 4ν8 state

The J → J + 1 transitions (J = 4, 5, 6, 7, 8) in the microwave spectra of methyl isocyanide and its ¹⁵N derivative have been obtained and analyzed in the 4ν₈ degenerate vibrational state. Theoretical analytical expressions are given for the rotational frequencies in a 4ν_E state, separately for the l = 0, ±2, ±4 values. These formulas could only be used as a starting point for the assignment and analysis, because of the complexity of the spectrum and the number of accidental resonances appearing in many l = 0 and ±2 lines for low K values. A detailed analysis was obtained through a diagonalization of the energy matrix. Many types of A₁A₂ doublings could be localized; in particular for CH₃¹⁵NC the K, l = ±1, ±4 doubling allowed the calculation of the g₆ coefficient of the 〈K, l|H|, l ± 6〉 term. As in the 3ν₈ state, some lines seem to undergo the effects of a vibrational resonance. A set of constants is given for both species, and a comparison is made with the other states.