New ground state constants of CH3Cl and CH3Cl from global polyad analysis

A global analysis of the infrared spectrum of chloromethane involving the ground state and the 13 vibrational states lying up to 2600 cm⁻¹ was recently achieved using high resolution Fourier transform spectra of pure isotopomers. More than 20 000 transitions (cold and hot bands) for each isotopomer ¹²CH₃³⁵Cl and ¹²CH₃³⁷Cl have been assigned and fitted with a standard deviation of about 3 × 10⁻⁴ cm⁻¹ close to the experimental precison. As part of this global effort, improved ground state constants up to sextic centrifugal distortion terms have been determined for each isotopomer taking advantage of the numerous allowed and perturtation-allowed transitions simultaneously fitted using our global model. The axial constants could be determined from ΔK ≠ 0 combinations arising from rovibrational local resonances within Polyads 3 and 5.