High-Resolution Infrared Spectra of ν5 and ν3 + ν6 of CH3I

The infrared spectra of the ${\nu }_5$ and ${\nu }_3{\mathrm{\hspace{0.17em}+\hspace{0.17em}\nu }}_6$ bands of CH₃I have been measured with a resolution of about 0.03 cm⁻¹. The excited states of these bands form a Fermi diad and, from the analysis of about 600 lines in the spectra, the matrix element of the Fermi‐resonance operator was determined. From this the cubic normal‐coordinate force constant $k_{356}$ was found to be 29.40 cm⁻¹. In addition to the Fermi resonance, a further perturbation due to an x‐y‐type Coriolis resonance between the states ${\mathrm{|\hspace{0.17em}\upsilon }}_5\text{\hspace{0.17em}=\hspace{0.17em}1\hspace{0.17em}〉}$ and ${\mathrm{|\hspace{0.17em}\upsilon }}_3{\text{\hspace{0.17em}=\hspace{0.17em}1, υ}}_6\text{\hspace{0.17em}=\hspace{0.17em}1\hspace{0.17em}〉}$ was discovered and analyzed.