Methylene chloride: The mid-infrared spectrum of an almost vibrationally unperturbed molecule

The infrared gas phase spectra of ¹²CH₂Cl₂, ¹³CH₂Cl₂, and ¹²CD₂Cl₂ have been studied in the region below 6200 cm⁻¹ under conditions of high resolution. Some 30 vibrational levels can be identified for each isotopic species and assigned unequivocally in terms of the band contours displayed. Direct observation has been made of the very weak ν₂ fundamentals in all species, and of the “inactive” torsion fundamental of CD₂Cl₂. Rotational analyses have been performed on the observed Q-branch features of over 30 bands. For each isotopic species, it is found, with one exception, that all vibration levels fit accurately the simple second-order perturbation expression involving ν′s and x′s. The sole exception in each species is the overtone region of the CH₂(CD₂) stretching vibrations. Here anharmonicity effects bring vibrationally interacting levels into close enough proximity for resonance effects to become just slightly more than of second-order importance. Full analyses including Fermi resonance are made. The effects of the Darling-Dennison resonance between the overtones of the CH stretching fundamentals are observed and corrected for in terms of a simple assumption. Most of the resulting anharmonicity constants bear isotopic relationships similar to those established for H₂O and D₂O. It is concluded that, with the exception of the CH(CD) stretching overtone region, methylene chloride isotopomers behave as vibrationally unperturbed molecular systems in the mid-infrared region.