Potential Energy Constants, Rotational Distortion Constants, and Thermodynamic Properties of Methylacetylenes

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Abstract
A special iterative method of obtaining the potential energy constants from vibrational and rotational spectra has been developed. The method has been applied to determine the potential energy constants of some methylacetylenes. The fundamental vibrational frequencies of the deuteriated and tritiated isotopic forms of the acetylenes have been calculated by transferring the force constants. The nonvanishing moments of the inertia tensor derivatives have been obtained for a CY3C≡CX type of molecule. Using a perturbation treatment, the rotational distortion constants DJ, DJK, and DK for these symmetric rotor molecules have been evaluated. These values have been compared with the values from microwave experiments where such data are available. The thermodynamic properties (heat capacity, heat‐content function, free‐energy function, and entropy) have been computed from the vibrational wave numbers for all the molecules. These calculations are based on a rigid‐rotor harmonic‐oscillator approximation for the ideal gaseous state at 1 atm pressure and for the range 100° to 1000°K.